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Triplet APO or Doublet APO?

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#1 JoeM101

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Posted 07 May 2013 - 08:28 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?

#2 orlyandico

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Posted 07 May 2013 - 09:18 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.

#3 Gregg Carter

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Posted 07 May 2013 - 09:26 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.

#4 maknewtnut

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Posted 07 May 2013 - 11:08 PM

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.

#5 Mike Clemens

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Posted 07 May 2013 - 11:17 PM

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.

#6 olivdeso

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Posted 08 May 2013 - 06:19 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).


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-teles...Strehlkurve.jpg

to the strehl of the 152mm triplet

http://www.apm-teles...letKurve600.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;

#7 Jon Isaacs

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Posted 08 May 2013 - 06:44 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.


:waytogo:

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

#8 jrbarnett

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Posted 08 May 2013 - 02:55 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

#9 CounterWeight

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Posted 08 May 2013 - 03:07 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)

#10 JoseBorrero

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Posted 08 May 2013 - 03:16 PM

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.


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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#11 JoseBorrero

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Posted 09 May 2013 - 03:57 PM

Same star with a triplet FPL-53.

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#12 JoseBorrero

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Posted 09 May 2013 - 04:04 PM

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

#13 terry59

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Posted 09 May 2013 - 04:26 PM

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


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

#14 A. Viegas

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Posted 09 May 2013 - 04:49 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... :question:

Al

#15 JoseBorrero

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Posted 09 May 2013 - 05:31 PM

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

#16 JoeM101

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Posted 10 May 2013 - 04:46 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...-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...

#17 JoseBorrero

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Posted 10 May 2013 - 05:48 PM

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

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#18 JoseBorrero

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Posted 10 May 2013 - 05:49 PM

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

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#19 JoseBorrero

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Posted 10 May 2013 - 06:42 PM

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

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#20 crazyqban

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Posted 10 May 2013 - 07:07 PM

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

#21 Arizona-Ken

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Posted 10 May 2013 - 09:05 PM

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

#22 Mentor

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Posted 11 May 2013 - 07:42 AM

LOL now Jose is just showing off ;)

Awesome pictures, very well done!

#23 kevint1

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Posted 11 May 2013 - 10:38 AM

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

#24 JoseBorrero

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Posted 11 May 2013 - 03:44 PM

Thanks Kevin, So far like both refractor :waytogo:

#25 JoseBorrero

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Posted 11 May 2013 - 03:45 PM

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






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