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Equipment Discussions >> Refractors

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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
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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Reged: 08/11/10

Loc: Northern New Mexico
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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Reged: 10/08/06

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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Reged: 11/26/05

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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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Reged: 02/20/11

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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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Reged: 09/04/09

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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5848113 - 05/08/13 04:16 PM Attachment (188 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 (147 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
scholastic sledgehammer
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Reged: 03/05/12

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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


Reged: 04/09/12

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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Reged: 09/04/09

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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5852515 - 05/10/13 06:48 PM Attachment (118 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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Loc: Puerto Rico Island
Re: Triplet APO or Doublet APO? [Re: JoseBorrero]
      #5852517 - 05/10/13 06:49 PM Attachment (117 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? [Re: JoseBorrero]
      #5852623 - 05/10/13 07:42 PM Attachment (114 downloads)

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

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crazyqban
Carpal Tunnel
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Reged: 12/16/08

Loc: Miami, Florida
Re: Triplet APO or Doublet APO? [Re: JoseBorrero]
      #5852665 - 05/10/13 08:07 PM

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

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