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Equipment Discussions >> Refractors

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JoeM101
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Triplet APO or Doublet APO?
      #5846672 - 05/07/13 09:28 PM

I'm curious, why would one spend upwards of 1 to 3k for a triplet apo if a doublet apo, there are a couple of awesome ones out there, with zero CA were available?

What advantage if any would a triplet offer? I know the doublet would cool a lot faster, so that's not working in the triplet's favor... anyone care to chime in?


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orlyandico
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5846755 - 05/07/13 10:18 PM

i understand it's because a doublet can't have zero CA... (i think it can only bring 2 of the colors together, the third one isn't in focus). unless it is long.

i think an f/12 APO doublet would be color-free. a fast APO doublet... certainly not.


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Gregg Carter
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5846773 - 05/07/13 10:26 PM

Joe, you might say there are different degrees of apo. A three-lens design gives the optical designer more options for reducing optical aberrations. For example, chromatic aberration can be reduced more in a three-lens design than in a two-lens design.

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maknewtnut
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5846932 - 05/08/13 12:08 AM

Aperture and focal ratio should be factors in such a decision. Smaller aperture and/or slower focal ratio will be more forgiving in regards to CA (in general).

You'll have to narrow down your desired aperture range because some folks might spend $3K or more for an apochromatic finderscope.


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Mike Clemens
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Re: Triplet APO or Doublet APO? new [Re: maknewtnut]
      #5846948 - 05/08/13 12:17 AM

Different people will have radically different opinions about what is color free also. Observing with a fluorite doublet like the Tak FS is certainly a contrasty pleasure, and many will say there is no color.

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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: Mike Clemens]
      #5847232 - 05/08/13 07:19 AM

Quote:

Aperture and focal ratio should be factors in such a decision. Smaller aperture and/or slower focal ratio will be more forgiving in regards to CA (in general).






Yes the triplet will have less spherochromatism i.e. It will be corrected over a widder spectrum, which is important for photo use. Math rules here; not only a matter of opinion.

for instance you can compare the strehl of a large ED doublet (the APM 152mm, which is quite afordable)

http://www.apm-telescopes.de/media/images/popup/Strehlkurve.jpg

to the strehl of the 152mm triplet

http://www.apm-telescopes.de/media/images/popup/LZOS152-1200TripletKurve600.jpg

you can see that the triplet is diffraction limited over the full visual spectrum, with marging (on the paper at least) while it is not the case of the ED doublet.

For visual use, the doublet can still do the job, if the diameter is not too big and the focal ration not too short;


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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: maknewtnut]
      #5847257 - 05/08/13 07:44 AM

Quote:

Aperture and focal ratio should be factors in such a decision. Smaller aperture and/or slower focal ratio will be more forgiving in regards to CA (in general).

You'll have to narrow down your desired aperture range because some folks might spend $3K or more for an apochromatic finderscope.






In the simplest terms, color free means hiding the chromatic blur behind the Airy disk. Since a large scope has a small Airy disk, hiding it becomes more difficult.

To my eye, an 80mm F/7 FPL-53 doublet is color free on tough objects like Venus, a 4 or 5 inch F/7 doublet would show some CA.

Jon


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jrbarnett
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5848069 - 05/08/13 03:55 PM

The only doublet APOs I'm aware of with "zero CA" all cost as much or more as triplet APOs with similar correction, at a given aperture.

Apochromatic doublets are few and far between. There are plenty of ED and fluorite doublets. Few of those, though, achieve truly color free performance. It's a lot easier to achieve a given level of color correction with three elements than with two, using similar materials.

Regards,

Jim

Edited by jrbarnett (05/08/13 03:56 PM)


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CounterWeight
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Re: Triplet APO or Doublet APO? new [Re: jrbarnett]
      #5848095 - 05/08/13 04:07 PM

my own experience with the Tak doublets which are my favorite (though FPL-53 doublet can also be very nice - loved my Onyx80) -as opposed to- triplets, the one thing that stands out in my mind is more to do with bightness/magnification.

I think as you increase mag and object dim the triplet holds up better when you start 'getting up there' if that is possible/meaningful for the object. As Mike mentioned the CaF2 has a color warmth FPL53 doesn't have (at least with the Tak doublets)


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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5848113 - 05/08/13 04:16 PM Attachment (194 downloads)

Well, I'll finish my own answer soon, recently got a FPL-53 doublet and a FPL-53 triplet.

Because I'm basically Imaging, using filters in Monochrome,I choosed a common star this time is Alkaid (eta Ursa Major). I image the star on every channel ex. L,R,G,B,Ha,OIII and SII using a bahtinov mask and observe the central spike how it get displaced by channel.

First test was the FPL-53 Doublet, next test is the FPL-53 Triplet on same star using same filters, Baader.


Quote:

I'm curious, why would one spend upwards of 1 to 3k for a triplet apo if a doublet apo, there are a couple of awesome ones out there, with zero CA were available?

What advantage if any would a triplet offer? I know the doublet would cool a lot faster, so that's not working in the triplet's favor... anyone care to chime in?




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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5850460 - 05/09/13 04:57 PM Attachment (152 downloads)

Same star with a triplet FPL-53.

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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5850475 - 05/09/13 05:04 PM

an Arcturus star test to compare on both scopes are posted here: http://www.flickr.com/photos/70224675@N06/

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terry59
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5850508 - 05/09/13 05:26 PM

Quote:

an Arcturus star test to compare on both scopes are posted here: http://www.flickr.com/photos/70224675@N06/




The doublet looks better to me. I see no reason to pay up for that particular triplet

Edited by terry59 (05/09/13 05:32 PM)


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A. Viegas
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5850542 - 05/09/13 05:49 PM

Jose-

Thank you for posting those images, I am curious what are you looking at to distinguish the quality of the objectives in each of the various images - i see very little difference in the difraction spikes - sure the triplet is sharper... i wonder how this same star test would look using a SCT...

Al


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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: A. Viegas]
      #5850617 - 05/09/13 06:31 PM

focus only on the central spike, as this is used for focusing.

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JoeM101
scholastic sledgehammer


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Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5852395 - 05/10/13 05:46 PM

I'm intrigued, the doublet sure seems much more contrasty than the triplet, at least that's how it looks to me...

the reason for the original post, take this scope for instance
http://www.celestron.com/astronomy/celestron-sky-watcher-pro-120ed-apo.html

the reviews on this are only good to amazing.. forget about the fact that it's not a premium scope, irrelevant, wouldn't this setup be the most contrasty, unlike what a previous poster said, the more glass you put in the path the dimmer the view, unless somehow the lens is amplifying the light

close attention to the headings
- Virtually Eliminates Residual Chromatic Aberration
- Schott Glass Used in Selected Models for Crown Element

i know, i know, whatelse they going to say right, but then there are reviews like this one Sky-Watcher Pro

curious to hear more thoughts on these observations because it seems logical to conclude that a doublet with high quality lenses, engineered to the highest standards, etc...


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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5852515 - 05/10/13 06:48 PM Attachment (120 downloads)

here's my recent sample on the Sky watcher 100 mm ED f/9 Doublet

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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5852517 - 05/10/13 06:49 PM Attachment (122 downloads)

And this sample done with the Orion ED80 Triplet. spikes were done with Starspike pro on Photoshop

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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5852623 - 05/10/13 07:42 PM Attachment (121 downloads)

I have spent more than a year with my ED 80mm triplet so here's another.

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crazyqban
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5852665 - 05/10/13 08:07 PM

Jose, that Rosette is amazing, very nice!!!

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Arizona-Ken
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5852833 - 05/10/13 10:05 PM

Quote:

And this sample done with the Orion ED80 Triplet. spikes were done with Starspike pro on Photoshop




Jose:

A beautiful picture and very well done. However, it is funny to me after reading all the posts about how much better refractors are than any kind of newtonian that someone puts star spikes in the picture to "fake" a spider vane. I know it is done for esthetic purposes, but it is funny nonetheless.

But if you have other pictures of this quality, please feel free to post as many of them as you like. Again, very nice job.

Arizona Ken


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Mentor
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Re: Triplet APO or Doublet APO? new [Re: crazyqban]
      #5853376 - 05/11/13 08:42 AM

LOL now Jose is just showing off

Awesome pictures, very well done!


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kevint1
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Re: Triplet APO or Doublet APO? new [Re: Mentor]
      #5853625 - 05/11/13 11:38 AM

Those are stunning photos, Jose. Your work is amazing!

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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: kevint1]
      #5854105 - 05/11/13 04:44 PM

Thanks Kevin, So far like both refractor

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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: Arizona-Ken]
      #5854110 - 05/11/13 04:45 PM

Hi Ken,
feel free to visit my link galleries on my signature.


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joelimite
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Re: Triplet APO or Doublet APO? new [Re: crazyqban]
      #5854163 - 05/11/13 05:27 PM

Absolutely stunning images, Jose!

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Eddgie
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5855167 - 05/12/13 10:35 AM

Quote:

more glass you put in the path the dimmer the view, unless somehow the lens is amplifying the light




Not really. Modern coatings make the transmission of at Triplet very close to that of a doublet.

Even my 25 year old oil spaced triplet has 96% transmission and it only used a simple single coating.

Almost all refractors have some amount of chromatic aberration.

The only question is "How bad is it at a given wavelenth of light?"

With an achromatic doublet, you can only achieve your best color correction for two colors at a time. You can have other colors close enough that they fall inside the Airy Disk, but it is almost impossible to get the reds and blues to do so, so the designer usually allows blue/violet to be unfocused so that the energy is spread out over the field making it hard to see.

With glasses of special dispersion characteristics (ED glasses) used at small apertures and moderate focal ratios, we can greatly improve things so that more than two colors can cross best focus and most others are close enought that when used visually, we don't see these unfocused colors easily.

If the focal ratio is long, even if we use a camera, the amount of fringing may be acceptably low.

But imagers want to use a fast system to reduce exposure time, but when you do this, it makes it harder to keep all of the colors reaching the same focus.

Things that are not visable to the eye become glaring when the camera shutter is open for 20 minutes.

The solution is to use a triplet. The triplet lens can be made somewhat faster and still keep the light from the different colors well focused.

For most visual use, it simply is not necessary to go to a triplet. Performance advantages over an f/8 or f/9 ED doublet are meaningless for visual astronomy. The colors all come to a close enough focus that unless the obejct is very very bright, we see the image as color free, and the amount of contrast lost to chormatic aberration is to low to be meaningful.

But for imaging, where shorter exposure times are very important, those wishing to see stars as close to their natural color as possible, and as free as possible of any fringing even on bright stars, a triplet becomes more and more necessary.

The number of elements do nothing to lower the contrast just because of the light loss. That is meaningless today (and has been for some very long time now).

The tendency of a glass element to act like an infinite number of prisms to disburse the different colors of light coming though the objective though is the real issue, and we are talking about a huge spectrum of colors.

The more elements, the more colors you can get to be in perfect focus at the same time.

Again, unnecessary for all colors to come to the exact same focus for visual use, but the long exposure astro-camera sees things that our eyes can't see.

There are many other factors here though.

For imageing, fast scopes have more steeply curved fields, and even if the color correction is near perfect, it may still become necessary to use special lenses so that all of the stars in the field will be in focus.


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junomike
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Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5855485 - 05/12/13 01:24 PM

Quote:

Again, unnecessary for all colors to come to the exact same focus for visual use, but the long exposure astro-camera sees things that our eyes can't see.




Eddgie, Excellent Informative post (as per usual), but I have to disagree with the above statement.

IME, I have detected CA in all the ED Doublets I've looked through. I haven't had the privilege of viewing through the Tak FS-series scope, so I can't comment on them, but I have had experience with the Orion/SW FPL-53 Doublets. The 100mm F9 was very close to being visually CA free, but at higher powers It was noticeable, although you did have to look for It.

My biggest complaint though is the tonal change(s) associated with and directly related to magnification.
Using the Moon as an example, At lower mags, the Image is near perfect, but as you move up 150X - 200X you can see two things happen:

1) The pure white color of the Moon's surface starts to change to a slight cream color. The higher the magnification, the stronger the color.
I have noticed this in my AT111EDT also, but to a much lesser degree)

2) The dark black colors (Craters) can change to a dark purple or off black.

It's easier to detect these If you have a Dob or other CA free telescope nearby for comparison.

Overall I felt that this was robbing me of potential contrast and/or detail. For this reason I upgraded to a Triplet. Was I happy about forking out an excessive amount of $$$ for a Triplet Refractor when I could've obtained a Doublet for less than half the cost? No, but I am happier than If I had used half my scope funds on a Doublet I would not have been happy with.

Mike


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terry59
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Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5855787 - 05/12/13 03:40 PM

What about when imaging with a mono CCD and focus is adjusted for each filter? Is there any benefit to a triplet then?

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orion69
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Re: Triplet APO or Doublet APO? new [Re: terry59]
      #5855845 - 05/12/13 04:03 PM

Well, probably there would be very little difference between doublet and triplet when imaging with mono camera.
Regarding AP, biggest advantage that triplet has over doublet is that it's much easier to manufacture faster scope for the same color correction. IMHO


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azure1961p
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Re: Triplet APO or Doublet APO? new [Re: crazyqban]
      #5856909 - 05/13/13 07:25 AM

Jose,


Stunning photos !!!! That its through an 80mm lens is amazing.
Pete


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Eddgie
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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5857004 - 05/13/13 08:53 AM

Quote:

but I have to disagree with the above statement.




I did not say that there would be no CA.

I said that it was unnecessary to have a triplet to get good visual performance.

The amount of fringing on modern ED refractors is so small that it will likley only be noticed on bright stars.

But for visual use, the question is does that degrade the amount of detail that the visual observer will see.

Of course it does, but is it enough that the visual observer will miss detail when using a moderate focal ratio doublet vs a triplet?

In other words when using visually, will the triple allow them to see more datail?

It is not necessary to have a tripet to get accepeable visual performance. If one wishies to be able to view the brightest stars in the sky and not see any fringe, then maybe they want a triplet.

But if the goal is to see all of the detail in all of the targets one points the telescope at, a triplet is not really necessary.

And if one wants completly color free images, one can buy a reflector.

But for visual use, I stand by my belief that unless the system is very large and very fast, for visual use, there is no more detail to see using a triplet than an ED doublet.


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Eddgie
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Re: Triplet APO or Doublet APO? new [Re: terry59]
      #5857024 - 05/13/13 09:01 AM

To be 100% honest, I don't know the answer.

I suppose it is a value proposition. Why use a filter wheel and take three exposurese rather than use a scope with a broad range of correction and take one exposure?

I have over the years seen many superb photos taken with achromats using filters, so of course it is possible.

Looking at some of the pictures taken with state of the art large aperture air spaced triplets though, you can see why people want them.

For visual use though, which is what the OP was concerned with, unless the apeture is relatively fast and large, it is doubtful that he would see any detail in a triplet that would not be visisble in an equal apeture, equal quality doulet of slightly more moderate focal ratio.

I believe that the OP is trying to determine if it is essential to have a triplet for visual use, and my own belief is that the advantage is only important for larger, fast refractors used for imaging.

For visual use, a 4" ED triplet will show you pretty much everthing that a 4" f/9 ED scope will show you.

He seemed to believe that the two elements of the ED scope would give better contrast, and of course it won't.


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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5857048 - 05/13/13 09:18 AM

Quote:


I believe that the OP is trying to determine if it is essential to have a triplet for visual use, and my own belief is that the advantage is only important for larger, fast refractors used for imaging.




Color correction depends on both aperture and focal ratio so I think one cannot characterize the color correction of an ED doublet based on aperture alone. Glass type matters too.

What a triplet allows for visual is a shorter, more compact scope of the same aperture with excellent color correction. A 4 inch F/9 probably has quite acceptable color correction for most observers but it's also about 3 feet long. A 4 inch F/6 or even F/7 is about 2 feet long, is handier to use, requires less substantial mount and may even be airline portable.

Jon


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bobhen
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Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5857092 - 05/13/13 09:51 AM

Quote:



For visual use, a 4" ED triplet will show you pretty much everthing that a 4" f/9 ED scope will show you.






That statement is at the heart of a lot of these posts. Is “pretty much” good enough? And, I presume, is what the OP is asking.

Why spend $4,500 on a Takahashi 120 when you can get a SkyWatcher 120 ED for less that $2,000. And, of course, then why would you spend for a SkyWatcher 120 ED when you can get a Celestron 120 mm achromat for a few hundred dollars?

Why pay Zambuto for one his 10-inch mirrors when you can get a 10-inch Orion Dobsonian for less?

The answer is that telescopes are interments that are pushed and used at the LIMIT of their performance. The objects observed are dim and very far away and “fine” details can be fleeting. High quality optics will ALWAYS show you more under ANY conditions (all else being equal).

If you only ever look at Saturn at 100x then you might not need a top quality optic (of any design). But if you are always pushing your scope to the limit, then higher quality optics will reward.

Those detail differences can be subtle, fleeting, and for many may not be worth the cost. BUT when pushed, the higher quality optic will be able to go the extra mile.

Bob


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PhilCo126
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Re: Triplet APO or Doublet APO? new [Re: bobhen]
      #5857143 - 05/13/13 10:17 AM

Summary: Apochromatic doublets are few and far between.... Apo = triplet

Basically, the quality of any apochromatic refractor depends on four items: (1) aperture, (2) glass quality , (3) lens configuration and (4) Focal length.

Astrophotography = an ED doublet will work but preferably get a true APO (air-spaced triplet).
Visual observations = an ED doublet will do very fine!


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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: PhilCo126]
      #5857183 - 05/13/13 10:43 AM

Quote:

Summary: Apochromatic doublets are few and far between.... Apo = triplet




"APO" is a definition and both doublets and triplets are capable of meeting the definition.

It's worth noting that a doublet can have better color correction than a triplet... of course the focal length and glass type may not be identical...

Jon


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BarrySimon615
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Re: Triplet APO or Doublet APO? new [Re: PhilCo126]
      #5857298 - 05/13/13 11:47 AM

Summary: Apochromatic doublets are few and far between.... Apo = triplet

Not all doublets are mere achromats and not all triplets are automatically apochromats.

Basically, the quality of any apochromatic refractor depends on four items: (1) aperture, (2) glass quality , (3) lens configuration and (4) Focal length.

.....and the optician's skill, some do it better than others.

Astrophotography = an ED doublet will work but preferably get a true APO (air-spaced triplet).

Define "true apo" AP labels their doublets like my Star 12ED from the early 90's as apochromats. Are we now looking at a different definition where this is no longer true?

Visual observations = an ED doublet will do very fine!

Correct, but they, at least some of the better ones, do just dandy for astrophotography as well.

(Responses in italics and color are made to each statement from a previous post.)

Barry Simon


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audioaficionado
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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5857459 - 05/13/13 12:59 PM

Quote:

...A 4 inch F/9 probably has quite acceptable color correction for most observers but it's also about 3 feet long. A 4 inch F/6 or even F/7 is about 2 feet long, is handier to use, requires less substantial mount and may even be airline portable.

Jon




This might be the direction I go in the future upgrading my refractor. My 3' scope isn't very heavy, but it is somewhat unwieldy to handle at times.


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junomike
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Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5859908 - 05/14/13 01:19 PM

Eddgie, Your posts have been informative to me in the past and I always enjoy the way you fully explain yourself, but I am disappointed in you putting your opinion across as fact.

Quote:

I said that it was unnecessary to have a triplet to get good visual performance.




Subjective! For you yes, for others maybe not!

Quote:

The amount of fringing on modern ED refractors is so small that it will likley only be noticed on bright stars.




Again, subjective? "likely only" is an uncertainty! If one doesn't see anything, you'd be correct, If one does see something, you'd be wrong.

Quote:

But if the goal is to see all of the detail in all of the targets one points the telescope at, a triplet is not really necessary.

And if one wants completly color free images, one can buy a reflector.




I find this particular statement ridiculous! "Not really" is "grey area" (again) and the mention of a reflector is largely out of category, unless you know of a Doublet or Triplet Reflector?

Quote:

But for visual use, I stand by my belief that unless the system is very large and very fast, for visual use, there is no more detail to see using a triplet than an ED doublet.




Ah, ok, now some realism. Although you have forgotten more than I'll even know about this Hobby, "your belief", although strongly respected is not fact. It's an opinion and no more correct than mine.

Mike

(no emoticons used and no harsh words, just a friendly rebuttal!)


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BarrySimon615
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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5860038 - 05/14/13 02:14 PM

The post above is very interesting. Like Eddgie I have years of objective experience to back up my objectively based personal assessment that Eddgie's postulates are essentially true. I cannot fault them because my experience agrees with his (and I have owned 3 triplet apo's plus another half dozen or so scopes, while only doublets, considered to be apochromatic.

Experience and statements based upon that experience (without detailed test data) is by it's very nature subjective.....but have confidence that it is well meaning and honestly offered and for many, as good as fact.

Barry Simon


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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5860211 - 05/14/13 03:19 PM

so after reading the many opinions / factoids, I find myself wondering if anyone has something to say about the claims made by some reviewers that the SW 120ED Schott/FPL-53 Doublet is in-fact a true APO able to hold it's own against any similar aperture triplet for both visual and AP..

i suspect this may ignite something.. i'll find the reviews i refer to, just don't have them handy yet, but i'm sure a few of you have come across one or two..

at f/7.5 it's not so bad for AP, obviously a faster scope is highly preferred but the price makes this scope a serious contender..imho, just mho


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cjc
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860271 - 05/14/13 03:56 PM

My Sky-Watcher Equinox 120mm F/7.5 doublet with S-FPL53 and a Schott manufactured crown glass of unknown type (but perhaps N-ZK7) is nearly colour free in practice. The image is sharp with only a very slight violet (or perhaps indigo) splash around bright stars. The focused image is stable and white even under unsteady atmospheric conditions (which we have often here in the UK). Out of focus, on a star test, colour can easily be seen, but with a solar continuum (540nm) filter, it shows very clean diffraction rings.

The baffles (push fit I understand) are placed correctly, so far as I can tell. It is vignetted by the very long, 205mm, 2" draw tube with a diagonal (112mm for my 2"). I estimate (misusing a diagonal calculator here) that the clear field for an eyepiece with its focal plane at the eyepiece barrel shoulder, is only about 12mm in diameter. For a field of 27mm (maximum 1.25" eyepiece field) the outer rim will be vignetted by about 0.12 magnitude and for a 45mm field (maximum 2" eyepiece) by about 0.4 magnitude, or to about 70%.

The scope cools quickly and weighs just 15lb. It is reasonably well constructed and is nice to handle. I find it a delight to use visually but that might not hold for imaging.


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860277 - 05/14/13 03:58 PM

I own an EON 120 which is essentialy the same as the SW 120ED. Both are doublets with FPL-53 which has the same optical properties as Fluorite. In focus, my EON shows no color at all. Out of focus a little, but that would be true of triplets also and who the heck uses their scope out of focus? And don't let people tell you that an EON or equivelant can't be used for AP. There is a guy on CN who uses an EON to produce stunning deep sky and solar system photos. His user name here is hfjacinto, which if you click on it under "User List" will take you to the astrophotos that he has taken with his EON 120. They are stunning.

I also used to own a FS-102 Tak doublet. It too, was essentialy perfect.

And here was a little factoid that most people don't want to talk about. Back in the late '80s when Roland Christen was starting Astro Physics his triplets showed quite a bit of color. Now I totally respect what Roland has done for amatuer astronomy, but my point is, it is the designer who determines how good or bad a doublet or triplet is based on his design. Some doublets are excellent as are some triplets. Some doublets aren't so good as are some triplets. You have to try and rely on people who own the scopes to give you an accurate representation of how the perform. And that can't always be counted on. Also some people will try to convince you that their 5 in. Astro Physics is better that anything else. Again, not true.

Good luck in your quest.


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BarrySimon615
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5860312 - 05/14/13 04:21 PM

Check these out, taken with a 120 mm ED doublet


www.cloudynights.com/ubbthreads/showthreaded.php/Number/5857373

Barry Simon


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greju
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5860333 - 05/14/13 04:32 PM

Someone much smarter than me once said "a good doublet is better than a bad triplet". And it used to be said that it takes three lenses to even come close to the definition ( ) of apochromatic. Unless technology has surpassed that statement doesn't it still hold true?

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CounterWeight
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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5860356 - 05/14/13 04:44 PM

Those are beautiful images (color on the cluster a bit odd but it's clear) and show what can be done. I think in all things it's important to try these options and see for yourself were or if something has value, and to what degree. IMO for imaging things like the focuser quality, cost and availbility of flattenrs and reducers that will work with your imaging chip loom large, these IMO not same importance for visual only.

In my link below are some of those same object through a budget FPL53 triplet... IMO in ways images are more about the imager than than the object. There are difference in imaging with a doublet, maybe more critical 'OSC', but again that is my being subjective.

I don't see it as a good new / bad news scenario unless it's that bad news is there are a lot of choices in the recent years, and can make decision difficult?


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Re: Triplet APO or Doublet APO? new [Re: CounterWeight]
      #5860364 - 05/14/13 04:48 PM

My ST80A quit showing CA when Jim's shadow passed over it as he walke by....now it wouldn't dare.....

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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: csrlice12]
      #5860457 - 05/14/13 05:13 PM

In an effort to come back to my original question, taking all into account, would it be fair to say that an f/7 doublet with the highest quality glass, i.e.LZOS, Schott, etc., with test report showing minimum standard of 95% Strehl, etc, maybe throw in a field reducer like a Riccardi (f/5.2), would be just as good as a premium triplet of the same aperture for both visual and AP? oh, and much cheaper?

Edited by JoeM101 (05/14/13 05:16 PM)


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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860515 - 05/14/13 05:24 PM

I don't know and I have been using refractors since the early 80's. I think, in other words it is my opinion, that doublet ED scopes can be great scopes, especially for visual observations. Takahashi FS fluorite doublets have a wonderful reputation that I believe is well deserved. Having said all that, I believe the best of the best refractors are the apochromatic refractors from the top apochromatric triplet makers like AP, TEC, LZOS and Takahashi. And the best views the observer can get will come from these scopes. My Opinion.

JimP


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BarrySimon615
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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5860641 - 05/14/13 06:12 PM

Here is something to consider in being well informed: A little overview of the subject (with a shifting definition). Apparently if an apo doublet does not quite meet the definition, neither would most "apo" triplets that are marketed as apos.

Additionally with a somewhat less stringent and perhaps more realistic definition, doublets do exist that do meet the criteria to be considered apos (as do triplets) in the context of the current discussion.

http://rohr.aiax.de/chapter%204a.htm

I find the discussion much like the purchase of the very best golf clubs or tennis rackets. Even the very best equipment in the hands of virtually all the players really does not improve your "game" all that much. It might look nice, but how much do you really gain?

Barry Simon


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junomike
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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5860671 - 05/14/13 06:30 PM

Quote:


I find the discussion much like the purchase of the very best golf clubs or tennis rackets. Even the very best equipment in the hands of virtually all the players really does not improve your "game" all that much. It might look nice, but how much do you really gain?




Barry, I agree! One has to decide where the want to get off of the "performance to cost" train.
But there's more to It than just improving one's game.
We're talking about something other than visual appearance. And how much one feels they "really gain" is a personal preference related to the cost/performance train.

Some people will spend small fortune on Premium eyepieces that eek out that last 5% - 10% detail. Others would never spend that and rather upgrade the scope.

For me the difference between a Doublet Apo and Triplet Apo above 4" is detectable and sometimes obvious.
Others appear to be quite content with Doublets and all the power to then!

One question for the Imagers of the 120ED's. Are those pics "raw" or processed? Curious?

Mike

Note: I really wish I knew someone with a Tak FS as I'd love to see what the views through one of them are like?


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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5860677 - 05/14/13 06:32 PM

well put Barry, how much of a difference is there really?

Has anyone had the chance to do a side-by-side? Has a serious comparison been done?

Edited by JoeM101 (05/14/13 07:05 PM)


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SteveG
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Re: Triplet APO or Doublet APO? new [Re: greju]
      #5860768 - 05/14/13 07:20 PM

Quote:

Someone much smarter than me once said "a good doublet is better than a bad triplet". And it used to be said that it takes three lenses to even come close to the definition ( ) of apochromatic. Unless technology has surpassed that statement doesn't it still hold true?




No not true. You can come close to an APO with 2 lenses.


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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: SteveG]
      #5860779 - 05/14/13 07:25 PM

wasn't it established that you can have a 2 lens APO?

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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860894 - 05/14/13 08:31 PM

The answer is no. Given a top notch designer a triplet adds 4 more degrees of freedom which can be used to reduce color error, field curvature, etc. There are two new radii of curvature, another glass type, and the spacing. The key is the top notch designer. And a triplet will most definitely not be cheaper than a doublet if they are both manufactured to the same standards.

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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5861424 - 05/15/13 03:11 AM

Quote:

I also used to own a FS-102 Tak doublet. It too, was essentialy perfect.





Rodger,

May I ask you for what reasons you seem to have replaced your essentially perfect FS-102 Tak doublet by a TSA-102 Tak triplet? Is it also "essentially perfect", or is it sth. more?

Chris


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ValeryD
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5861490 - 05/15/13 05:26 AM

Quote:

In an effort to come back to my original question, taking all into account, would it be fair to say that an f/7 doublet with the highest quality glass, i.e.LZOS, Schott, etc., with test report showing minimum standard of 95% Strehl, etc, maybe throw in a field reducer like a Riccardi (f/5.2), would be just as good as a premium triplet of the same aperture for both visual and AP? oh, and much cheaper?




Yes. But not much cheaper. In the reality can be even somewhat more expensive than a triplet.
And only fluorite doublets.
All depends of the designs of a doublet and a triplet. As farther from F/7 to slower F/D, as doublet becomes more and more advanced vs triplet.


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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5861576 - 05/15/13 07:32 AM

Quote:

In an effort to come back to my original question, taking all into account, would it be fair to say that an f/7 doublet with the highest quality glass, i.e.LZOS, Schott, etc., with test report showing minimum standard of 95% Strehl, etc, maybe throw in a field reducer like a Riccardi (f/5.2), would be just as good as a premium triplet of the same aperture for both visual and AP? oh, and much cheaper?




I think that you will find that at F/7, for a scope 100mm or larger, there are no high quality doublets using the highest quality glasses, (OK-4, FPL-53, Fluorite) they are all triplets. Valery could probably make you one but as he said, it would represent a cost savings.

It probably doesn't make sense to invest the time and expense fussing with the ED glass and not make it a triplet.

Jon


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5861712 - 05/15/13 09:16 AM Attachment (25 downloads)

I have an ED doublet and love it, I see no reason to spend so much for a triplet when most doublets work so well.

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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5861713 - 05/15/13 09:16 AM Attachment (40 downloads)

Still get diffraction spikes on bright stars

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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5861720 - 05/15/13 09:20 AM

BTW in my opinion, these doublet/triple posts get so over the top that they really are without benefit, those that believe a triplet is better will never see a doublet as a true APO and those that see no benefit from a triplet think the doublet as a good cost option. In the end, both are right.

A doublet can be essentially color free, but a triplet gives a designer more options. Both can be excellent.

As can be seen in photos, both work really well.


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5861729 - 05/15/13 09:24 AM Attachment (42 downloads)

With a 120MM ED EON.

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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5861760 - 05/15/13 09:41 AM

Sure I'll tell you why I replaced the FS-102 with the TSA-102. About 4 years ago OPT offered me a very good trade-in offer for the FS-102 on a C-14. So off it went. The longer I didn't have it, the more I missed it because it really was optically and mechanically a perfect scope. Finally after regretting it long enough I started looking to buy a pristine FS-102. Two popped up on Astromart, the first seller, who shall remain nameless because he frequently contributes here, withdrew his offer. The next FS-102 had a couple of flaws which I pointed out to the seller and he replied with a nasty email saying that "...obviously you want a perfect scope." A yeah! I had mine for 7 years and it had like one really small paint chip and that was it. So anyway, I was looking at the OPT website and they had 2 TSA-102s for sale, both in pristine condition. So I bought one. It had nothing to do with the TSA-102 being a triplet. I would have been just as happy with a pristine FS-102. If you'll also notice I do still have my FS-152 which is also a magnificant scope that I probably will never sell.

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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5861954 - 05/15/13 11:37 AM

I beg to differ with the poster that said there is no benefit to such posts, I think that anyone looking into buying a refractor would benefit, at the very least, from reading the various posts/ opinions CNers have... These posts shed light on some of the questions people may have about APOs.

Jon, I think you are mistaken when you say that a doublet cannot be an apo.... There are manufacturers like TS and APM/LZOS that offer True APO doublets... Adding a third element certainly does offer the optician options for correcting the light path but this can be achieved with 2 element design i.e. Schott or ok4 with FPL-53 ED... Why would reviewers and manufacturers claim zero to almost undetectable color in an f/7.5 SW 120pro ED, the obvious answer is not always the only one


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Binojunky
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5861963 - 05/15/13 11:42 AM

I had two ED 80 scopes, one a triplet(Meade 5000) the other a doublet (Orion ED80, the one that started the low cost revolution)the triplet was rubbish optics wise, the Orion I still have and use,the number of lens elements do not make or break for that matter a good scope,DA.

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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: Binojunky]
      #5861997 - 05/15/13 11:59 AM Attachment (39 downloads)

Some one asked for an UNPROCESSED image, which is imposiible as I can't load FITS files, so all are processed. But here is one that is just stretched so one can see the image, resized and converted to JPG.

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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5862000 - 05/15/13 12:00 PM Attachment (35 downloads)

After processing.

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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5862007 - 05/15/13 12:02 PM

Again the debates between which is better are really not worth it. So many scopes are killer, stop worrying about it and use the scope, most are so close you can't tell. The premiun scopes give you better harder, but a lot of the lower cost APOs work really well.

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Uela
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5862075 - 05/15/13 12:41 PM

I've got a doublet apo, skywatcher 120Ed Apo black diamond, and I'm very happy.
Eyepieces celestron.
The moon is very clear in any single dettails. Saturn is perfect clear, so clear that I can see the black space between the planet and the rings!
in my opinion skywatcher 120 Ed apo is a good telescope!!!
Higly recommended !!


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KaStern
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5862176 - 05/15/13 01:12 PM

Hi Joe,

Quote:

with zero CA




there is no apochromat with zero colour aberration.
Neither triplett, nor doublet.
If you want zero ca you have to buy a pure mirror scope.

The number of lenses is only one point, glas pairing is another one.
There are tripletts with more ca than doublets.
If glass pairing is the same the triplet has the same longitudinal ca
but lesser spherochromatism.

Cheers, Karsten


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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: KaStern]
      #5862990 - 05/15/13 07:40 PM

I just want to point out i'm that i'm not motivated by any particular reason but to know why one would opt for a triplet over a doublet... but i do thank you all for your opinions, i just wanted to see what informed CNers had to say.

i'm sure more opinions will follow, but i'd like to see a real star test side-by-side.. will try to get to a local star party and see what i can peep through


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BarrySimon615
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5863297 - 05/15/13 10:06 PM

Here is a post of mine from 2009 in which I report about my comparison of my Orion ED 100 to other scopes and finally to my new (at that time) Takahashi TSA 102 triplet apo. My conclusions as reported may seem contradictory in view of what I have written about doublets vs triplets in this thread. I would argue however that what I wrote then does not contradict what I have recently written. In spite of the differences seen I would still consider the Orion ED to be an apo and a very good scope at that. Now having said this, the Tak TSA, at least mine, was superior to my Orion ED 100 in several ways. Those ways included -
1) build quality
2) sharpness/resolution
3) ability to take magnification
4) apparent feeling that it could reach fainter magnitudes, a function I think of tighter stars with concentrated light, perhaps a function of better sharpness/resolution

I know some will read my post from 4 years ago as an endorsement of triplets over doublets and I can see why. Having said that I still feel strongly that in many instances a moderately priced doublet can be competitive with a more expensive triplet (witness my comparison of my Orion ED 100 with my Takahashi FCT 76 triplet. Anyway, here is what I wrote 4 years ago:

The Orion ED100 is a great entry level ED tube assembly that approaches true apochromatic performance and meets it if you go by many definitions.

I got mine within a day or two of when they were first announced (summer of 2004). Mine essentially replaced a Vernonscope Brandon 130 mm f/8 triplet apo tube that was sold due to non-use.

The Orion ED 100 had the advantage of light weight and a tube diameter of 100 mm that could easily be gripped and carried with one hand. I found the build quality to be generally quite good. I was ok with the non-retractable dew shield as I have never been a big fan of retractable dew shields. My main quibble was with the focuser which was quite rough and sloppy in mine, requiring some major surgery on the roller surface before the scope could even be used. I quickly replaced the focuser with a FeatherTouch which put the scope in a totally different league. My tube assembly initially cost about $1000 back in 2004. Add the FeatherTouch and it's interface and you now have a $1500 tube assembly. With today's price and a ready to fit Moonlite focuser, I believe you can do essentially the same thing for about $1000.00

In respect to performance the ED 100 did very well during a refractor shoot out that was done in 12/04. The other participant scopes included a TeleVue NP101, TeleVue 102, Takahashi FC100N, Vernonscope Brandon 94 Apo, and an Orion-Vixen 102 f/6.5 ED. We found that the top 3 scopes listed plus the ED100 were very close with the NP101 and the Tak FC100N being slightly better. The TV 102 and Orion ED100 were virtually the same optically with the FeatherTouch on the ED100 blowing away the standard TV focuser. The Brandon and the Vixen-Orion 102, being faster scopes could not keep up in respect to color correction, but they were clearly better than achromats.

I later compared my ED100 to my Tak FCT76 and found the resolution and color correction to be virtually the same on these scopes. The later sale of the FCT76 and it's P2Z mount had me contemplating the purchase of a new TAK TSA 102 (fall 2007). As good as the ED100 was I wondered if the TSA could be better? I did buy that TSA 102 and a shootout between it and the ED100 confirmed that it was no contest. Stars that appeared to be color free thru the ED100 suddenly had color when compared to the TSA 102. In addition the superior triplet objective concentrated light into smaller brighter points which allowed one to go .1 to .2 magnitude fainter. The tighter stars with the TSA 102 made the stars look a litte mushy with the ED100.

The question then becomes - Is the quality difference worth the current $2000+ differential in price ($680 vs $2699)? For some it is, and for others, it is not.


Barry Simon


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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5863379 - 05/15/13 10:36 PM

Thanks Barry... most informative!

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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5863846 - 05/16/13 07:27 AM

Quote:


Jon, I think you are mistaken when you say that a doublet cannot be an apo.... There are manufacturers like TS and APM/LZOS that offer True APO doublets... Adding a third element certainly does offer the optician options for correcting the light path but this can be achieved with 2 element design i.e. Schott or ok4 with FPL-53 ED... Why would reviewers and manufacturers claim zero to almost undetectable color in an f/7.5 SW 120pro ED, the obvious answer is not always the only one




Joe:

I think there is some confusion here.

- I did not say a doublet could not be an apo, in fact I had earlier said that it could be. You had asked about high quality F/7 doublets using the highest quality glasses and I had pointed out:

"I think that you will find that at F/7, for a scope 100mm or larger, there are no high quality doublets using the highest quality glasses, (OK-4, FPL-53, Fluorite) they are all triplets."

The important factor here is the F/7 focal ratio.. I don't know of any F/7, FPL-53 or OK-4 doublets that are 100mm or greater. I was not stating an opinion, I was simply stating a fact.

- The Synta 120mm F/7.5 is close to F/7 but it does not fall under what I consider high quality and having had experience reading reviews for the past 20 years, I take "zero to almost undetectable false color" claims with the entire salt shaker. I have read reviews of fast achromats that made similar claims.

- 80mm F/7 with FPL-53, to my eye, it is possible to hide the chromatic blur behind the Airy disk. The 80mm Synta ED-80 was F/7.5 and is considered color free by most. The 100mm Synta ED-100 was slowed to F/9, a little math shows this is consistent with maintaining the same level of color correction as the ED-80. With the ED-120, the designers faced a dilemma. If they slowed the ED-120 to maintain that same level of color correction, the scope would have to be F/11.2 with a 1320mm focal length. That would be a large, heavy, unwieldy scope. So, I think made they decision to compromise the color correction and go with the lighter, more manageable 900mm focal length.

- Color correction is a function of both focal length and aperture. Historically, 4 inch FPL-53, OK-4 and fluorite doublets have been F/8 to F/9, not F/7. Smaller doublets can be faster, larger doublets need to be slower.

Valery D. worked some magic with his 7 inch F/8 fluorite doublet by using an exotic mating element and according to Valery, it did not make the optics any less expensive than they would have been if it had been a triplet. One cannot use Valery as a valid data point, he works outside the normal envelope, whether it's the Chromacorr, ion milling 1 meter optic sets for Ritchey–Chrétien telescopes or designing the optics for the legendary Astro-Physics 10 inch Mak, he does things that others can't do.

Jon Isaacs


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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5863860 - 05/16/13 07:48 AM

Quote:

Stars that appeared to be color free thru the ED100 suddenly had color when compared to the TSA 102.




This was my experience as well. If one doesn't compare against a better color corrected scope, then It doesn't become obvious. Once you do see the difference It is hard to forget it.

Quote:

In addition the superior triplet objective concentrated light into smaller brighter points which allowed one to go .1 to .2 magnitude fainter.




I found that my slower Apo Tiplet seems to go deeper than my fast Achro Doublet while still giving up almost 10mm Aperture.

Quote:

The tighter stars with the TSA 102 made the stars look a little mushy with the ED100.




This again is really only seen when one compares two scopes. I was quite happy with mine Achro until.......

Mike


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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5863883 - 05/16/13 08:19 AM

Quote:

- The Synta 120mm F/7.5 is close to F/7 but it does not fall under what I consider high quality and having had experience reading reviews for the past 20 years, I take "zero to almost undetectable false color" claims with the entire salt shaker. I have read reviews of fast achromats that made similar claims.




Jon, I have viewed thru the 120ED's and I agree. I feel the relative claims of CA (or there lack of) is one of the issues with this scope. The problem is not only the fact we all experience CA differently (as well as specimen variation), but also the "pawning off" of the visible CA as minor or non-Intrusive. IMO, If It's there, It's there. If It isn't, It Isn't. For me personally there's little room for the "grey area", but I'm a sensitive stickler when It comes to CA.

Quote:

- 80mm F/7 with FPL-53, to my eye, it is possible to hide the chromatic blur behind the Airy disk. The 80mm Synta ED-80 was F/7.5 and is considered color free by most. The 100mm Synta ED-100 was slowed to F/9, a little math shows this is consistent with maintaining the same level of color correction as the ED-80. With the ED-120, the designers faced a dilemma. If they slowed the ED-120 to maintain that same level of color correction, the scope would have to be F/11.2 with a 1320mm focal length. That would be a large, heavy, unwieldy scope. So, I think made they decision to compromise the color correction and go with the lighter, more manageable 900mm focal length.




This is exactly how I see the SW/Orion/ WO scopes as well. I've viewed thru them all and consider the 80mm and 100mm CA free as well (YMMV ). The trade off with the 120ED is "bang on" IMO and IME.

Although I personally would have preferred the longer, thus better color corrected version of the 120ED, I believe the designers made the correct choice (trade off) as It has to be one of the best selling Refractors I know of. Most owners are quite pleased with It's performance.

Mike


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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5863890 - 05/16/13 08:24 AM

Jon,

Quote:

- The Synta 120mm F/7.5 is close to F/7 but it does not fall under what I consider high quality and having had experience reading reviews for the past 20 years, I take "zero to almost undetectable false color" claims with the entire salt shaker. I have read reviews of fast achromats that made similar claims.




Have you actually looked through one?

I think my pictures pretty much show my sample is essentially color free.


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junomike
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5863944 - 05/16/13 09:02 AM

Helder, I have actually looked through two Skywatcher 120ED's. The first one showed quite a bit of CA on the Moon (mainy dark crators). The second one was much better, but did reveal a little CA when pushed in Magnification.

Quote:

I think my pictures pretty much show my sample is essentially color free




No offense Helder, but here we go again! "essentially", "basically", "relatively".

For most that's fine, but It's not the same as CA free. I strongly feel I can see that last little bit of unfocused color.

I agree, your pics are fantastic and I was the one asking for a "raw" image as I was curious If the CA was somehow cancelled out (not saying It was or wasn't, just wondering)

Mike


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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5863988 - 05/16/13 09:23 AM

Mike,

No offense taken but look at the moon image, they have no color, I have used over 300 power before I noticed some CA and that was with a Nagler. With an Ortho I could see no false color. The camera is much more sensitive than the eye and it essentially captures no CA.

My friend had a TAK 130 and an FSQ 106. The FSQ106 is color free, but the TAK 130 was as color free as was the 120MM. After looking through my 120, he sold the TAK 130 and brought a mount and a 9.25 as we all know aperture rules on DSO

I know some people are sensitive to CA but for some of us, its not that big deal and we don't see it.

I also have expeirenced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".

What I am saying is that while a triplet can be good, a lot of people use doublets and they work really well also. I have also looked through a lot of doublet and triplets and the only pure color free refractor that I ever looked through is an FSQ.


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junomike
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5864091 - 05/16/13 10:13 AM

Helder, If your pics are unedited (to remove CA) then they are impressive. I do see some CA in this pic though (from about 2 o'clock - 6 o'clock). It not intrusive (as they say) but It does bother me.

Quote:

My friend had a TAK 130 and an FSQ 106. The FSQ106 is color free, but the TAK 130 was as color free as was the 120MM. After looking through my 120, he sold the TAK 130 and brought a mount and a 9.25 as we all know aperture rules on DSO




My experience was the opposite! CA never really bothered me until I viewed through Maks and Dobs. Then I craved that "pure, clean" Image in a Refractor as well.

Quote:

I know some people are sensitive to CA but for some of us, its not that big deal and we don't see it.




This is the main key I believe! What one actually sees and what bothers that same person is also an issue. You may see CA and not bother commenting on It (or to a lesser degree) because It's minor to you. Or you may really not see It at all.

Quote:

I also have expeirenced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".




This probably does happen, but there's also the flip side where someone has to settle (due to finances, mount limitations, etc) and tries to rationalize the performance of their scope to one they can never obtain.

Too many times "good enough for me" or "similar to" is often put forth as "equal to" or "the same as".
I'm not saying you're doing this, but in general It happens with Astronomy gear all the time.

Quote:

What I am saying is that while a triplet can be good, a lot of people use doublets and they work really well also. I have also looked through a lot of doublet and triplets and the only pure color free refractor that I ever looked through is an FSQ.




Agreed! The Doublets can work very well. I will however add that sometimes the idea of having one can be different than actually making the plunge to own one. What I'm referring to is the Markus Ludes 152ED. When It was In the discussion phase there were many people interested in one.
Now that It's available, where are all the potential buyers?
Is It that the economy is not as good as It was last year?
Or Is It that for $4G's they'd rather forgo the extra 20mm and get a TMB 130SS with FPL-53? (Especially after James Ling's reivew/comparison)
I know which one I'd be more happy with?

There's also the true situation of the fellow who had an FS-152, sold It, acquired a TEC 140 and Immediately missed the FS-152 (he was lucky enough to find another one close by).

I guess It's a matter of taste. Because I see CA doesn't mean others will....But because you don't see It doesn't mean others won't either.

Mike


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5864152 - 05/16/13 10:46 AM

Mike,

I see you and I "actually" agree on a lot.

As to this "Helder, If your pics are unedited (to remove CA) then they are impressive. I do see some CA in this pic though (from about 2 o'clock - 6 o'clock). It not intrusive (as they say) but It does bother me. "

There is CA in that picture, that was actually one of my first images and it used eyepiece projection. Its not good.The focus is also soft. That was done in 2010.

Compare it to this one. This one was in November 2012, http://www.astrophotogallery.org/moon-and-sun/p10662-moon-on-11-29-2012.html

I got a little better imaging I also don't run any CA removal routines. I actually don't really over process my images, I think lots of the AP out there while nice is not really accurate, the tricolors are beautiful but they don't exist in nature (although I admit I do make star spikes for artistic reasons )

About Premiun optics, I also agree with you, I looked through a TEC 140 and that essentialy was a perfect scope for me. I also would love the NP127.

But I have access to a 178MM Astro Physics that shows CA and the 127MM ES (while a really nice scope) shows more CA than the 120MM EON. So not every premium scope or triplet is perfect.

But even doublets have issues, my 80MM EON has lots of field curvature so you need a field flatner to make it work, actually all my images use a Hutech field flatner, which probably also removes some CA.

So to the OPs original question, a Triplet "can" be better than a doublet, but a doublet can also be a great scope. As to whether the $$$$ is worth it, only you can make that decision.


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5864318 - 05/16/13 11:54 AM

a) If made to exacting standards, doublets with one element of fluorite are better than with FPL-53, because it is more difficult to find an appropriate partner-glass for the latter.
b) In "Telescopes, Eyepieces, Astrographs" (WB-2012) they confirm that a 100mm fluorite doublet should be no faster than f/9 - which has been realized by Vixen. By scrutinizing the spot diagrams it becomes obvious that f/8 is the fastest focal ratio which allows a 100mm-doublet to have essentially perfect color-correction - which has been realized by Tak.
c) Besides Tak's FC100 f/8 there was also an f/10 made (the FC100N), suggesting that Tak themselves considered f/8 a bit sub-optimal for color-correction.
d) Tak's FC150, in spite of being f/11.3, was rumoured not to be totally free of false colors, demonstrating that scaling up objective's diameter should go together with scaling up focal ratio proportionally for same color correction.
e) Tak's FS-152 f/8 doublet was given score 88 by Mr. Yoshida's group testing planetary performance. By comparison, TOA-150 and Zeiss APQ-150 triplets got scores 95 and 96.
f) Tak's FS-128 fluorite doublet and TSA-120 FPL-53 triplet were scored equal (73 both), which also demonstrates the triplets' bigger potential, once the objectives' diameter having grown substantially over 100 mm.
g) In comparison with f), TOA-130 and APQ-130 triplets scored 84 each.

Of course, this is no mathematical proof - however these are all evidences pointing into the same direction...

Chris

PS My FCT100 f/6.4 triplet is distinctly better color corrected (inside, outside plus IN focus) than any FC100 or FS102 f/8 doublet which I have ever seen...


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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5864362 - 05/16/13 12:09 PM

Is there a guy that can put down any triplet is this guy http://www.starhopper.at/

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audioaficionado
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5864426 - 05/16/13 12:39 PM

http://www.astrophotogallery.org/data/878/Closer_Moon.jpg

hfjacinto I see a little CA in your shot, but it is very acceptable to my eyes.
However I haven't been spoiled by $$$$ optics yet LOL.


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: audioaficionado]
      #5864479 - 05/16/13 12:57 PM

Quote:

http://www.astrophotogallery.org/data/878/Closer_Moon.jpg

hfjacinto I see a little CA in your shot, but it is very acceptable to my eyes.
However I haven't been spoiled by $$$$ optics yet LOL.




LET ME POST AGAIN, this is an OLD image using Eyepiece Projection, compare to a new image. http://www.astrophotogallery.org/moon-and-sun/p10662-moon-on-11-29-2012.html


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Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5864505 - 05/16/13 01:05 PM

Quote:

Quote:

What I am saying is that while a triplet can be good, a lot of people use doublets and they work really well also. I have also looked through a lot of doublet and triplets and the only pure color free refractor that I ever looked through is an FSQ.




Agreed! The Doublets can work very well. I will however add that sometimes the idea of having one can be different than actually making the plunge to own one. What I'm referring to is the Markus Ludes 152ED. When It was In the discussion phase there were many people interested in one.
Now that It's available, where are all the potential buyers?
Is It that the economy is not as good as It was last year?
Or Is It that for $4G's they'd rather forgo the extra 20mm and get a TMB 130SS with FPL-53? (Especially after James Ling's reivew/comparison)
I know which one I'd be more happy with?

There's also the true situation of the fellow who had an FS-152, sold It, acquired a TEC 140 and Immediately missed the FS-152 (he was lucky enough to find another one close by).

I guess It's a matter of taste. Because I see CA doesn't mean others will....But because you don't see It doesn't mean others won't either.

Mike




Regarding the 152ED I'm not sure what you are expecting to see that you aren't seeing. There are only 50 scopes (plus some lens cells) being made in this first production run, worldwide, and they haven't been delivered to customers yet (though it shouldn't be much longer now). I'm waiting for one myself. Now that I'd made my purchasing choices and am just waiting for delivery there isn't a lot about the scope for me to discuss on forums. I can't say what my opinions of the scope are until I have it, people can't ask me questions with a view to making their own decision to purchase one, etc etc. Empty speculation seems futile when I'm soon to take delivery of the item.

What fraction of a forum community "lurks" (reads while rarely or never posting), and what fraction post a lot? It must vary from forum to forum but I believe I've seen figures in the past demonstrating that the posters are vastly outnumbered by the lurkers. If there are only 50 scopes worldwide, how many are going to be owned by active members of this forum? I myself am more of a lurker and am probably not going to be all that vociferous when the scope does arrive, as I'm not very knowledgeable and would not want to give anybody inaccurate information. I'll post whatever general impressions I feel I am qualified to give, which might not amount to a lot.

You're right about the attraction of the 152ED being a matter of taste. I for example love the idea of a 6 inch refractor but view traditional 6" f/8 achromats as being too compromised by CA to make much sense, while I view 6" apo triplets as being too far off the deep end in terms of value for money. The doublet is only "somewhat" cheaper than a triplet, but it has good enough performance for me, plus better cooldown times and lower weight (it is bound to be less front-heavy in particular). To me, it's a sensible compromise for a visual user with reasonable expectations who wants something moderately better than an achromat. I admit though that before the 152ED came into existence I wondered whether there could possibly be enough people fitting that description to make it financially viable. I hope there will turn out to be but we don't know yet and the minority (or majority) will probably remain pretty silent until we have some hardware in our hands to talk about.


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5864513 - 05/16/13 01:12 PM

Quote:

Is there a guy that can put down any triplet is this guy http://www.starhopper.at/




Same scope I have except mine is the EON. Shows what a doublet can do. Like I said in probably every post here, a doublet can be an excellent scope. The people that post that the CA is not acceptable in a doublet for imaging are plain wrong.


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audioaficionado
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5864585 - 05/16/13 01:34 PM

Quote:

Quote:

http://www.astrophotogallery.org/data/878/Closer_Moon.jpg

hfjacinto I see a little CA in your shot, but it is very acceptable to my eyes.
However I haven't been spoiled by $$$$ optics yet LOL.




LET ME POST AGAIN, this is an OLD image using Eyepiece Projection, compare to a new image. http://www.astrophotogallery.org/moon-and-sun/p10662-moon-on-11-29-2012.html




Awesome moon mosaic.


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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5864715 - 05/16/13 02:20 PM

Quote:


No offense taken but look at the moon image, they have no color, I have used over 300 power before I noticed some CA and that was with a Nagler. With an Ortho I could see no false color.




My hope here is that there an unbiased discussion of the realities of the differences between doublets and triplets. I have some, limited experience with the 120mm F/7.5, I think it's a reasonable scope and were I interested in a 5 inch ED/apo, it's the one I would most likely choose, it seems to be a good value and a good scope.

But in my mind, this discussion is about the differences at the high end, how good is the color correction under the most difficult conditions, not 300x viewing the moon, which is relatively forgiving, but 300x viewing Venus... in my experience Venus is the ultimate test.

Jon Isaacs


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StarsAbove
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5865521 - 05/16/13 06:41 PM

That is simply beautiful!

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Re: Triplet APO or Doublet APO? new [Re: StarsAbove]
      #5865955 - 05/16/13 09:50 PM

What about a Hatori-Honso triplet?

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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5865973 - 05/16/13 10:01 PM

Quote:

300x viewing Venus... in my experience Venus is the ultimate test.

Jon Isaacs




x300 on Venus ?! the atmophere will have a hudge chromatic dispersion effect. This will be the first limiting factor.
You will have to use a well tuned ADC (Atmospheric Dispersion Corrector) to cancel the chromatic error, then you might be able to juge the scope chromatic error.

Another way to cancel the atmosphere chromatic dispersion effect is to use a filter which will narrow the spectrum, but this will also cut the secondary spectrum of the refractor if any.

Olivier


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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5866419 - 05/17/13 04:14 AM Attachment (9 downloads)

Quote:

Quote:

300x viewing Venus... in my experience Venus is the ultimate test.

Jon Isaacs




x300 on Venus ?! the atmophere will have a hudge chromatic dispersion effect. This will be the first limiting factor.
You will have to use a well tuned ADC (Atmospheric Dispersion Corrector) to cancel the chromatic error, then you might be able to juge the scope chromatic error.

Another way to cancel the atmosphere chromatic dispersion effect is to use a filter which will narrow the spectrum, but this will also cut the secondary spectrum of the refractor if any.

Olivier




Olivier:

I am curious if you have spent much time observing Venus at higher magnifications? Certainly there are situations when Venus shows a significant amount of atmospheric dispersion but this is not a given, the maximum elongation of Venus is about 45 degrees so it's can be free from the effects of atmospheric effects.

At it's brightest, Venus is about 2.3 magnitudes brighter than the moon at it's brightest, that's 8 times brighter than the moon at it's brightest. This is what makes Venus such a challenge for a refractor.

Even when Venus is lower on the horizon and atmospheric effects a visible, it takes quite good color correction to show the orange side that results from the atmospheric dispersion because it is ovewhelmed by purple haze of the refractors chromatic aberration. Out of focus tests can also be instructive.

Of course there are situations where a test using Venus is not particularly useful.

Jon

Edited by Jon Isaacs (05/17/13 04:20 AM)


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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5867176 - 05/17/13 01:16 PM

Quote:


I am curious if you have spent much time observing Venus at higher magnifications? Certainly there are situations when Venus shows a significant amount of atmospheric dispersion but this is not a given, the maximum elongation of Venus is about 45 degrees so it's can be free from the effects of atmospheric effects.

Jon




Hi Jon,

Until now I never had a chance to see Venus free of atmospheric dispertion here in France.
Even last year when it was well positioned, the color dispersion was obvious with the TEC160ED for instance.

The paradox with high end large APO like the TEC160ED is that they show a very high contrast and excelent color correction that result also in high sensitivity to atmospheric dispersion.
Even on Mars, the dispersion is enoying as soon as Mars is more than 1h away from the meridian.

So I invested in an ADC at the end.

That said I will try again, following your advice, but I a not optimistic at all. The analysis will not be easy anyway...migh be it could be easier in a drier place...

Olivier


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5867210 - 05/17/13 01:36 PM Attachment (30 downloads)

Last year when Venus was its highest, I was able to capture it, air was stagnant and seeing was essentially perfect. I was also new at this planetary imaging so the picture isn't the best, but when the atmosphere is stable the planet can lose most its shine and color, this was imaged right at sunset and a red filter was used to dim the view. I only shot red so this is only the red channel.

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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5867268 - 05/17/13 02:04 PM

Quote:

At it's brightest, Venus is about 2.3 magnitudes brighter than the moon at it's brightest, that's 8 times brighter than the moon at it's brightest. This is what makes Venus such a challenge for a refractor.


Jon




Perhaps I'm misunderstanding you here? Isn't the moon at about magnitude -13 always brighter than Venus at about magnitude -4?

Edited by SteveG (05/17/13 02:10 PM)


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SteveG
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Re: Triplet APO or Doublet APO? new [Re: SteveG]
      #5867273 - 05/17/13 02:07 PM

I had my best view ever of Venus last year when it was fairly high in the sky and through a very slight upper atmosphere haze. Rock steady and no false color - and I detected the chevron shaped faint markings on the cloud tops.

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Mike Clemens
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5867319 - 05/17/13 03:08 PM

Quote:

After processing.




This is what I'd expect from a doublet (awesome results) but Iota Orionis there at mag 2.77 has a glowing blue sphere around it.


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Re: Triplet APO or Doublet APO? new [Re: SteveG]
      #5867363 - 05/17/13 03:28 PM

Quote:

Quote:

At it's brightest, Venus is about 2.3 magnitudes brighter than the moon at it's brightest, that's 8 times brighter than the moon at it's brightest. This is what makes Venus such a challenge for a refractor.


Jon




Perhaps I'm misunderstanding you here? Isn't the moon at about magnitude -13 always brighter than Venus at about magnitude -4?




I am sorry, I was referring to surface brightness. The moon is much brighter overall due to it's large surface area but Venus is much smaller so the intensity, the surface brightness, can be considerably greater. It's the surface brightness and it's small size that brings out the chromatic aberration when viewing Venus.

Jon

Edited by Jon Isaacs (05/17/13 03:29 PM)


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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5867378 - 05/17/13 03:33 PM

Quote:

Quote:


I am curious if you have spent much time observing Venus at higher magnifications? Certainly there are situations when Venus shows a significant amount of atmospheric dispersion but this is not a given, the maximum elongation of Venus is about 45 degrees so it's can be free from the effects of atmospheric effects.

Jon




Hi Jon,

Until now I never had a chance to see Venus free of atmospheric dispertion here in France.
Even last year when it was well positioned, the color dispersion was obvious with the TEC160ED for instance.

The paradox with high end large APO like the TEC160ED is that they show a very high contrast and excelent color correction that result also in high sensitivity to atmospheric dispersion.
Even on Mars, the dispersion is enoying as soon as Mars is more than 1h away from the meridian.

So I invested in an ADC at the end.

That said I will try again, following your advice, but I a not optimistic at all. The analysis will not be easy anyway...migh be it could be easier in a drier place...

Olivier




Olivier:

My experience suggests that the fact that the atmospheric dispersion is so apparent is a sign of the superior color correction of your scope. Were chromatic aberration of the objective an issue, you would not see the atmospheric dispersion.

Jon


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REC
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5867399 - 05/17/13 03:39 PM

Wow, they are amazing images, thanks for the link!

Bob


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audioaficionado
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Re: Triplet APO or Doublet APO? new [Re: SteveG]
      #5867459 - 05/17/13 03:55 PM

Quote:

Quote:

At it's brightest, Venus is about 2.3 magnitudes brighter than the moon at it's brightest, that's 8 times brighter than the moon at it's brightest. This is what makes Venus such a challenge for a refractor.


Jon




Perhaps I'm misunderstanding you here? Isn't the moon at about magnitude -13 always brighter than Venus at about magnitude -4?



The moon is gray. About the shade of asphalt pavement after it's aged a year or two. Venus is more white and a brighter surface even though the moon reflects much more light in total.


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Re: Triplet APO or Doublet APO? new [Re: CounterWeight]
      #5867574 - 05/17/13 04:19 PM

Quote:

What about a Hatori-Honso triplet?




Wow, wasn't aware he made scopes.. bet he made one for Bill though nice one Jim


I just got another opportunity to pick up a TMB130ss (triplet APO) at a great price but was looking at the SW120ProED (doublet APO).. 1400 cheaper.. wish i had 5k to blow right about now, would pick em both up and really give this forum a review...

hmm.. .anyone care to donate to the "Let's help get Joe the scopes so he can give us a review fund"... i accept Paypal

But in all seriousness, great images!

Edited by JoeM101 (05/17/13 04:20 PM)


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FaronD
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5867950 - 05/17/13 07:08 PM Attachment (12 downloads)

Quote:



My friend had a TAK 130 and an FSQ 106. The FSQ106 is color free, but the TAK 130 was as color free as was the 120MM. After looking through my 120, he sold the TAK 130 and brought a mount and a 9.25 as we all know aperture rules on DSO

I know some people are sensitive to CA but for some of us, its not that big deal and we don't see it.

I also have expeirenced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".





Helder, you are saying that a Synta doublet ED120 and my TOA130 with "two" FPL53 elements have equal optics?
Also, you say that your friend took one look through your Eon 120 and immediately sold the TOA130 to purchase a 9.25 SCT and mount?? This doesn't make sense to me? Shouldn't he have purchased an EON120 or a Skywatcher ED120mm perhaps?

Your comment "I also have expeirenced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".
Sorry but the TEC, AP, Taks are better than most Chinese 120mm doublets, not better than an 18" reflector of course, well, sometimes they are. Nothing really blows away the other anymore however, there is an improvement as you go up. Is it worth the extra thousands of dollars? Depends on who you ask. Do you truly believe it's only conspicuous consumption to purchase a top shelf refractor?
I'm not trying to be arrogant here, honestly, but an EON120 is not an Astro-Physics GT130, sorry. However, they are great scopes. Actually, I've viewed through the EON120, Skywatcher 80mm Equinox, and the Skywatcher ED120, Skywatcher 80ED, they are nice scopes.

Everyone try this for fun. Type "ST80 moon" in google then select images. Actually, I'll post a random shot here. We all know a ST80 is correct?
I used to love that little short tube 80mm and yes it has lots of ca.
My point is, don't judge optics by a photo alone, there are too many variables.

Faron


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: FaronD]
      #5867966 - 05/17/13 07:23 PM

"Helder, you are saying that a Synta doublet ED120 and my TOA130 with "two" FPL53 elements have equal optics? "

The objects we looked at, the view was comparable between the 120 and the 130MM. He also looked through my 9.25 and that was the scope he ended up buying. I never said he wanted an EON, he looked through the 120MM, looked through the 9.25 and now owns a 9,25 and an FSQ for imaging but no longer owns a 130. Mechanically the TOA is a superb scope, I had the option of buying it and I could have, I had the cash on hand, but the difference wasn't enough for me to spend that much money. I ended up getting a Lunt Solar scope and I am pretty happy with that. This isn't to say that a TEC, TAK or AP are not nice scopes they are, I just don't see the reason to pay so much, I mean I can get a C14 which will show me more of deep space objects, I don't do double stars and I am happy with my imaging rig.

" Your comment "I also have experienced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".
Sorry but the TEC, AP, Taks are better than most Chinese 120mm doublets, not better than an 18" reflector of course, well, sometimes they are."

Looking through an NP101, an AP178, Tec 140 and an 18" Dob at M42, there was no comparison, the 18" DOB showed more nebulosity and had a brighter image.

No comparison to a 120MM, the 18" DOB was so far ahead that I didn't even try to compare my scope. A few weeks ago, we looked through a C14 EDGE and that scope had one of the best views ever, better than anything else I've looked through. I was so impressed with the C14 EDGE that I would rather spend the money on that set-up.

But that's not the question, the question was can a doublet also be color free and I say yes it can. Or at least color free to me and many others.

Edited by hfjacinto (05/17/13 07:33 PM)


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FaronD
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5867986 - 05/17/13 07:34 PM

Quote:

"Helder, you are saying that a Synta doublet ED120 and my TOA130 with "two" FPL53 elements have equal optics? "

The objects we looked at, the view was comparable. I am not the one that owned the TOA130 and the owner no longer owns it, so I am assuming he somewhat agreed.

" Your comment "I also have experienced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".
Sorry but the TEC, AP, Taks are better than most Chinese 120mm doublets, not better than an 18" reflector of course, well, sometimes they are."

Looking through an NP101, an AP178, Tec 140 and an 18" Dob at M42, there was no comparison, the 18" DOB showed more nebulosity and had a brighter image.

Actually a few weeks ago, we looked through a C14 EDGE and that scope had one of the best views ever, better than anything else I've looked through.

But that's not the question, the question was can a doublet also be color free and I say yes it can. Or at least color free to me and many others.




Helder, that is the question but it's not your only answer. You are also suggesting that it's as good as any top tier scope out there, correct?
Hopefully, I'm not sounding combative, I certainly don't mean to be. I'm just chiming in.

Faron


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: FaronD]
      #5868016 - 05/17/13 07:46 PM

Faron,

I am in no way saying that the mass produced Chinese scopes are as good as the premium brands. They are clearly better on certain things, like for example Venus shows some color on my EON, but on the TAK it didn't show any. We split Sirius with both but on the TAK it was much easier.

Looking at M45, fainter stars appeared in the TAK, the focuser is WOW on the TAK. The heft and weight are nice and the scope is beautiful.

But thats when looking for items, so let me tell you a story, we once compared the 30MM ES 82* to the 31MM Nagler T5, I own the T5 and there were certain things that I could tell in the T5 (like a red star was really red in the Nagler but not in the ES and the extreme outer edge of the Nagler was better, no comatic stars), but I needed to look for these items, in the end, if I didn't own the T5, I doubt I would have spent the money to get it. I am not an intense observer, I like to look, spend a few minutes go to the next object. I don't need a $5000 scope to do that. The doublet I own is nice and has enough contrast to keep me happy and CA is very well controlled.


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timps
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5868191 - 05/17/13 09:37 PM

I don't think the TOA130 has two FPL53 elements.

As far as I am aware it has one FPL53 element between two crown & flint elements.


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FaronD
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5868193 - 05/17/13 09:38 PM

Helder, to be honest, I'm exactly like you. I'm quite happy looking through any scope. I'm not bothered by CA either, I certainly will notice it when it's there, but I can look pass it.
I've been fortunate enough to be able to purchase a few nice toys. If that were to turn around, I would still be quite content with an EQ6 and my old Orion 80ED for imaging. Would I notice the difference, absolutely.

Faron


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FaronD
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Re: Triplet APO or Doublet APO? new [Re: timps]
      #5868287 - 05/17/13 10:15 PM

Timps, you could be right. I've read it both ways. I remember reading it here and a few other places http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3473595/Main...
That certainly doesn't make it fact. It might be a fpl53 between two ED elements. Whatever the configuration is, it's certainly one of the best corrected designs out there.

Faron


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Re: Triplet APO or Doublet APO? new [Re: FaronD]
      #5868346 - 05/17/13 10:43 PM Attachment (16 downloads)

To each his own. I just came up on the list to order an AP 130 GT. Would I rather buy an EON 120 and save money? No. I just ordered the AP 130. I have been observing since 1965. I would never question your decision if you would rather purchase the EON 120 and save the money. As I said, to each his own. Some have the money to buy high end scopes and some do not. Others have the money but choose not to spend it on high end telescopes. Fine with me. I own a quite fine 20" Dob but would never choose it if I could only keep one of my scopes.
The TAK FS series Fluorite doublets are excellent visual telescopes. I agree that the degree of CA seen in a doublet varies from individual to individual and the mind will compensate for the CA after a while and appear far less obtrusive than initially noted. That was certainly the case with an RE Brandt 8" F/13 doublet I once owned.
Buy what you can or want to spend the money for and use it. You will be happy as long as you use the scope. My opinion.


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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5868375 - 05/17/13 10:59 PM

Jim I don't want to purchase an Orion EON120, I want an AP130 like you

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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5868558 - 05/18/13 12:31 AM

Quote:

The doublet I own is nice and has enough contrast to keep me happy and CA is very well controlled.






That seems like a very reasonable assessment of the 120mm F/7.5 Synta ED/APO scopes. Maybe not perfection but pretty darn good.

Thinking back, it's been nearly 10 years since the ED-80 arrived and transformed the refractor marketplace. At the time, many were skeptical that the ED-80 was the real thing but for many of us, it was our first taste of owning a very good quality refractor.. $500 for an 80mm FPL-53 apo sounds quite reasonable today, it's a sign times have changed.

Now you can buy a Sky Watcher 120mm Pro for about $1600 including rings, a 2 inch dielectric diagonal, two speed focuser, a case, a RACI finder.. That's a heck of a deal and a pretty good scope too.

Jon


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JoeM101
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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5869292 - 05/18/13 01:06 PM

there you have it...

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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5869502 - 05/18/13 02:51 PM

here's a video with 10" TMB Apo and as you can see, still have
CA. Or I guess is the video camera. http://www.youtube.com/watch?v=5p_Wx-0HYNs


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JoseBorrero
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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5869593 - 05/18/13 03:36 PM

Don't get tricked if narrowband is on the question because a good achromatic might trick you! look here: http://www.astrobin.com/users/FrancescoTallarico/

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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5869605 - 05/18/13 03:45 PM

Quote:

there you have it...




It's kind of a question whether you are satisfied with "pretty darn good" or not.

In general, I am OK with "pretty darn good" or even just "pretty good" and indeed, even "not half-bad" and "so-so" can be entirely satisfying.

But there is also no doubt that the difference between "pretty darn good" and something like the NP-101 is noticeable ... a thread like this one will hopefully allow one to get a feel for that difference.

Jon


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Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5869686 - 05/18/13 04:32 PM

That is my TMB 10" and that is NOT chromatic aberration! It is atmospheric dispersion. The images through my TOM 10" are absolutely color free.

Jim Phillips


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hfjacinto
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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5869762 - 05/18/13 05:20 PM

Again this is such a stupid argument, some people like triplets some like doublets. If you can't afford an AP, then a doublet is a really nice scope. Period end of conversation.

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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5869775 - 05/18/13 05:32 PM

I agree with you hfjacinto. There are some really nice scopes to choose from these days and some come with 2 lenses and others with 3 lenses. Pick one and head out and enjoy!
The only rfractors available when I started out were F/15 achromats. We have come a Long way from those days.


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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5869805 - 05/18/13 05:48 PM

The best scopes are the ones you have in hand and use frequently.

There's a price point and quality level for everyone. It's all good IMHO.

I'd love to get one of those nice $$$$ doublet or triplet scopes (someday maybe).

For now I'm really looking forward to my bargain low $$$ scope when it arrives.

This has been a very interesting and informative thread so far.


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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5870176 - 05/18/13 08:41 PM

Quote:

Again this is such a stupid argument, some people like triplets some like doublets. If you can't afford an AP, then a doublet is a really nice scope. Period end of conversation.




I believe the goal of a thread such as this is to understand the differences between doublets and triplets.

Jon


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5870349 - 05/18/13 10:00 PM

If you have the book "Telescopes, Astrographs, and Eyepieces" go to the chapter where they have on axis and off axis spot diagrams for 3 element APOs and you will see that even the highest quality triplets have CA. Even if they were made perfectly to the design, and they of course aren't. It's the nature of the beast when you bend light rays of different wavelengths through glass elements and try to bring them all to one focus. If you want CA free you need to use only reflective optical elements.

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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5870401 - 05/18/13 10:17 PM

Well, I suppose it depends on your definition of color free. If you mean what off axis spot diagrams in a book show, you may be right. If, on the other hand, color free means seeing absolutely No extraneous color at the eyepiece you are wrong as there are apochromatic refractors, many of which I have owned and used, which show no detectable extraneous color at the eyepiece. And, for me, it is the view at the eyepiece that counts. In addition, I image planets using a Skynyx color webcam (not RGB filters)and there is no extraneous color in my images. Therefore my apos fit the definition of color free meaning no detectable extraneous color at the eyepiece

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Re: Triplet APO or Doublet APO? new [Re: timps]
      #5870416 - 05/18/13 10:23 PM

Quote:

I don't think the TOA130 has two FPL53 elements.

As far as I am aware it has one FPL53 element between two crown & flint elements.




The TOA has a unique design : a cooke triplet 2 x FPL53 1x BS7 and a wide air space. Thanks to this design it has almost not any spherochromatism. This is the same for te TOA150.

This is the best performer among the 130 for photographic use especially if you would kike to do some CaK sun picture: the strehl at 400nm will be much higher than any other triplet of the market of simillar F/L

At little bit overkill for visual use, and its heavy and take time to cool down.

At My side I prefered the AP130GT, much smaller, shorter F/L, faster cool down, very good for APS-C sensor or smaller, exclent for wide field visual use.

But the Toa has a better optic for photo use, wider corrected spectrum, widder corrected field thanks to its 4" focuser and flatener.

You can see here test reports and have a look to the spherochromatism graph. The flatener test is also excelent at the Tak side, showing a very low focus shift on a wide field: it is diffraction limited at least at 27mm from the center and thus over the full visual spectrum. The strehl is still higher than 0.92 in the blue channel, where other classical triplet usualy give about 0.7. In the violet/UV channel, the difference is even greater.

http://www.airylab.com/contenu/mesures/astro/rapport%202011-05002-public.pdf

http://airylab.net/contenu/mesures/astro/rapport%202011-40001-a.pdf


Olivier


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BarrySimon615
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5870490 - 05/18/13 11:21 PM

Quote:

If you want CA free you need to use only reflective optical elements.




This statement begs the question: So how much then does the eyepiece influence chromatic aberration? (Given a pure image, because it was reflected by a mirror, does it remain pure going thru 3 or more elements in an eyepiece?)

Barry Simon


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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5870889 - 05/19/13 08:48 AM

Quote:

If you want CA free you need to use only reflective optical elements.




Reflective optics have their own set of aberrations and difficulties. Personally, I find refractive optics are best suited for small scopes, reflective optics for larger scopes. Each represents compromises, optimizations.

Jon


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5871326 - 05/19/13 12:54 PM

The answer is yes eyepieces introduce their own CA. In "Telescope Optics" by Rutten and VanVerooj they raytrace a few designs for three colors and clearly show how eyepieces behave from a CA standpoint.

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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5871584 - 05/19/13 02:15 PM

Quote:

The answer is yes eyepieces introduce their own CA. In "Telescope Optics" by Rutten and VanVerooj they raytrace a few designs for three colors and clearly show how eyepieces behave from a CA standpoint.




On axis, eyepieces are essentially color free, off-axis lateral color, we know about.

Jon


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5871586 - 05/19/13 02:16 PM

You say that many of the scopes you have looked though are color free. What you are ignoring is the response of the detector, your eye. Eyes are notoriously insensitive to wavelengths away from yellow-green, particularly red and blue. Everyone's eyes are different. However if you can't see color using triplets, you must also allow that some folks don't see it with doublets. I was only saying that when you look at a cold, hard ray trace analysis neither triplets nor doublets are color free.

As it relates to a one shot color camera, all CCDs are monochrome. To make a "color" camera the manufacturer inserts red, blue, and green color filters between the source and the ccd. So what you are recording is a pseudo-color image based on a summation of adjacent red, green, and blue pixels. And since you are only looking at three specific wavelengths it is possible for it to be color free. More likely is that the red and blue signals are just much weaker than the green and therefore don't show up in the final image.


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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5871590 - 05/19/13 02:18 PM

If any of you feel this is a stupid argument, I ask that you try to understand that the original intent was to understand the differences between the two and to get better informed.... Many people on here really don't understand what some of the glass types are and how they're arranged affects performance, let alone trying to grasp the, what seems to ne, loosely defined APO designation

If you have nothing empirical to add other than to express your opinion, try to make it constructive so that the readers can benefit somehow

Clear skies!


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5871599 - 05/19/13 02:22 PM

I believe that is what I am attempting to do. Empirically some people don't see color in doublets or triplets. Some do. When you look at it from a strict analytical standpoint, neither are color free.

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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5871841 - 05/19/13 04:08 PM

Thanks Rodger. Actually we agree. When I owned a Brandt 8" doublet F/13 achromat andthe color was "impressive". White stars appeared yellowish to me. After a while using the scope white stars appeared white to me! Many people would look through one of today's modern doublets and what little color they saw might "disappear" in time. Plus many people are far more color sensitive than others. My feeling is that if I see no extraneous color at the eyepiece the lens is color free. Of course I also feel if the double star companion of Antares looks green to me it's green (although we know it is blue and looks that way as a CONTRAST effect).
So, my opinion, based on almost 48 years of observing is that many of today's doublets, and I will use the Takahashi FS series (Fluorite) as an example, are as color free as you need for visual observations and they get more and more color free the longer you use them. Today's triplet apos like the Takahshi TOA and TSA models along with my favorite, Astro-Physics (and others)represent the height of the maker's art. They don't get any better. Is it necessary to own a triplet apo if you want a fine telescope to observe with? No. If you want the very best example of what can be done today I would say yes. Just my opinion based on experience.
I hope this is helpful.

best,

Jim

Edited by JimP (05/19/13 04:09 PM)


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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5872320 - 05/19/13 07:52 PM

Quote:

I believe that is what I am attempting to do. Empirically some people don't see color in doublets or triplets. Some do. When you look at it from a strict analytical standpoint, neither are color free.




I think there is more to it than just whether a scope is a doublet or a triplet, more to it than whether a scope uses FPL-53 or FPL-51.... The aperture and the focal ratio are important in determining the level of color correction in apo/ED scopes just as they are in determining the color correction in achromats and singlets.

Jon


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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5872325 - 05/19/13 07:53 PM

Concerning color correction there are 2 differences between triplets and doublets.

- ED triplets have lower spherochromatism than ED doublet of same diameter and same focal ratio. This means triplets are corrected on a wider spectrum, at least from blue to red and sometimes from violet to deep red, where ED doublet are corrected from light blue to light red.

All the refractors are optimized (i.e. "centered")at a single color, usually the green, or yellow-green. The red and blue channels are less corrected (one is over corrected and the other under corrected)

There is an exception : the takahashi TOA series that have almost no spherochromatism. The blue is as good corrected as the red and green.

For visual use, ED doublets are usually good enough, but the sperochromatism still quickly rises together with the diameter.

For CCD that are sensitive on a wider spectrum than eyes, the triplet makes more sense.

At the end, a triplet of same F ratio and same diameter and same ED glass focuses more energy in the diffraction pattern than a ED doublet resulting in an higher contrast.

The difference is marginal for long focal ratio and small diameter, like the TSA102 vs the FS102 for instance, but for larger diameter and shorter focal ratio, the triplet can take the advantage even for visual use.


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5873093 - 05/20/13 04:50 AM

Other than achros, apos produce much less longitudinal chromatic aberration. OTOH, as a result of their usually much shorter focal lengths they tend to be plagued by spherochromatism (=variation of spherical aberration with wavelength), first of all.
As can be seen on Wolfgang Rohr's Website "astro-foren.de", despite the TOA's virtually complete freedom from spherochromatism, they are OTOH, as far as longitudinal chromatic aberration is concerned, not better color-corrected than other top-notch apochromats! - Yes, they are good performers in that latter discipline as well, but with an RC-index of ~0.5 are no better than Tak-TSAs, for example...
Plus in this contex, their Strehl is distinctly lower in blue as the ones in green, yellow and red.

Chris


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ken svp120
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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5873217 - 05/20/13 08:25 AM

Quote:

If any of you feel this is a stupid argument, I ask that you try to understand that the original intent was to understand the differences between the two and to get better informed.... Many people on here really don't understand what some of the glass types are and how they're arranged affects performance, let alone trying to grasp the, what seems to ne, loosely defined APO designation

If you have nothing empirical to add other than to express your opinion, try to make it constructive so that the readers can benefit somehow

Clear skies!




Joe,

I too continue trying to understand this subject. Call me old-school but I believe in defining a word and then sticking to that definition. Once you start chipping away at or broadening a definition, the next thing you know the original word no longer means anything at all. That being said, I am of the understanding that the original definition of apochromatic was established by Abbe and that (in addition to spherical aberration and coma) it requires there be correction for 3 widely spaced wavelengths...i.e. bringing three colors to focus at the same point.

So the way I look at answering this question of "can a doublet be apochromatic" simply comes down to answering the question of "can two pieces of glass accomplish bringing 3 widely spaced wavelengths to focus at the same point while correcting for spherical at 2 and coma as well."

If the answer is Yes, then a doublet has the ability to be apochromatic.

If the answer is No, then they can't.

As an aside, I'm also not sure that bringing three colors to focus necessarily means color-free....does it? Or is that just a convention we're assuming to be meant by apochromatic but really isn't necessarily the case?

Anyway - good topic!


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5873222 - 05/20/13 08:28 AM Attachment (7 downloads)

Don't know where the Rohr's numbers come from, but Takahashi China says 130TOA has 0.01mm focus shift between g and C lines (F to C is only slightly less). That's like 1/50 of the C-F shift in an achromat. The graph they show implies more like 0.02mm, but it's still far in the apo land.

Vla


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5873252 - 05/20/13 08:48 AM

Just have a look yourself, for example (!) here:
http://www.astro-foren.de/showthread.php?11522-Takahashi-TOA-130-S&p=4604...
(scroll up and then down)
P.S. Measured and not calculated, plus focused on green!

Chris


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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5873339 - 05/20/13 09:42 AM

In this other report from AiryLab / France you can see on p12, that the focus shift between the 3 colors is 60µ. Very similar to concurent products. The TOA has a little bt longer Fl ratio which helps in containing the longitudinal chromatic error.

http://airylab.net/contenu/mesures/astro/rapport%202011-40001-a.pdf


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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5873407 - 05/20/13 10:22 AM

Well, what I see is that Rohr gives the relative defocus vs. e-line going from -68 microns in F to +32 microns in the C line. That's 100 microns, or 0.1mm, at least five, according to the graph, or ten, according to Takahashi China, times the TOA's advertised shift. If we look at the graph, the entire range is within less than 0.02mm. Can Rohr control measurements to that level of precision? He doesn't specify how does he determine best focus position. What I've seen of his site - admittedly, not much - doesn't make me confident. Takahashi Europe shows this same TOA130 chromatic shift graph. Chromatic correction is not subjected to significant changes vs. design, as long as it uses design glasses. That includes spherochromatism. E-line correction will be less good, but it is not a factor affecting secondary spectrum. Rohr's test is in disagreement with the official data, and if I am to take my pick, it's Takahashi. Still remember the mess that Rohr made of testing a simple system such as Dall-Kirkham (Tak Mewlon). The other testing site, the French one, didn't impress me either. Someone claiming he's from the site didn't show he knows why the Zernikes weren't used properly, or how their longitudinal aberration graph was obtained (CN thread). These days, sophisticated optics software is readily available, and may get used by those not quite up to the task.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5874264 - 05/20/13 05:18 PM

Quote:

Chromatic correction is not subjected to significant changes vs. design, as long as it uses design glasses.




I should have said as long as it uses design glasses and have power components of the objective near optimally balanced. If, for instance, the positive component is weaker, the blue end shifts away, and the red end comes closer. Still, this means that the error is induced to both ends, if starting from near-optimum balance (design).

On the other hand, the info on the Tak is not consistent as well, although not nearly as much. Confusion seems to be coming from vendors' interpretations and assumptions, but part of it is that the official info from Takahashi is scarce and incomplete. I am not aware of any glass combinations that would produce nothing even close to such a tight chromatic shift curve in a this large/fast triplet. The possibility is that it shows chromatic shift based on best, not paraxial (usual) foci. But one can only guess. It's kind of strange that people do not get specific info on anything that they are paying good money for.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5874527 - 05/20/13 07:49 PM

Quote:

The other testing site, the French one, didn't impress me either. Someone claiming he's from the site didn't show he knows why the Zernikes weren't used properly, or how their longitudinal aberration graph was obtained (CN thread). These days, sophisticated optics software is readily available, and may get used by those not quite up to the task.

Vla




Stange... the guy is a professional guy with profesionnal tools, he knows his job.
Whatever the risk of error on the focus plane measurement is very low.

But to me the most intresting in these test reports are not the absolute values, but the relative values from one scope to another tested in the same conditions by the same operator.


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5874974 - 05/20/13 11:43 PM

Quote:

Stange... the guy is a professional guy with profesionnal tools, he knows his job.




Well, take a look.

Quote:

Whatever the risk of error on the focus plane measurement is very low.




Anything specific?

Vla


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5875116 - 05/21/13 03:30 AM

Quote:

But to me the most intresting in these test reports are not the absolute values, but the relative values from one scope to another tested in the same conditions by the same operator.




So, here another TOA (this time a TOA-150) tested by the same operator:

http://www.astro-foren.de/showthread.php?12633-TOA-150-1100-ein-quot-Sahnestü...

Chris


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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5875332 - 05/21/13 08:43 AM

Quote:

Quote:

Stange... the guy is a professional guy with profesionnal tools, he knows his job.




Well, take a look.





You mean the spherochromatism graph calculated from 3,5 and 7...The graps seems a little bit strange to me at higher values than 70/80%, especially if we compare to the front wave map, but however, this graph is not so far from what is expected : for instance we can see a 70/80% crossing and the graph is roughly OK.
I don't know whether the "difference" at high values comes from the calculation hypothesis or from the real world difference to ideal optics/specs, but at the end this gives a good idea of how the optics work.
To me the most interesting is comparing the graphs from a refractor to another.


The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5875701 - 05/21/13 12:08 PM

Quote:

You mean spherochromatism graph calculated from 3,5 and 7...




No. What I mean is:
1-arbitrarily discarding Zernike terms, which shows no knowledge of how they work
2-mixing pupil size with the value of Zernikes, which is a factor in ophthalmology, not in telescope optics
3-evaluating primary spherical based on its Zernike term, which has little sense with apos where it is only a component in modeling the wavefront, routinely affected by the higher order terms, as well as other aberration terms
4-saying that the wavefront *phase* error is given in microns (that's zero level error)
5-saying that TOA and Maks are the only systems *not showing any spherochromatism* (another zero-level error)
6-not answering the obvious inconsistency between his explanation of how the LA graph was obtained and their wavefront maps
7- saying that in the AP155 5th and 7th order spherical (i.e. 6th and 8th in the modern nomenclature) are "quite the same over the spectrum" (take a look of the graph)

Quote:

The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?





Not really. The second test is for the 150/1100 TOA, with 10% longer f.l. which means that the secondary spectrum should be 10% greater only based on the f.l. And somewhat more than that due to secondary spectrum in apos being affected by the magnitude of spherochromatism, which increases exponentially with the f-ratio (f/7.3 vs. f/7.7). With the TOA130 tested at 100 microns F-C, this one should be more than 110 microns. Instead, it is 68 microns, 2/3 of the smaller/slower TOA's. Doesn't add up.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5877855 - 05/22/13 11:49 AM

Okay you have issues with some of the testing Rohr does. But as far as I can tell he is the only source for in-depth testing of astronomical optics on the net. I am interested in what he does because when I want to buy a telescope (new or used) I can go to his website and find out how well the optic was made. And if I remember he also gives the polychromatic strehl for refractors which I don't see very often. No one, and I mean no one, does more exhaustive and thorough testing of telescope optics. Maybe you should give him some competition?

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Jon Isaacs
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5878091 - 05/22/13 01:46 PM

Quote:

Okay you have issues with some of the testing Rohr does. But as far as I can tell he is the only source for in-depth testing of astronomical optics on the net. I am interested in what he does because when I want to buy a telescope (new or used) I can go to his website and find out how well the optic was made. And if I remember he also gives the polychromatic strehl for refractors which I don't see very often. No one, and I mean no one, does more exhaustive and thorough testing of telescope optics. Maybe you should give him some competition?




Or maybe some advice...

Testing is one thing but it must be done properly and with an understanding of the analysis.

Jon


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5878422 - 05/22/13 03:50 PM Attachment (24 downloads)

I'm not sure what is "in-depth" worth, if it is not reliable. Also, "exhaustive" is not necessarily a quality. To me, the test reports I saw are way to overcrowded with all kinds of software interfaces and numbers. All it takes is a few numbers that specify optimized line correction, chromatic defocus i spherochromatism for several selected wavelengths. Wavefront maps for those won't hurt.

Going back to the specifics, here's a 130mm f/7/7 triplet made of Ohara FPL53 and Schott ZKN7. The latter is available, inexpensive crown that happened to be the best match for FPL53 with respect to secondary spectrum reduction. The only significant chromatism comes from the spherochromatism (partial dispesion diagram indicates that Ohara BSM81 could be a bit better with respect to minimizing spherochromatism). But it is also negligible. The polychromatic Strehl (photopic, 430-670nm) is 0.983, and with the e-line correction error taken out, it degrades the Strehl by no more than 0.99 factor (this is not strictly optimized lens, but it can't be significantly better).

The paraxial foci from 400-800nm (and wider) are within 0.2mm. Best F/C foci, which nearly coincide, are at about 0.018mm, or 18 microns from the best e-line focus. That's 1/13 wave P-V of defocus in the blue, and 1/17 wave in the red.

At the location of best e-line focus, the F-line error is still only 1/9 wave P-V. According to Rohr's test, this error is four times larger longitudinally and nearly 1/3 wave P-V.

Why would Takahashi, going for a top apo make something that is significantly worse than readily available, relatively inexpensive and simple triplet? Of course, the actual objective will likely deviate from the design optimum. In fact, by slightly weakening its positive component (2.3% longer rear surface radius), best F and C foci come at nearly the exact distance from the e-line focus as in Rohr's measurement, with the e-line correction still better than 1/10 wave P-V. But the polychromatic Strehl is still just short of 0.96 with the degradation factor due to spherochromatism still over 0.98.

And Rohr gets 0.909 polychromatic Strehl, with 0.998 e-line Strehl, which means that it is chromatism (spherochromatism+secondary spectrum) accounting for all that degradation.

Just doesn't add up. Again, this triplet is supposedly not as good a design as TOA. Either is Takahashi running a big conspiracy scheme to purposely cheat on its customers, or Rohr's test results are a suspect. I opt for the latter.

Vla


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5879595 - 05/23/13 05:32 AM

Quote:

Quote:

The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?




Not really. The second test is for the 150/1100 TOA, with 10% longer f.l. which means that the secondary spectrum should be 10% greater only based on the f.l. And somewhat more than that due to secondary spectrum in apos being affected by the magnitude of spherochromatism, which increases exponentially with the f-ratio (f/7.3 vs. f/7.7). With the TOA130 tested at 100 microns F-C, this one should be more than 110 microns. Instead, it is 68 microns, 2/3 of the smaller/slower TOA's. Doesn't add up.
Vla




Vla,
Intercon-Spacetec, the German distributor of Takahashi, writes the following:
"Der TOA-150 ist nicht nur einfach der größere Bruder des TOA-130, sondern das Linsendesign ist völlig neu konzipiert und berechnet worden, um ein Höchstmaß an Korrektur der chromatischen Aberration zu gewährleisten."
Translation: "The TOA-150 isn't just the TOA-130's big brother but its lens design has been completely newly conceived and figured from scratch in order to guarantee the best possible correction of chromatic aberration."

=> So please do not expect the TOA-150 to be just an upscaled version of the TOA-130!

Quote:

...And Rohr gets 0.909 polychromatic Strehl, with 0.998 e-line Strehl, which means that it is chromatism (spherochromatism+secondary spectrum)...



Mr. Rohr doesn't think much of caring about the polychromatic Strehl. Instead, he prefers to measure the monochromatic Strehls in five or six different colors and (as in the case of the TOA-150) then demonstrates the human eyes' differing sensitivity for different colors by the width of the Strehl-bars in a chart.
There is another German called Kurt Schreckling who is after the polystrehl and has (interferometrically) measured and calculated it for about a dozen well-known apos. He did so by measuring the monostrehls in 11 colors and then weighting these results according to the human eyes' sensitivity for each of the measured wavelengths.

Mr. Rohr also uses a (Bath-) interferometer and if you want to gather how exactly he does so then please study
the first ~twenty entries here:
http://www.astro-foren.de/showthread.php?6084-Verzeichnis-optischer-Beroichte...
because of course he doesn't explain it all over again with each new telescope...

Vla, you certainly are a competent expert in optical theory... But did you ever personally measure and interpret real-world (!) instruments? If so, then pleeease give us not hundreds (as Mr. Rohr did) but at least half a dozen examples of what for us laymen is at least as, if not more important than bloodless theory.

Chris


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RodgerHouTex
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5879992 - 05/23/13 11:03 AM

I stand by my comments. Mr. Rohr is "the best source" for optical testing of amatuer telescope optics for those of us who don't have the resources or time to do what he does. It is always easy to criticize another's work. That was why I suggested that maybe Vla could start performing a similar service.

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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880000 - 05/23/13 11:12 AM

Quote:

"The TOA-150 isn't just the TOA-130's big brother but its lens design has been completely newly conceived and figured from scratch in order to guarantee the best possible correction of chromatic aberration."

=> So please do not expect the TOA-150 to be just an upscaled version of the TOA-130!




This is what they say, completely:

"The TOA-150 is not just the big brother of the TOA-130, but the lens design has been completely re-designed and calculated to provide maximum correction of chromatic aberration. In this case, not only visible wavelengths have been included, but also the near ultraviolet and the infrared. This provides a perfect platform for CCD applications. The large back focus of 214mm allows the connection of diverse accessory from the CCD camera with Robofokus to the dissecting microscope."

Which is simply that TOA130 is visually optimized, and andn TOA150 CCD-optimized. All it takes is a slight adjustment in radii (even single radius), in order to pull violet and deep red closer. This typically comes at the price of increasing the error in F and C, so the bigger TOA would have it larger than what would come from its longer f.l. and faster f-ratio alone. That's what is likely. I wouldn't pay much attention to the marketing talk, "completely new", "from scratch" etc. It uses the same FPL-53 between two crown elements. It cannot have significantly better correction than another top apo using same glasses and configuration. It can be optimized for CCD, and that's all.

Quote:

Mr. Rohr doesn't think much of caring about the polychromatic Strehl.




Is that explaining why its polychromatic Strehl puts Tak TOA130 at the level of semi-apo? BTW, plychromatic Strehl is the single most important and most informative number with respect to the level of chromatic correction. It also cannot be in conflict with the line Strehls, simply because it is obtained from them.

Quote:

There is another German called Kurt Schreckling who is after the polystrehl and has (interferometrically) measured and calculated it for about a dozen well-known apos.




I've seen it a few years ago. Wasn't impressed.

Quote:

Mr. Rohr also uses a (Bath-) interferometer and if you want to gather how exactly he does so then please study
the first ~twenty entries here:
http://www.astro-foren.de/showthread.php?6084-Verzeichnis-optischer-Beroichte...
because of course he doesn't explain it all over again with each new telescope...




The basic technique is using color filter to measure best focus for the bandwidth. The question is how precise it is. Another question is how appropriate is the use of software, e.g. Zernikes. Bath has nothing to do with determining best focus location for a wavelength - it measures wavefront for a given focus point. And, yes, one can err with the Bath as well.

Quote:

Vla, you certainly are a competent expert in optical theory... But did you ever personally measure and interpret real-world (!) instruments? If so, then pleeease give us not hundreds (as Mr. Rohr did) but at least half a dozen examples of what for us laymen is at least as, if not more important than bloodless theory.




Well, that "bloodless theory" actually enables you to see inconsistencies and errors in tests results, instead of merely believing it, or not. Like, for instance, that doesn't make any sense that the TOA130 is - according to Rohr's test - semiapo. That a plain FPL53 triplet beats it hands down. That it claims measuring e-line for it at 0.998 Strehl (comparable to less than 1/40 wave p-v of primary spherical). That has the error in F line 4.5 times the error in C line (very unlikely to impossible). And so on.

Anyone can believe in anything. But if someone with a different view comes up with specific objections, the believer should face them and respond directly instead of responding with general unverified and unverifiable assumptions. It is really irrelevant whether I ever did any actual measurements or not. The subject here are the measurements itself.

Vla


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5880084 - 05/23/13 12:02 PM

Quote:

Quote:

"The TOA-150 isn't just the TOA-130's big brother but its lens design has been completely newly conceived and figured from scratch in order to guarantee the best possible correction of chromatic aberration."

=> So please do not expect the TOA-150 to be just an upscaled version of the TOA-130!




... so the bigger TOA would have it larger than what would come from its longer f.l. and faster f-ratio alone. That's what is likely. I wouldn't pay much attention to the marketing talk, "completely new", "from scratch" etc. It uses the same FPL-53 between two crown elements. It cannot have significantly better correction than another top apo using same glasses and configuration....




Citation Takahashi Europe, and Takahashi China as well:
"The objective of TOA-150 (D=150mm, F=1100mm, F/D 7,3) is an air-spaced triplet of which two of the lenses consist of ED glass with very weak chromatic dispersion."

=> The TOA-150 is obviously said (by people who should know) to be a pnp-objective and not an npn such as the TSA. (p = positive and n = negative lens)
=> Are you really sure you know everything already???

Chris


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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880200 - 05/23/13 01:06 PM

Chris, that last sentense wasn't neccesary! Vla's arguments are oviously backed up on knowledge and insight. Always interesting readings vla!!

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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880400 - 05/23/13 02:39 PM

Good point actually Chris.

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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5880416 - 05/23/13 02:42 PM

It is not the critic who counts: not the man who points out how the strong man stumbles or where the doer of deeds could have done better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly, who errs and comes up short again and again, because there is no effort without error or shortcoming, but who knows the great enthusiasms, the great devotions, who spends himself for a worthy cause; who, at the best, knows, in the end, the triumph of high achievement, and who, at the worst, if he fails, at least he fails while daring greatly, so that his place shall never be with those cold and timid souls who knew neither victory nor defeat.

– “Citizenship in a Republic,” Speech at the Sorbonne, Paris, April 23, 1910 Teddy Roosevelt

I think that about sums it up.


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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5880625 - 05/23/13 04:09 PM

Quote:

Citation Takahashi Europe, and Takahashi China as well:
"The objective of TOA-150 (D=150mm, F=1100mm, F/D 7,3) is an air-spaced triplet of which two of the lenses consist of ED glass with very weak chromatic dispersion."

=> The TOA-150 is obviously said (by people who should know) to be a pnp-objective and not an npn such as the TSA. (p = positive and n = negative lens)




Astronomics says it's FPL53 between two crowns; Astromart says the same; Optcorp says the same; ScopeCity says the same (also that it is like TOA130 "absolutely color-free"); and others. So many people think they know everything!

The fact is, it doesn't matter what triplet configuration is TOA150, but whether it is different than 130. And Takahashi says the two are "similar in design". That kind of excludes one of them being a reverse design.

Vla


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: roadi]
      #5880999 - 05/23/13 08:03 PM

Thanks, Roadi. Worry not - just another day on CN

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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5881058 - 05/23/13 08:34 PM

Quote:

The fact is, it doesn't matter what triplet configuration is TOA150, but whether it is different than 130. And Takahashi says the two are "similar in design". That kind of excludes one of them being a reverse design.




Hi Vla,

2009 Jan 19 Takahashi Japan TOA 130 renewal announcement, TOA 130N

The announcement says that TOA 130 has two FPL53 elements out of three. I remembered that it was FPL53-BSL7-FPL53 configuration but I can't find a reference to it. So don't quote me on that

I believe that TOA 150 has two FPL53 elements like TOA 130 but I am not sure what glass the middle element is.

I hope Art chimes in for definitive answer.

Tammy


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Re: Triplet APO or Doublet APO? new [Re: Tamiji Homma]
      #5881220 - 05/23/13 10:10 PM

so i take it a triplet wins every time when it comes to CA? providing of course the quality of glass and aperture is similar.. is this the skinny on this out of control thread no offense to the CNers who are discussing Taks..

personally, i'm not sure I would go for a Tak, i think at that price range i'd much rather go for an APM/LZOS triplet or maybe a TEC140, TV NP127.. jmho


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Re: Triplet APO or Doublet APO? new [Re: Tamiji Homma]
      #5881269 - 05/23/13 10:40 PM

Is that what this says?

"TOA-130 full detachable triplet telescope, was reborn in TOA-130N you change the S-FPL53 that are stable supply with two super ED glass.
In the visible light region, and include as before the full aberration-free, which is a feature of the TOA-130, the chromatic aberration near-ultraviolet wavelength region in (400nm), has improved about 20 percent.
Due to a change in glass material, then a gradual curvature sensitive critical surface aberration, it has been improved to strong cell structure to impact from the side. As a result, the ratio of the optical axis shifts will be less than ever before. It should be noted that, for the rich accessory lens of TOA series adapted as ever, to shooting in the cooled CCD digital camera or from visual observations, to provide the optical performance of the best for any purpose. Lineup is also two types of Type F of the eyepiece body specification of large that can be used by the 645 and 67 reducer flattener and Type S standard specification ever the same."


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Re: Triplet APO or Doublet APO? new [Re: Tamiji Homma]
      #5881626 - 05/24/13 04:32 AM

Quote:

2009 Jan 19 Takahashi Japan TOA 130 renewal announcement, TOA 130N

The announcement says that TOA 130 has two FPL53 elements out of three. I remembered that it was FPL53-BSL7-FPL53 configuration but I can't find a reference to it. So don't quote me on that
I believe that TOA 150 has two FPL53 elements like TOA 130 but I am not sure what glass the middle element is.
I hope Art chimes in for definitive answer.

Tammy




Tammy,
I read it also and wrote down the glass-specifications for my own information: TOA was announced to be a (pnp-) triplet with small airgap between 1st and 2nd lens plus wide airgap between 2nd and 3rd lens. The glass material was written to be FPL53-BS7-FPL53.

The name of the (original) basic design was not given. At first I thought it was kind of a Cooke-triplet - which, because 1st airgap is small, was wrong. Meanwhile I have come to the conclusion the TOA-objectives probably are based on the classic "VAF"-design.

The last development and offer by Dieter Lichtenknecker in Hasselt, Belgium, before passing away ~20 years ago was an aspheric variant of the VAF-objective: The VAF125/750 which (according to Uwe Laux' "Astrooptik") already had extremely low spherochromatism, and the published spot-diagrams of which (by Lichtenknecker) showed there remained some longitudinal aberration, resulting in slightly larger blurs in blue (486 nm) and still more so in violet (436 nm) than in red (656 nm) - ~in accordance to what Rohr has measured for those TOAs that got on his optical bench.

In my today's personal opinion, Takahashi have developped the VAF-design further on and have realised an FPL53-version of it - whether aspheric (as realised by Lichtenknecker) or not I have no idea of. If yes, this could possibly be an additional cause (besides the need of two FPL53-blanks) for the much higher price of TOA-130 in comparison to TSA-120... Anyway, as everybody knows they called their newly designed objective "Takahashi ortho-apochromat"...

@ Vla: It would be very (!) interesting (and not bloodless at all - sorry...) if you could calculate VAF-variants with the glasses mentioned by Tammy and so far also confirmed by myself.

Just my 2c...

(By the way: the glass material of the closely spaced TSA-(npn)-objectives was given to be BS7-FPL53-BS7.)

Chris


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Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5881892 - 05/24/13 10:04 AM

Quote:

I stand by my comments. Mr. Rohr is "the best source" for optical testing of amatuer telescope optics for those of us who don't have the resources or time to do what he does. It is always easy to criticize another's work. That was why I suggested that maybe Vla could start performing a similar service.




+1

thought I may not fully agree with all Rohr results or measurement method, his work is very interesting and I learnt a lot reading all his test reports.
More over he shares his valuable work and experience at no cost which is becoming rare nowaday...

Concerning AiryLab test reports, it is more recent but very usefull also. The process is a little bit different (zygo interferometer) and Frederic Jabet (the tester) refuses to remove some Zernike terms (coma) unlike Rohr. each of them has a different opinion, why not...


Quote:

I read it also and wrote down the glass-specifications for my own information: TOA was announced to be a (pnp-) triplet with small airgap between 1st and 2nd lens plus wide airgap between 2nd and 3rd lens. The glass material was written to be FPL53-BS7-FPL53.




I read it also somewhere, may be in the tests result sheets/graph. Besides, Takahashi Europe website states that the TOA150 has 2 ED elements.
I am not sure the TOA 130 and the 150 have the same kind of spacing. Not sure, but I thought the TOA130 had similar and wide spacing between the element which does not seem to be the case for the TOA150. So may be the 130 is a cooke triplet and the 150 is different?

Quote:

so i take it a triplet wins every time when it comes to CA? providing of course the quality of glass and aperture is similar.. is this the skinny on this out of control thread no offense to the CNers who are discussing Taks..




Whatever is the language (same kind of thread on French forums, especialy when it is cloudy ), speaking of refractor deals with passion and almost always results in a endless loop.

Quote:


personally, i'm not sure I would go for a Tak, i think at that price range i'd much rather go for an APM/LZOS triplet or maybe a TEC140, TV NP127.. jmho




all of them make sense and are excelent, depends on your needs, they all have some pro and cons, assets and issues...
None is stritcly better than the other in all the areas.

Doublet or triplet...depends also on your needs, there are also many other parameters that define a refractor, the focal ratio is a very important parameter as well. Of course if you choose an ED doublet, you shoud better take a long focal ratio one especially if you choose a large diameter. But a long ED doublet is often good enough for visual use. If your needs are deep sky astrophotography wide field, you may prefer a shorter focal ratio. In that case, a short focal ratio triplet like the AP or TEC FL, would make sense.

Olivier

Edited by olivdeso (05/24/13 10:50 AM)


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5882574 - 05/24/13 03:48 PM Attachment (33 downloads)

Chris,

OK here's what these glasses give. What really matters for chromatic correction is the glasses themselves. The arrangement only matters for spherochromatism, in that more relaxed radii (e.g. triplet) induce less higher-order spherical and consequently generate less spherochromatism. Going from NPN to PNP triplet doesn't change nothing significantly. And so doesn't spacing itself. Aspherizing could have appreciable effect, but I still don't see these glasses in a triplet (quadruplet, yes) producing nothing close to what Takahashi states in terms of focal shift.

On the other hand, the difference is "no difference", since even the plane PNP triplet with these glasses gives 0.97 polychromatic Strehl, and NPN triplet 0.98+ (possibly a bit more either, if fully optimized). BSL7 is not as good a match for FPL53 as ZKN7, but the residual defocus error is less than 1/10 wave p-v in F, undetectable visually (the best Ohara match is BSM81, but any significant benefit could come from lower spherochromatism, which partial dispersion graph indicates). Aspherizing did produce reduction in spherochromatism across the spectrum. Not dramatic, nor really significant in the magnitude for visual observing, but maybe marginally significant for photography.

Still puzzled by how these glasses in a triplet can give that incredibly tight chromatic shift. Maybe some computer design acrobatics, but I'm skeptical.

Vla


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: Tamiji Homma]
      #5882594 - 05/24/13 03:55 PM

Hi Tammy -

Yes, guess they won't give us any specifics so that we can let our imagination run loose. Makes it more interesting

Vla


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5882709 - 05/24/13 05:01 PM

Vla,

Thank you for your analysis which is highly interesting!
Maybe the TOAs are in fact not quite up to what they are said to be on the paper...?

Well, let us all go on speculating and investigating what could be behind "the secret of the TOA" as well as of "OIT" (= other interesting telescopes)!

Chris

By the way: I appreciate your great HP "telescopeOptics.net".


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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5882785 - 05/24/13 05:40 PM

Chris,

My pleasure. And thank you, too. I believe it all boils down to higher-order spherical. I noticed that its magnitude varies significantly with different configurations vs. that implied by its Abbe differential. In theory (or, I should say, in general), this dispersive power differential determines how strongly curved the surfaces need to be in order to minimize secondary spectrum The larger differential, the more relaxed radii, the less higher-order spherical. But obviously, lens configuration is also a factor. Tak may have found particular comfiguration (which could use odd lens shapes, aspherics and/or wider spacings) to significantly reduce spherochromatism vs. standard configurations. With the secondary spectrum negligible, it would really tighten up those foci.

But, like I said, the benefit of all it over the standard triplet configurations is marginal at best. Except possibly for some very special purposes.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5883057 - 05/24/13 08:18 PM

Now you're talking Vla, thanks for the analysis and +1 on the web pages "telescopeoptics.net". I refer to it often.

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Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5883088 - 05/24/13 08:53 PM

http://www.astrobin.com/users/FrancescoTallarico/ doublet achromat in narrowband and pseudo RGB.

http://www.starhopper.at/ double fpl-53 using ccd and dslr


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5883182 - 05/24/13 10:07 PM

Quote:

. Going from NPN to PNP triplet doesn't change nothing significantly.




And what it gives in term of cost? I why if they chose an higher cost solution, there should be a reason somewhere...

whatever, very interesting analysis

The Airylab repport also shows on p12 a graph close to the takahashi specifications

http://airylab.net/contenu/mesures/astro/rapport%202011-40001-a.pdf

http://www.takahashi-europe.com/images/products/toa130/700/TOA-130_correction...

impressive...

edit:

Vlad, you said

Quote:

Going from NPN to PNP triplet doesn't change nothing significantly. And so doesn't spacing itself.




but in the particular case of the Cooke triplet, I guess the spacing impacts the spherochromatism. Increasing the distance(starting from a thin air gap) should reduce the spherochromatism.

Edited by olivdeso (05/24/13 11:04 PM)


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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5883937 - 05/25/13 12:49 PM Attachment (35 downloads)

Olivier,

Thinking of how spherochromatism can be minimized leads to the two surfaces that generate 90% of spherical aberration. The disparity in ray height caused by chromatism results in less than perfect correction for non-optimized wavelength. In the PNP triplet configuration, after the front positive lens blue rays hit the mid (negative) lens lower than green, picking up undercorrection, and red rays hits higher, picking up overcorrection. In order to correct for that, this has to be reversed with the negative lens. For this to happen, positive refraction has to be concentrated on the 2nd surface, and negative refraction on the following, 3rd surface. This minimizes the path over which the rays diverge "positively" and makes it the easiest to reverse the effect with the negative lens.

I thought the higher-order spherical is main culprit, but should have known better. Lower-order is always significantly greater, and even its residuals due to refractive index variations are still the dominant portion of spherochromatism.

Following above logic, I let the front lens be plano-convex, and the middle lens plano-concave. Adequate spacing does indeed take away spherochromatism due to lower-order spherical. All that is left is negligible higher-order residual and secondary spectrum. The problem is not really secondary spectrum, which is strictly defined as the separation between common F/C focus and green focus, but the defocus in the violet and deep red. After spherochromatism is taken away, this becomes the limiting factor to the level of correction. And it is glass-match dependent.

This configuration is not Cooke, because it utilizes only one spacing, that between the two surfaces generating most of spherical aberration. The other interspace has little effect. With the spherochromatism taken away, the focal shift curve begins to resemble the one published by Takahashi. This particular arrangement only required slight aspherization, which means that it can be all-spherical (I just didn't want to waste time on slight bending of the lenses that would have done it). It is aplanat, so no coma, but it does have somewhat more astigmatism and field curvature than the standard PNP triplet. The colors can be further tightened by slightly strengthening the rear lens (bottom). Color correction is great, less than 1/4 wave at 0.4 microns (h-line). Red is not that good, hits 1/2 wave at 0.77 microns, but that's still very good.

Of course, this is not the actual TOA, but it comes close enough to be considered illustrative of its design. It probably can be mirror-reversed, i.e. made with the spacing between the 2nd and 3rd lens element.

Needless to say, with this strongly curved surfaces perfect collimation is critical, and it has to be top-notch mechanically.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5883963 - 05/25/13 01:12 PM

Forgot to put the poly-Strehl: 0.9933 visual (photopic, 25 wavelengths from 430 to 670nm) for the top, and 0.9788 for the bottom. CCD poly-Strehl (15 wavelengths from 400 to 750nm) 0.8869 and 0.9031, respectively. All this hoopla brings no detectable difference visually, and maybe very marginally detectable on CCD vs. standard PNP or NPN triplets with these same glasses

Vla.


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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5884183 - 05/25/13 03:15 PM

Quote:

. Going from NPN to PNP triplet doesn't change nothing significantly.

And what it gives in term of cost? I why if they chose an higher cost solution, there should be a reason somewhere...





The answer is in the arrangement, but it may not be obvious. Reversing/canceling spherochromatism generated by lower-order spherical at the stronger positive element requires nearly as strong negative element. It cannot be done in the standard NPN triplet configuration, since the power of negative component (which is about a third weaker than the positive power component) is split between two negative elements. It needs all negative power in a single lens, and that's PNP configuration.

Vla


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5885152 - 05/26/13 06:14 AM

Vla,
Congratulations!!!

a) If your design is reversed, then it seems to be quite exactly the VAF-design used by Lichtenknecker in his VAF-125/750 using FK-glass, 25 years ago. In his book "Astrooptik" Uwe Laux confirms that: "... (the VAF) must have a very solid mounting in order to realize the theoretically high quality of the image also in practice" {translation German => English} - which also is the case with TOA.
Lichtenknecker had published a scaled technical drawing of their (PNP) VAF-125/750. The radii are approximately as follows: r2 < r1; r3 < r2; r3 ~ r4 ~ r5; r5 < r6; r6 ~ r1
(=> first small airgap is concave-convex plus wider on axis, second wide airgap convex-concave ~ parallel). One (probably 2nd or 5th?) surface aspheric.
Of course glasses are different, however maybe this can still be useful to you.

b) I wonder if Takahashi possibly didn't communicate (maybe) having replaced one of their two FPL53-lenses by a different ED-glass, in order to keep at least part of the TOA-construction principle secret?

Chris


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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5885708 - 05/26/13 02:42 PM

Thanks, Chris -

Got lucky on the first try It's good to know it is doable, reducing spherochromatism to a non-factor. I tried it in the reverse configuration, with the two strong lenses and the gap in rear, but it doesn't work as well. There is imbalance between higher and lower order spherical going in the opposite direction, which cannot be corrected. Higher order cannot be taken out, only minimized, and the LA graph looks similar to Mak's. Not much difference with non-optimized wavelengths, but the optimized cannot be better than 1/14 wave P-V. Bending two lenses away from the plano shape worsens it. Putting Schmidt-like aspheric on the last surface takes out higher order spherical, but the lower-order spherochromatism remains, and cannot be nearly cancelled as with the first arrangement, only reduced, and only in the violet. That is probably because the negative lens comes last in the train, with a longer positive divergence, beginning from the front surface of the front lens (front positive lens also generating somewhat more of spherical than when it comes last).

That makes more likely the front-gap arrangement.

VAF design sounds like one modification of Taylor's photo-visual triplet. The triplet I posted before this last one I started from the Zeiss B shell, which is modified Taylor, with some 6mm rear gap, threw in the alleged TOA glasses, adjusted negative-to-positive power until it produced best chromatic correction, bent the lenses to correct for monochromatic aberrations, and that's what came out. It is not capable of significantly minimizing higher-order spherical or lower-order spherochromatism by increasing the rear gap, because has a different distribution of power.

Can't do much with the radii outline for the WAF125/750. It is much easier to start with the glasses and basic configuration known, and figure the radii.

Takahashi wouldn't make much by replacing FPL53 element with FPL52, and FPL51, which is cheaper (should I say "less expensive") has different, less favorable partial dispersion, that would worsen secondary spectrum. Still, the glasses do remain questionable, since the limit to secondary spectrum set by the dispersive characteristics of BSL7 and FPL53 is much greater toward the red end of the spectrum (about 4-5 times at 800nm) than what the Tak's graph shows (~0.05mm).

Vla


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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5885810 - 05/26/13 03:53 PM

Quote:

Olivier,

Needless to say, with this strongly curved surfaces perfect collimation is critical, and it has to be top-notch mechanically.

Vla




Thank you very much Vla for the time you taken to analyse and write a detailed answer, your explanation is excelent: high level, but still very clear, I do like it Top!

I was thinking exactly the same, this configuration requires a excelent mechanical side. This seems really critical. I hear that sometimes, the user have to shake a little bit the TOA to release the constraints and this seems to help the lenses sitting well in the cell.
That said, a famous astrophotographer told me he performed several road trips all around Europe, and his TOA remained well colimated.

Olivier


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5887049 - 05/27/13 12:49 PM Attachment (28 downloads)

Thank you, Olivier. I think it's safe to say it wouldn't have happened if all of us weren't at it. Collimation-wise, it is more like a fast doublet, than a triplet, so it's definitely more hard to stay close to the design maximum in the actual instrument. But Takahashi should know how to handle that.

Here's another take, all spherical and with all surfaces curved (it can be all spherical with plano surfaces as well). The lines are nearly vertical, which means that all zones focus nearly into a point. Slight curling comes from less than perfect balance in the 6th and 8th order spherical. It can be further reduced by slightly adjusting the gap width, but it is already negligible. The only chromatic error to speak of is secondary spectrum and it is nearly entirely determined by the glass match; no lens bending or change in separation available in triplet like this one will appreciable affect it. These two glasses can only be so good - which is very good. The focal shift from h to C line is nearly identical to that advertised by Takahashi. To get tOA's advertised magnitude of secondary spectrum reduction in the far red, it has to use different glass combination.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5887274 - 05/27/13 03:01 PM

Vla,

Great!

Is it possible for you to identify the design that you hereby have found by means of Rohrs big photograph of the TOA-150's objective right at the beginning of the following thread?

http://www.astro-foren.de/showthread.php?12633-TOA-150-1100-ein-quot-Sahnestü...

And would you think this last design of yours is now in better accordance with what Rohr wrote to have measured in his same thread (for example: chromatic focal shift)?

Chris


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5887922 - 05/27/13 09:08 PM

Chris,

Quote:

Is it possible for you to identify the design that you hereby have found by means of Rohrs big photograph of the TOA-150's objective right at the beginning of the following thread?

http://www.astro-foren.de/showthread.php?12633-TOA-150-1100-ein-quot-Sahnestü...





For what I can tell, if the dark lines are the lens edge, it does look like having wider gap between the first two elements. That leaves little doubt about TOA design. Nice find!

Quote:

And would you think this last design of yours is now in better accordance with what Rohr wrote to have measured in his same thread (for example: chromatic focal shift)?




Not really, since from C to h it's nearly identical to what Takahashi advertised. But it is possible that this particular system, if less than perfectly executed, would come closer to it. There's just to many "ifs", because design optimum and actual units are two (or many) different things. Considering how tricky these measurements can be if all conditions aren't strictly met, one shouldn't expect them to be perfect, only close. And, again, it's always telling more about the unit, than the design.

Vla


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ValeryD
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5888191 - 05/28/13 12:25 AM

We can shuffle the imaginary and the real benefits of triplets indefinitely. But when it comes to important practical aspects of the comparison of triplets and doublets, in many cases, the doublets are superior to the triplets.
First of all, within the most popular sizes (130 to 180mm) and F/D (7,5 to 9) there are several doublet designs that come so close to triplets in color correction that both types will be practically indistinguishable. However the doublets will have more practical advantages.
1. Doublets have less glass path - hence, they have lower light scattering
2. Doublets have less air to glass surfaces - hence, they have lower light scattering and lower potential wave front errors, especially on small scale of irregularities.

the summary of 1 and 2 is better image contrast.

3. Doublet cools faster than triplets. As larger objective size, as larger the difference.
4. Doublets weight less (better telescope balance).
5. Doublets are MUCH easier to collimate than triplets. If for whatever reason a doublet has it's lenses decentered, any user can perform collimation easily, using three screws. The same problem in a triplet can be fixed only by very experienced person of only in an optical laboratory.

Triplets have only two advantages:
1. They can be made faster than doublets and F/6 and faster is a triplet territory.
2. They have lower spherochromatism at the same color correction and F/D.

So, if a telescope is a visual instrument or a universal instrument (visual and photo), then better if it is a doublet. If a telescope is strictly a moderately fast astrograph, then better if it is a triplet.


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olivdeso
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5888321 - 05/28/13 03:02 AM

This analysis is applicable to air spaced triplet but oil spaced triplets are a litlle bit different:

- they cool down fast
- they only have 2 air to glass surfaces
- their lenses self center, so only the whole triplet is colimatable, even easier than air spaced doublet.

However it is possible to align an air spaced doublet or even triplet using the Rodier software and an artificial star. (it takes time...)


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stanislas-jean
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5888329 - 05/28/13 03:18 AM

I followed the forum with profit and your post highlighted some concerns about the possibility to show tiny and very low contrasted details on planets. This is my reading.
You said theory and optic applications are a field and it remains the application on sky. This is an important matter often not discussed.
May I bring some observationnal results about features on Uranus where the said apo were unable to show banding as a simple achromat does, of good facture.
On Venus the tendency is quite the same except the fact that leasibilty is lowered than on Uranus with the apo.
6" on each case.
My 2 cents to the concern.
Stanislas-Jean


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Re: Triplet APO or Doublet APO? new [Re: olivdeso]
      #5888980 - 05/28/13 01:31 PM

Quote:

This analysis is applicable to air spaced triplet but oil spaced triplets are a litlle bit different:

- they cool down fast
- they only have 2 air to glass surfaces
- their lenses self center, so only the whole triplet is colimatable, even easier than air spaced doublet.

However it is possible to align an air spaced doublet or even triplet using the Rodier software and an artificial star. (it takes time...)




Lenses in air spaced triplets CAN"T self center. They are not alive and they don't have a brain. Lenses in an oiled triplet require centering in any case. They better keep their centering - yes, but they can't self center.

And time to time an oiled system can leaks. Not all, not always, may be rarely, but still they can. They are prone to thermal shock.

The align procedure for a doublet is easy like 1-2-3 ready! Not so for ANY triplet.


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Re: Triplet APO or Doublet APO? new [Re: stanislas-jean]
      #5888983 - 05/28/13 01:33 PM

Quote:


May I bring some observationnal results about features on Uranus where the said apo were unable to show banding as a simple achromat does, of good facture.





I personally don't believe that a good achromat will show equal or more details than the same aperture and same good quality apochromat.

All peoples can mistaken and your are not any kind of exception, Stanislas.


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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5889044 - 05/28/13 02:05 PM

Respectfully Valery, this was not just on a trial, side to side observations, the apo triplet was in a certain difficulty to reveal such feature (banding pattern on Uranus) as the doublet did (with colour filters indeed, I am doing only with).
Stanislas_Jean


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greju
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5889110 - 05/28/13 02:48 PM

Doesn't this;

"So, if a telescope is a visual instrument or a universal instrument (visual and photo), then better if it is a doublet. If a telescope is strictly a moderately fast astrograph, then better if it is a triplet."

and this seem to contradict one another?

"I personally don't believe that a good achromat will show equal or more details than the same aperture and same good quality apochromat."

What would your scope for visual be? A doublet or a triplet? Thanks


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Re: Triplet APO or Doublet APO? new [Re: greju]
      #5889316 - 05/28/13 04:35 PM

The conclusion I acquired from OTA use on sufficient period is, less optical surfaces we have, better this is, and, when the acuracy is near the top, the better situation for collecting features on the case I said.
Now I am on reflectors with 2 optical surfaces only.
N8" does a same job as a R150 can do and may be a little better, on the optical axis area.
Stanislas_Jean


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ValeryD
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Re: Triplet APO or Doublet APO? new [Re: greju]
      #5890214 - 05/29/13 04:50 AM

Quote:



What would your scope for visual be? A doublet or a triplet? Thanks




A fluorite doublet with F/D about 9. During about a decade I have owned two identical mechanically 7" F/8 apos. One was the triplet Fluorite, the second one was the doublet Fluorite. I recently sold the triplet. The doublet is better for visual and is same good for CCD deep sky photos.


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5890219 - 05/29/13 05:30 AM

Quote:

During about a decade I have owned two identical mechanically 7" F/8 apos. One was the triplet Fluorite, the second one was the doublet Fluorite.




Valery,

Both of the same make (=> "Aries")?

I owned a Tak-FCT100 f/6.4 fluorite triplet with "excellent" startest (intra- vs. extrafocal identity) and a Vixen F102 f/8.8 with "good" startest, both with uncoated fluorite-lens - other lenses multi-coated.
I had both simultaneously side by side each on his own mount and compared them several times on Jupiter and other objects. When having reached thermal equilibrium (after ~1 hour), the FCT was slightly better. However, F102 needed only half of FCT's time until reaching peak performance.

OTOH, the FCT kept staying dew-free for about twice as long as the FS102 (sometimes quite important in my climate)... The Ucraine of course is a much dryer place than Switzerland.

As for me, I kept the triplet. And haven't had any collimation-problems since 1992 when I had bought it...

Chris


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5890988 - 05/29/13 04:42 PM

Quote:

5. Doublets are MUCH easier to collimate than triplets. If for whatever reason a doublet has it's lenses decentered, any user can perform collimation easily, using three screws. The same problem in a triplet can be fixed only by very experienced person of only in an optical laboratory.




I agree with the other points, generally, but don't see how a doublet can be so much easier to collimate - and keep collimated - than a standard triplet of similar f-ratio, using similar glasses. Triplet allows for significantly less strongly curved surfaces, thus also significantly lower tolerances for given collimation error.

Specifically, for 0.2mm decenter, 130mm f/7.7 FPL53/ZKN7 doublet and triplet will generate 2.8 and 0.24 waves p-v of coma. For 1/20 degree tilt, 4.2 waves and 0.13 wave p-v of coma, respectively.

I certainly wouldn't consider easy adjusting the doublet to less than 0.01mm decenter error.

The doublet will also always have more higher-order spherical, i.e. larger minimum error in the optimized wavelength (in this case design minimum for the doublet is about 1/12 wave p-v of balanced higher-order spherical).

Last, not least, the doublet has much tighter radii tolerances for given wavefront error.

That's the reality of it. 130mm is not a large apo, nor is f/7.7 fast one. But it is already pushing the doublet to require near perfect fabrication and alignment.

For comparison, TOA-like triplet would generate 1.6 wave and 1.8 wave of coma, respectively.

Vla


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5891146 - 05/29/13 06:54 PM

Vla,

If the objective is decentered then it is ALREADY decentered and one must fix this decentereing.
If you see the coma in a triplet and all three lenses do not have a common optical axis. Which of these three lenses one need to adjust? First, middle, back one?

In a doublet all is MUCH easier. One element is hold by the cell and the second one is movable. The second lens must be moved along the direction between the head and the tale of the coma. Very easy, like 1-2-3.
There were no problem to fix coma in doublets. Doublets with internal radii of curvative as short as 1,45 D are relatively easy to collimate. Slower ones are not a problematic at all.

Also there are a number of glass combinations for doublets where no high order spherical aberration or it is very low.


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5891235 - 05/29/13 07:59 PM

Valery,

I don't think any owner is looking for collimating their apos by themselves in the first place. Most apo makers discourage their customers from doing it. So what we are talking about is in the first place the consequence of (1) inevitably less than perfect factory collimation, and (2) possible misalignment that took place after the instrument left factory. For any given nominal misalignment doublet will have significantly larger error induced than comparable triplet. If the error induced to the doublet is tenfold that to the triplet, or more, then the latter is likely not to need any intervention even when the error in the doublet is quite obvious.

Btw. this particular ZKN7/FPL53 doublet has the radius ratio of less than 1.3. And there is no glass match for FPL53 that would produce less spherochromatism, except short flints, but that would come at a price of larger secondary spectrum (and more money, of course).

Anything significantly faster than f/9 is beginning to push the limits of a doublet, the exception to some extent being the smallest apertures. The faster lens, the more the triplet comes ahead.

Vla


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Mike Clemens
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5891242 - 05/29/13 08:04 PM

How APO could an 8" f/12 doublet be made?

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ValeryD
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Re: Triplet APO or Doublet APO? new [Re: Mike Clemens]
      #5891799 - 05/30/13 04:38 AM

Quote:

How APO could an 8" f/12 doublet be made?




Quite easily. With very good color correction. The problem is only one - too small market for such a long telescopes. Such a beast will require a very heavy mount and massive, tale pier.


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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5891809 - 05/30/13 05:03 AM

Quote:

Valery,

I don't think any owner is looking for collimating their apos by themselves in the first place. Most apo makers discourage their customers from doing it. So what we are talking about is in the first place the consequence of (1) inevitably less than perfect factory collimation, and (2) possible misalignment that took place after the instrument left factory. For any given nominal misalignment doublet will have significantly larger error induced than comparable triplet. If the error induced to the doublet is tenfold that to the triplet, or more, then the latter is likely not to need any intervention even when the error in the doublet is quite obvious.

Btw. this particular ZKN7/FPL53 doublet has the radius ratio of less than 1.3. And there is no glass match for FPL53 that would produce less spherochromatism, except short flints, but that would come at a price of larger secondary spectrum (and more money, of course).

Anything significantly faster than f/9 is beginning to push the limits of a doublet, the exception to some extent being the smallest apertures. The faster lens, the more the triplet comes ahead.

Vla




It is difficult to design a cell which will keep the lenses in collimation forever. If to use a silicon gel which hardens enough, but never completely hardens, this gel will holds lenses in a doublet in perfect collimation same reliably as in a triplet with radii of curvature close to what they are in a doublet. The tolerances in the doublets are the same as in the triplets with the same radii of curvature . Triplets based on a single FPL53 with F/6 normally have about the same radii of curvature as F/7 fluorite triplet with a short-flint mate element.

And, again, I sold my 7" F/7.8 Fluorite triplet and keep my 7" F/7.8 Fluorite doublet. Color correction is quite similar - no one can distinguish images in focus in these telescopes. But the doublet has all other advantages and little more contrast images.


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wh48gs
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5891884 - 05/30/13 07:27 AM

Quote:

And, again, I sold my 7" F/7.8 Fluorite triplet and keep my 7" F/7.8 Fluorite doublet. Color correction is quite similar - no one can distinguish images in focus in these telescopes. But the doublet has all other advantages and little more contrast images.





You are not specifying the matching glass. Comparison of a doublet vs. triplet is only meaningful if the glasses are similar.

Vla


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ValeryD
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Re: Triplet APO or Doublet APO? new [Re: wh48gs]
      #5891891 - 05/30/13 07:42 AM

Quote:

Quote:

And, again, I sold my 7" F/7.8 Fluorite triplet and keep my 7" F/7.8 Fluorite doublet. Color correction is quite similar - no one can distinguish images in focus in these telescopes. But the doublet has all other advantages and little more contrast images.





You are not specifying the matching glass. Comparison of a doublet vs. triplet is only meaningful if the glasses are similar.

Vla




Yes, this is better approach. But this is very specific case - to use only the same two glasses in a doublet and in a triplet.
And we again will juggle with positive and negative sides of each type objectives, but the main sense is that on a practice a doublet in most cases can be better choice than a triplet.


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Fomalhaut
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Re: Triplet APO or Doublet APO? new [Re: ValeryD]
      #5893680 - 05/31/13 05:56 AM

Quote:

...a doublet in most cases can {!} be better choice than a triplet.




...Depends on choice of glasses, execution, fabrication tolerances, centering, quality of lens cell, temperature gradients... and last but not least also on the question if one of the contenders is having this or that design in the pipeline, just ready for touting on the market .

IOW, the advantages and disadvantages of these or those products also depend quite directly on marketing considerations. Example: Do most of potential consumers have a 4-incher now? => Then let's propagate and sell 4.5- to 5-inchers! And in a few years we are gonna go back again with leight-weight 4-inchers with plastic focusing knobs for portability (and so on, without end...). => Certainly an advantage at least for growth of economies, I have to agree ...

Just my 2c...

Chris


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charles genovese
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Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5934239 - 06/22/13 08:55 AM

Just read through this whole thread and have a few comments - Have been in the hobby 50 years and currently have 20 scopes of all types (and have bought and sold 20 or 30 more).
Had a FS 102 for many years and then bought a SW120ED. Compared them visually and photographically for a year and sold the FS 102.
Field curvature is easily corrected with inexpensive correctors, and of course they can also lower the f/ratio- it doesn't have to be corrected by full aperature expensive extra lenses( not sure why there arn't some sub-aperature correctors (chromacore) for these to make them perfect!
prime focus deep sky imaging doesn't require Strels of .99-typically the finest details cover several arc sec.
My bigger scopes are always better for powers over 50 or so- not to knock the grab and go" of the 120ED on a LXD 75 type mount for a quick roll out/ back in (essentially no cool down needed)
Why would anyone observe Venous near the horizon? Always observe it when it is near zenith like every other planet- it was easy enough to see in the daytime before Go-to ( I was observing it this way with my Rv-6 in the '60s. BTW it's best to be in the shade of a building or tree.


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Re: Triplet APO or Doublet APO? new [Re: charles genovese]
      #5935225 - 06/22/13 07:52 PM

I'm a proud owner of a 1984 Tasco 60mm apochromatic crown and flint doublet, I joke not...lol, no color at any magnifications and I use Vega to look at the nearly perfect star test, but normally just use it to look at the moon at 40x-67x the intensity and whiteness of the airy disk with 2 faint rings is a good indication of a high strehl probably in the mid to upper 90's, It trips me out, and I'm lovin it.

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