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FCD100 triplet vs F-PL53 doublet, 6"?

Astrophotography Refractor
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#1 GTom

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Posted 25 September 2023 - 06:39 PM

Considering trading my smaller refractors for one larger light cannon, not giving up on quality though.

Came across an FCD100 offering in Altair's product line (wave 152, f8), is that a disadvantage compared to the "classical" f-pl53 and CaF2?

 

Moreover, the triplet is considerably more expensive than the F-PL53 based doublet. Does it worth to invest in the triplet?



#2 ris242

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Posted 25 September 2023 - 07:46 PM

If I look at the TS website (they have the weights although theres is a 155mm).

 

its 9kg vs 13kg

 

its 2600 vs 4250 (if you include the vat on the Altair site).

 

visually I'd be happy with the doublet.

 

 

CaF2 - you're talking a different thing again.


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#3 Jethro7

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Posted 25 September 2023 - 07:58 PM

Hello GTom,

First off, I would hang on to at least one of the smaller fracs. With that being said. The Altairs are Manfactured by Kungming United Optics and are well manufactured. I own several scopes that KUO, manufactured and the manufacturing quality is as good as it gets at the price point of these scopes. 

 

Some say that FCD 100 lenses have a bit of color fringing on bright objects. The opinions on this seems to be personal. Some can't stand the color fringing to the point that they sold their scopes and others, the color fringing is not a problem. I have a Altair 152 F/5.9 achro that uses Ohara and Schott glass lenes and and the CA is there but it is minimal so I suspect that the F/8 Doublet will show even less CA. From my experience, minimizing CA seems to be an attribute with the KUO scopes.

 

For the most part if you are a viewer and not doing AP, a good doublet should be just fine. Doublets most often are cheaper and will certainly be lighter and will cool down a lot quicker, say 20 minuets vs 45 minuets for the triplet. A doublet can also use a lighter mount. The Triplet is going to have better color correction and should be totally free of CA. I have a TS Opik C-F 155 APO triplet with FPL 53 Glass and the views are a marvel but the scope is a 37 Lbs beast loaded. With that in mind, I prefer to use my Tak TSA 120 most  often simply because of the weight factor. This is something to concider.  But the TS 155 is awesome on the planets and I am happy that I have this scope. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

 

The Big Red One

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The Beast

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#4 scout

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Posted 25 September 2023 - 08:34 PM

Some say that FCD 100 lenses have a bit of color fringing on bright objects. The opinions on this seems to be personal.

Are you sure you're not thinking of FPL51 ED glass?

FCD100 and FPL53 have basically identical refractive indexes. Roland Christen uses FCD100 in his Astro-Physics refractors.

 

IMG 0272
IMG 0271

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#5 Jethro7

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Posted 25 September 2023 - 08:37 PM

Are you sure you're not thinking of FPL51 ED glass?

FCD100 and FPL53 have basically identical refractive indexes. Roland Christen uses FCD100 in his Astro-Physics refractors.

 

Hello Scout,

You are correct. I was thinking of the FCD 1.

 

Thanks for the Correction KEEP LOOKING UP Jethro


Edited by Jethro7, 25 September 2023 - 08:57 PM.

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#6 Jeffmar

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Posted 25 September 2023 - 09:12 PM

I am not that knowledgeable on types of glass, but from what I have read up on this topic a bit. I have also talked to people I know who seem to know what works. The general idea is It’s more about how the different types of lenses are put together than whether one glass is marginally better than another. My experience with refractors is you usually get what you pay for. I haven’t worried too much about which glass is better.

 

That is my expert opinion!!lol.gif


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#7 Oldfracguy

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Posted 25 September 2023 - 10:54 PM

I've had ED Triplets as well as SD Doublets, from 80mm up to 130mm.  I just do visual observation, and I have never seen any bluish fringing (CA) around the brightest objects using SD (FCD-100 / FPL-53 and Lanthanum) Doublets, but I have seen some CA with ED (FK-61 / FPL-51 / FCD-1) Triplets.  In addition, those big Triplets are really front-heavy.  In fact, the objective of the 130mm ED Triplet I had not too long ago weighs more than that of the awesome Big Red's 152mm Doublet Achromat objective seen in Post #3 above.

 

If you are a visual observer, an ED or SD Doublet will be fine.  If you are interested in Astrophotography, the slightly lower focal ratio offered by the Triplets might be an important consideration.


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#8 ris242

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Posted 26 September 2023 - 02:56 AM

Jethro - how often do you use big red, if you have the blue beast? I mean although the TS is heavier, its still quite a simple setup..... and surely the view is improved?

[I am looking at Red vs '53 F8 doublet. I dont need a big triplet]

#9 Jon Isaacs

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Posted 26 September 2023 - 03:09 AM

Are you sure you're not thinking of FPL51 ED glass?

FCD100 and FPL53 have basically identical refractive indexes. Roland Christen uses FCD100 in his Astro-Physics refractors.

 

 

:waytogo:

 

FCD-100 and FPL-53 are essentially identical in terms of color correction.  For equal aperture and focal ratio, an FCD-100 triplet will have significantly better color correction than a FPL-53 doublet.

 

Color correction also depends on aperture and focal ratio, the larger the aperture, the poorer the color correction (for the same design) the faster the focal ratio, the poorer the color correction. A 6 inch F/8 would benefit from an FCD-100 triplet.

 

Jon


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#10 GTom

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Posted 26 September 2023 - 04:29 AM

Many thanks, that was the information I was after. Unfortunately, mating glasses are most often not disclosed.

Are you sure you're not thinking of FPL51 ED glass?

FCD100 and FPL53 have basically identical refractive indexes. Roland Christen uses FCD100 in his Astro-Physics refractors.

 

FCD-100 and FPL-53 are essentially identical in terms of color correction.  For equal aperture and focal ratio, an FCD-100 triplet will have significantly better color correction than a FPL-53 doublet.

Color correction also depends on aperture and focal ratio, the larger the aperture, the poorer the color correction (for the same design) the faster the focal ratio, the poorer the color correction. A 6 inch F/8 would benefit from an FCD-100 triplet.

Jon

 

 

 

Thank u, I am going for photography with the gear and definitely do not want to compromise on B-channel performance. Yes, at this aperture, I do not plan to go below f/8 even with a triplet. Weight is not a huge concern for me as my camera is super-heavy anyway (Moravian g3, almost 2kg)lol.gif .

I've had ED Triplets as well as SD Doublets, from 80mm up to 130mm.  I just do visual observation, and I have never seen any bluish fringing (CA) around the brightest objects using SD (FCD-100 / FPL-53 and Lanthanum) Doublets, but I have seen some CA with ED (FK-61 / FPL-51 / FCD-1) Triplets.  In addition, those big Triplets are really front-heavy.  In fact, the objective of the 130mm ED Triplet I had not too long ago weighs more than that of the awesome Big Red's 152mm Doublet Achromat objective seen in Post #3 above.

 

If you are a visual observer, an ED or SD Doublet will be fine.  If you are interested in Astrophotography, the slightly lower focal ratio offered by the Triplets might be an important consideration.


Edited by GTom, 26 September 2023 - 04:40 AM.


#11 Jethro7

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Posted 26 September 2023 - 06:04 AM

Jethro - how often do you use big red, if you have the blue beast? I mean although the TS is heavier, its still quite a simple setup..... and surely the view is improved?

[I am looking at Red vs '53 F8 doublet. I dont need a big triplet]

Hello ms242,

The Altair 152 Achro is very 8" Dob like with its faster focal ratio and 6" of unobstructed aperture. It is a great scope choice for night vision Astronomy. Even though the big red one is brighter and has a wider field of view I still prefer to use either the Tak 120, TS 155 or the Dob simply because the Altair 152 is not as good as my other scopes for the Moon and planets. I keep the big red one around for use at the beach, or for one of those quick viewing sessions.

 

HAPPY SKIES AND KEEP LOOKING UP Jethro



#12 Oldfracguy

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Posted 26 September 2023 - 08:27 AM

waytogo.gif

 

FCD-100 and FPL-53 are essentially identical in terms of color correction.  For equal aperture and focal ratio, an FCD-100 triplet will have significantly better color correction than a FPL-53 doublet.

 

Color correction also depends on aperture and focal ratio, the larger the aperture, the poorer the color correction (for the same design) the faster the focal ratio, the poorer the color correction. A 6 inch F/8 would benefit from an FCD-100 triplet.

 

Jon

I agree with Jon here.  A few months ago I purchased a new Astro-Tech AT130EDT.  That is a 130mm f/7 ED Triplet with "regular ED" FK-61 glass, or its optical equivalent, as the center element.  I sent it back for a refund after a couple days.  That scope showed as much bluish fringing around bright targets as would be expected (and tolerated) with a 4" f/10 achromat, and I only do visual observation.  I don't want to imagine what a camera sensor might have seen.  The larger in aperture these scopes get, the more critical it is for them to have SD glass objectives, whether they are Doublets or Triplets.  Here are some notes I put together regarding Doublets that help to predict how well a scope should perform regarding its level of residual CA:

 

Color Blur JPEG Notes.jpg


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#13 daquad

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Posted 26 September 2023 - 02:32 PM

I agree with Jon here.  A few months ago I purchased a new Astro-Tech AT130EDT.  That is a 130mm f/7 ED Triplet with "regular ED" FK-61 glass, or its optical equivalent, as the center element.  I sent it back for a refund after a couple days.  That scope showed as much bluish fringing around bright targets as would be expected (and tolerated) with a 4" f/10 achromat, and I only do visual observation.  I don't want to imagine what a camera sensor might have seen.  The larger in aperture these scopes get, the more critical it is for them to have SD glass objectives, whether they are Doublets or Triplets.  Here are some notes I put together regarding Doublets that help to predict how well a scope should perform regarding its level of residual CA:

 

attachicon.gif Color Blur JPEG Notes.jpg

Using your values for the secondary spectrum of different glass combinations, it is easy to figure the color blur for a given generic design, assuming the color blur for ordinary crown/flint achromats will meet the Sidgwick criterion  (CB = 3XAiry disc) if the focal ratio is 3 times the aperture in inches.  Thus a 5" crown/flint achromat will have a CB of 3 if its focal ratio is 3X5 = 15.

 

Or we can use the Conrady criterion, which requires a CB = 1.8 with a focal ratio equal to 5 times the aperture in inches.  Thus a 5" crown/flint combo will need to be f/25 for a CB of 1.8X the Airy disc.

 

For the more modern designs the CB can be determined relative to either of these two criteria.  First note that the CB is inversely proportional to the focal ratio.  Second, we see that for a given focal ratio, the modern glass combinations will provide a smaller CB based on the ratio of the secondary spectra.  For example, the ED combination will yield a CB 1/3 that of crown/flint for a given focal ratio.  2000/6000 = 1/3.

 

For my AT 125 EDL I use the secondary spectrum ratio of 1/6.5 (2000/13000).  However, its focal ratio is 7.8, so if it were a crown/flint design its CB would be (15/7.8)X3 = 5.77.  But the FCD100/ Lanthanum glass reduces the secondary spectrum by 1/6.5.  So the CB for the AT 125 EDL is 5.77/6.5 = 0.887.

 

Note that we get the same result if we use the Conrady criterion, (5" needs to be f/25 for a CB = 1.8).  Doing the math:  (25/7.8)X1.8/6.5 = 0.887.

 

If you want a formula (using the Sidgwick criterion):  CB = (F1X3)/(F2XS), where F1 is the focal ratio of the modern design, F2 is the required focal ratio of the crown/flint design for a color blur of 3, and S is the inverse ratio of the secondary spectra.  (For the 125 EDL S = 6.5).

 

My 102 f/11 Altair Starwave is an ED design with FPL51; thus, I use S=3, 1/ (2000/6000).  Its design color blur is:

 

CB = (12X3)/(11X3) = 1.1.

 

Dom Q.


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#14 Oldfracguy

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Posted 26 September 2023 - 05:45 PM

Using your values for the secondary spectrum of different glass combinations, it is easy to figure the color blur for a given generic design, assuming the color blur for ordinary crown/flint achromats will meet the Sidgwick criterion  (CB = 3XAiry disc) if the focal ratio is 3 times the aperture in inches.  Thus a 5" crown/flint achromat will have a CB of 3 if its focal ratio is 3X5 = 15.

 

Or we can use the Conrady criterion, which requires a CB = 1.8 with a focal ratio equal to 5 times the aperture in inches.  Thus a 5" crown/flint combo will need to be f/25 for a CB of 1.8X the Airy disc.

 

For the more modern designs the CB can be determined relative to either of these two criteria.  First note that the CB is inversely proportional to the focal ratio.  Second, we see that for a given focal ratio, the modern glass combinations will provide a smaller CB based on the ratio of the secondary spectra.  For example, the ED combination will yield a CB 1/3 that of crown/flint for a given focal ratio.  2000/6000 = 1/3.

 

For my AT 125 EDL I use the secondary spectrum ratio of 1/6.5 (2000/13000).  However, its focal ratio is 7.8, so if it were a crown/flint design its CB would be (15/7.8)X3 = 5.77.  But the FCD100/ Lanthanum glass reduces the secondary spectrum by 1/6.5.  So the CB for the AT 125 EDL is 5.77/6.5 = 0.887.

 

Note that we get the same result if we use the Conrady criterion, (5" needs to be f/25 for a CB = 1.8).  Doing the math:  (25/7.8)X1.8/6.5 = 0.887.

 

If you want a formula (using the Sidgwick criterion):  CB = (F1X3)/(F2XS), where F1 is the focal ratio of the modern design, F2 is the required focal ratio of the crown/flint design for a color blur of 3, and S is the inverse ratio of the secondary spectra.  (For the 125 EDL S = 6.5).

 

My 102 f/11 Altair Starwave is an ED design with FPL51; thus, I use S=3, 1/ (2000/6000).  Its design color blur is:

 

CB = (12X3)/(11X3) = 1.1.

 

Dom Q.

I never was able to see any color with my old AT125EDL, even at absurd magnifications like 585x on bright targets.  On the other hand, when pushing it with a TS-Optics 102mm f/11 ED (regular FPL-51 glass I believe TS uses) I used to have, I could just see a trace of CA, but only on the very brightest targets.  So, it seems that the formula for CB being either below 1.00 or slightly above 1.00 really do predict what you see with varous refractors.


Edited by Oldfracguy, 26 September 2023 - 05:48 PM.


#15 GTom

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Posted 26 September 2023 - 06:22 PM

Visually seeing the actual CA depends on a handful of variables, photography is much more reliable.

I never was able to see any color with my old AT125EDL, even at absurd magnifications like 585x on bright targets.  On the other hand, when pushing it with a TS-Optics 102mm f/11 ED (regular FPL-51 glass I believe TS uses) I used to have, I could just see a trace of CA, but only on the very brightest targets.  So, it seems that the formula for CB being either below 1.00 or slightly above 1.00 really do predict what you see with varous refractors.

 

Surprised to see such a significant difference between CaF2 and SD-glass (f-pl53/55). Especially in light of the data shown in #4.

I agree with Jon here.  A few months ago I purchased a new Astro-Tech AT130EDT.  That is a 130mm f/7 ED Triplet with "regular ED" FK-61 glass, or its optical equivalent, as the center element.  I sent it back for a refund after a couple days.  That scope showed as much bluish fringing around bright targets as would be expected (and tolerated) with a 4" f/10 achromat, and I only do visual observation.  I don't want to imagine what a camera sensor might have seen.  The larger in aperture these scopes get, the more critical it is for them to have SD glass objectives, whether they are Doublets or Triplets.  Here are some notes I put together regarding Doublets that help to predict how well a scope should perform regarding its level of residual CA:

 

attachicon.gif Color Blur JPEG Notes.jpg



#16 daquad

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Posted 27 September 2023 - 02:06 PM

I never was able to see any color with my old AT125EDL, even at absurd magnifications like 585x on bright targets.  On the other hand, when pushing it with a TS-Optics 102mm f/11 ED (regular FPL-51 glass I believe TS uses) I used to have, I could just see a trace of CA, but only on the very brightest targets.  So, it seems that the formula for CB being either below 1.00 or slightly above 1.00 really do predict what you see with varous refractors.

The constant 735 in the formula for CB is for the Airy disc in green light (0.55 microns).  And more importantly, it is for the diameter of the diffraction pattern to the first minimum, which is larger than the actual visible disc.  The visible disc is usually taken to be full width at half maximum (FWHM).  Using FWHM as the criterion for CB relative to the disc, the constant in the formula would be around 900, which means the CB values would be about 22% larger relative to the FWHM disc.

 

AP aficionados prefer a CB less than the FWHM, which requires a triplet design or focal ratios that are too large for AP with a doublet.  Personally, some of the work I've seen here by CN members using high quality doublets are outstanding.  But for the very meticulous AP'ers a triplet design is the ticket.  (Assuming high quality, of course.)

 

Dom Q.


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#17 GTom

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Posted 27 September 2023 - 06:05 PM

The constant 735 in the formula for CB is for the Airy disc in green light (0.55 microns).  And more importantly, it is for the diameter of the diffraction pattern to the first minimum, which is larger than the actual visible disc.  The visible disc is usually taken to be full width at half maximum (FWHM).  Using FWHM as the criterion for CB relative to the disc, the constant in the formula would be around 900, which means the CB values would be about 22% larger relative to the FWHM disc.

 

AP aficionados prefer a CB less than the FWHM, which requires a triplet design or focal ratios that are too large for AP with a doublet.  Personally, some of the work I've seen here by CN members using high quality doublets are outstanding.  But for the very meticulous AP'ers a triplet design is the ticket.  (Assuming high quality, of course.)

 

Dom Q.

Most great AP results from doublets are likely using restrictive B and L filters, wasting all photons below 420nm I suspect. I rather go with a smaller, high quality triplet that allows the old "ccd"-type, or even "L1" type filters.

 

Also, the real-life limitation might be more allowing than the Airy disc. Deep-sky AP is more often limited by either the pixel size (short FL) or the seeing (long FL).


Edited by GTom, 27 September 2023 - 06:06 PM.

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#18 GTom

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Posted 28 September 2023 - 08:30 PM

Most great AP results from doublets are likely using restrictive B and L filters, wasting all photons below 420nm I suspect. I rather go with a smaller, high quality triplet that allows the old "ccd"-type, or even "L1" type filters.

 

Also, the real-life limitation might be more allowing than the Airy disc. Deep-sky AP is more often limited by either the pixel size (short FL) or the seeing (long FL).

Contradicting myself, found this very impressive result: https://www.astrobin.../full/s9okh1/0/

As I understood, both B and L filters used start below 400nm.



#19 scout

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Posted 29 September 2023 - 02:36 PM

I never was able to see any color with my old AT125EDL, even at absurd magnifications like 585x on bright targets.  On the other hand, when pushing it with a TS-Optics 102mm f/11 ED (regular FPL-51 glass I believe TS uses) I used to have, I could just see a trace of CA, but only on the very brightest targets.  So, it seems that the formula for CB being either below 1.00 or slightly above 1.00 really do predict what you see with varous refractors.

I don't see any color with my AT102EDL either, except a tiny bit outside of focus on Venus or Sirius. But in focus, which is all that matters visually, even on Venus and Sirius (above the atmospheric dispersion), the EDL scopes are color-free. Really remarkable refractors for the price.


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#20 Drothgeb

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Posted 29 September 2023 - 06:55 PM

I don’t see any color fringing with either my FCD100 or FPL53 triplets (2 of each), even when using a reducer and with bright stars . My one FPL53 doublet does have a bit of color.


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#21 RichA

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Posted 29 September 2023 - 09:10 PM

waytogo.gif

 

FCD-100 and FPL-53 are essentially identical in terms of color correction.  For equal aperture and focal ratio, an FCD-100 triplet will have significantly better color correction than a FPL-53 doublet.

 

Color correction also depends on aperture and focal ratio, the larger the aperture, the poorer the color correction (for the same design) the faster the focal ratio, the poorer the color correction. A 6 inch F/8 would benefit from an FCD-100 triplet.

 

Jon

S-FPL-53 you mean.  FPL-53 index was 1.4387




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