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Improving Achromat CA with a Prism Diagonal

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#51 Sarkikos

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Posted 23 August 2009 - 09:31 PM

TimD,

Soon after this thread came out, I tried my ST80 using the half-a-dozen diagonals in my collection: (1) Celestron 1.25", mirror, 90 deg, no CI; (2) Smart Astronomy (GSO?) 1.25", mirror, 90 deg, dielectric, no CI; (3) Orion 1.25", prism, 90 deg, CI, BK-7?; (4) Unknown provenance 1.25", prism, 90 deg, no CI, BAK-4?; (5) Orion 2", mirror, 90 deg, no CI, dielectric; (6) Kson 1.25", 45 deg, CI, BK-7? Verdict: In every case, no discernible improvement. No miracles seen.

On the other hand, since my prism/mirror trials, I have taken my ST80 to a dark site (green/blue zone) and discovered for myself the short tube achromat's true forte: Wide field views of nebulae and star fields. I upgraded to a single-focus Crayford (why would I need a dual-focus for low powers?). I fitted it with a Knight Owl UWA 30mm 2" 80 deg AFOV eyepiece, which yielded 13x, 6 deg TFOV, 6mm exit pupil, and a brightness factor of 1.1. And I screwed on a Celestron 2" OIII filter. The results: Excellent views of the North America, Pelican and Veil Nebulae, in fact the first view of them I'd ever managed to have. I could see all three major parts of the Veil with no difficulty. My wife and daughter also had no problem seeing these nebulae, so it was not a matter of my visual-observing skills. My personal conclusion based on my experience: These short tube achromats are best used for wide field Milky Way vistas and nebulae. Or you could rumage through your own collection of prisms and hope to get lucky. But wouldn't it just be better to use a long focus refractor or a good Newt or Cat for the planets and Luna? Just my two cents. YMMV. :grin:

May you have better luck with your prisms,
Mike

#52 Dan84

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Posted 25 August 2009 - 07:16 AM

...I was totally unprepared for what it did on a colorful achormat!!

I just picked up an f/5 80mm Achromat and of course expected color. Turning to Saturn at powers near and over 100x and it was a mushy view at best. I was using both an AT 2" dialectric or an AT 1.25" dialectric. So tonight I decided to give my old Celestron (prism) Star Diagonal 1-1/4" #94115-A a try. Again, using my 9mm Ortho on top of a 2.5x Siebert Barlow (111x) and in the slot of the Prism diagonal and expecting perhaps a marginal difference and was instead presented with a beautiful crisp planet, nicely defined rings, easy spot of the dark space between the planet and the rings as they circled behind the planet, and even hints of the polar shading and one band. OMG! From a fast Achromat! ...

-Bill


For a month or so I was trying to find a light weight and inexpensive go-to 100mm refractor with good optics and was encouraged to stay away from the Celestron's NexStar 102 SLT Computerized Telescope for $380 that I had decided on, because there would be nothing I could do to get rid of the overwhelming CA in the f/6.5 scope.

I do not have very much extra income to spend and needed something known to be good yet light weight, go-to [because of my eyes - can't see the stars very well any more to star hop], and inexpensive.

After asking questions here and at other forums I was finally lead away from my first idea to a Sky-Watcher - 100mm ED f/9, it is 34" inches long and weighs about 10.5 pounds.

Because of confusion caused by iOptron and their statements concerning their SmartStar -E mount [$242 from Amazon.com], refractor specs were under 40" long and <11 pounds, I thought that I would be able to mount the Sky-Watcher. Unfortunately after ordering and receiving the Sky-Watcher I have found that the iOptron SmartStar -E can only control a refractor that weighs up to 7 pounds.

After making a phone call I was informed that I will need the Sky-Watcher iOptron Mini-Tower which is closer to $800 after shipping cost.

This thread should be a sticky - in red color. It would have saved me a great deal of trouble. As it is, I may have to wait until next year before I can save the money for the $800 mount, as I am already over budget on the Sky-Watcher OTA.

The Sky-Watcher is a nice scope and I am trying to find a light weight Alt-az mount that might work, at least until I can afford the more expensive mount. I know that I could have purchased the Sky-Watcher and EQ go-to mount for about $1100, however, the mount is too expensive, too heavy and because of my poor health I do not feel like making 3 or 4 trips just to set up a scope. That is not grab and go.

Over the past 34 years I have sold all of my scopes except my last purchase, a 10" DOB that has outstanding optics, unfortunately it is too heavy for me, however, the optics are so good that I can't talk myself into letting it go.

In any event, my quest for an inexpensive 100mm achromatic refractor, and light weight grab and go, has now become a morass.

God willing, I hope to find a way to use it soon.

#53 astroducky

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Posted 25 August 2009 - 10:29 AM

Hi,

I tried using a Celestron prism diagonal and it does improve the sharpness and detail of the planets noticeably compared to the mirror diagonal.

BK7 and 1.25" diagonal works best for fast ED refractors as it imparts just enough SA to null the undercorrected green, and improves the wavefront for red. BAK4 or higher refractive index glass will cause too much overcorrection in green and thus lead to no or negative improvement.

Also, if it is a BK7 2" diagonal, I would think it will impart too much SA due to the longer optical path. It is a clear 'no' to using 2" diagonal for this 'trick'.

I would think the main reason why there is rather noticeable improvement on the planets is that planets tend to be 'warm' in colour being mainly brown. Which is compose of yellow and red. Since modern fast ED refractors are designed to be well corrected in blue-green and expense of yellow and red at focus, the planets will not be as sharp as 'majority' of the colours of the planets are 'unfocused'. Using the prism diagonal improves the view noticeably because it shifts the correction more towards red at the expanse of blue. I do easily see violet and red (though less) CA on Jupiter with my scope. In the past it was almost purely red CA. This correction in colour and nulling of the green (which tend to be undercorrected by design), improves the view noticeably. Now I just use my prism diagonal instead of my dielectric for planetary.

However, there are a few drawbacks to prism diagonal:
1. Since they tend to be single coated, transmission is noticeably less than the dielectric diagonal but is quite inconsequential for a bright planet and outweighed by better contrast (due to more colours from the planet in focus)
2. Ghosting, especially eyeball reflection can be a bit annoying.

However, the pros much outweigh the cons in planetary viewing, so thumbs up to the 'cheap' BK7 1.25" prism diagonal. :)

#54 deSitter

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Posted 25 August 2009 - 12:57 PM

I am wondering if people are seeing here the lack of diffuse scattering from the mirror, which is stronger in blue light. I personally always use a prism with 1.25" eyepieces unless they vignette from the barrel stop (32mm and 40mm Plossls). There is no comparison on the crisp view to be had with the glass, but I can't really chalk it up to a change in correction. With my 90mm f/10 the view is so much more washed out with the mirror that I would never get so far as to compare the CA - I will try some tests with my 127mm. In that I tend to prefer my prism-based turret which is big enough avoid vignetting with any 1.25" eyepiece. Comparo coming! The contenders are a very good Japan-made Meade 930 UHTC mirror, a Meade 918 prism, and a Meade mystery turret prism.

-drl

#55 Sarkikos

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Posted 25 August 2009 - 12:58 PM

BK7 and 1.25" diagonal works best for fast ED refractors as it imparts just enough SA to null the undercorrected green, and improves the wavefront for red. BAK4 or higher refractive index glass will cause too much overcorrection in green and thus lead to no or negative improvement.


BillP, the originator of this thread, used a prism to help correct the color error in his f/5 80mm achromat, I suppose something similar to my ST80, if not the same model. You are talking about a fast ED refractor. I'm not an expert on refractors by any means, but these are two different animals. I have an ST80 f/5 achromat and the chroma is pretty substantial on bright objects and higher powers are a bit "mushy." As I said above, though, it's a decent scope for low-power, wide vistas at a dark site. Improving the color in a short tube achromat by just changing the diagonal is a significant feat. I'm beginning to believe, though, that it might be better to accept the ST80 as a Rich Field Telescope, and use other scopes for the planets and Luna. Would it be worthwile for me to take a chance and buy a Celestron diagonal of a type that has been reported to improve chroma in some specific achromats but not necessarily in others, hoping that it might allow me to view planets more sharply with my ST80? Is it worth it? What's the point? As always, YMMV. It is a fun experiment, but I don't know if it's worth the mulah for a new diagonal. :grin:

Mike

#56 Clive Gibbons

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Posted 25 August 2009 - 01:25 PM

Hi,

I tried using a Celestron prism diagonal and it does improve the sharpness and detail of the planets noticeably compared to the mirror diagonal.



(Much very good explaining snipped for brevity's sake)

However, the pros much outweigh the cons in planetary viewing, so thumbs up to the 'cheap' BK7 1.25" prism diagonal. :)



Thanks very much for posting your insights, Yang. :bow:

They echo my experiences, using relatively "fast" ED doublet refractors with BK7 1.25" prism diagonals. :waytogo:

#57 Sarkikos

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Posted 25 August 2009 - 01:42 PM

I suppose I could repeat my diagonal trials but on my C4-R refractor instead of the ST80. I never noticed much color error in the C4-R, but then I wasn't really looking for it, as some refractor afficionados seem to do. It's a 4" achromat f/9.8. Maybe the prisms will surprise me and give a sharper view of the planets. Now that would be worthwhile. An 80mm, no matter how sharp, never struck me as a proper planetary scope.

Mike

#58 astroducky

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Posted 25 August 2009 - 11:25 PM

Since your scope is f/10, the amount of colour shift is small due to the light rays not being so 'steep'. Added spherical aberration overcorrection is about 1/4 wave PV at red if using a BK7 1.25" prism diagonal. The faster the telescope, the more the SA overcorrection, and the more colour shift.

#59 Sarkikos

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Posted 26 August 2009 - 07:54 AM

Yang,

Since your scope is f/10, the amount of colour shift is small due to the light rays not being so 'steep'. Added spherical aberration overcorrection is about 1/4 wave PV at red if using a BK7 1.25" prism diagonal. The faster the telescope, the more the SA overcorrection, and the more colour shift.


Yes, I realize longer f ratio refractors have less chromatic aberration. That's why the pre-achromat refractors - the "aerial telescopes" - were made so monstrously long. So .... do you think a BK7 1.25" prism diagonal might sharpen the planetary images in my 4" f/10 achromat? Or some other type of prism diagonal? I could always pop it in and make a comparison, but what do you think the results might be? Would the attempt be worthwhile? I don't want to limit my sky time with futile experiments, although they can be fun. :rainbow:

Mike

#60 ibase

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Posted 26 August 2009 - 09:48 AM

Bought this used prism diagonal:

Posted Image

Can anyone confirm if this is indeed a Vixen prism diagonal? There are no markings saying so. Thanks.

Best,

#61 wh48gs

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Posted 26 August 2009 - 01:20 PM

Since your scope is f/10, the amount of colour shift is small due to the light rays not being so 'steep'. Added spherical aberration overcorrection is about 1/4 wave PV at red if using a BK7 1.25" prism diagonal.



That's way too much. The effect of a plane-parallel plate in converging cone is described by:

s = [(n-1)t/n] + (n^2-1)t/8(F^2)n^3

with left side giving paraxial, and the right side marginal focus shift. In effect, the right side is added longitudinal aberration. For BK7 n=1.51432, the in-glass path for 1.25" prism t~25mm, and with F=10 the right side is 0.0116mm. That corresponds to best focus PV wavefront error of 0.116/64F^2=0.00000182mm or 1/361 wave @ 656nm. Since the longitudinal aberration LA increases with F^2 and the wavefront error is proportional to LA/F^2, the latter changes in proportion to F^4 for given plate thickness. Even at f/5, the induced overcorrection in red is still less than 1/20 wave. At f/4 it is little over 1/10 wave.

0.1 wave PV of induced spherical can significantly affect performance. For instance, if a telescope is 0.25 wave overcorected, it deteriorates to 0.35 wave of overcorrection. And the other way around, if it is 0.25 wave udercorected, it improves to 0.15 wave undercorrection.
At f/5, 1.25" prism induces less than 1/20 wave of overcorrection, which is still not quite negligible. A 2" prism would induce twice as much, so it has comparable effect at f/6.

The left side of the above relation implies that plane-parallel plate causes color foci shift according to it/n^2 relative to the green focus, with "n" being the green (usually e-line) index, and "i" the index differential relative to the green. Taking BK7 gives index differential of -0.0044, 0.00366 and 0.00797 for the red (C-line), blue (F-line) and violet (g-line). Substituting these values for t=25mm (1.25" barrel prism) gives that focus separation between green and red is reduced by 0.07mm, and increased by 0.06mm and 0.13mm for the blue and violet lines, respectively.

The separation due to secondary spectrum in a standard 80mm f/5 achromat is about 0.2mm for the blue and red, and about 0.5mm for the violet g-line. With the prism added, focus separation decreases to 0.13mm in the red, while increasing to 0.26mm and 0.63mm in the blue and violet, respectively.

If a Barlow is added after the prism, the linear foci separation quadruple, and color defocus error remains nearly unchanged. Barlow lens itself, being of negative power, probably also tends to extend blue, while reducing red focus separation, but the effect is generaly minor, as the plots from OSLO show. The combined effect of a prism and Barlow will tend to add up, but still no significant change in chromatic correction: it is somewhat better in the red, but worse in blue/violet. Barlow lens with less than perfect achromatism will also improve one end of the spectrum, while worsening it on the other - no free lunch here.

Vla

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#62 k5apl

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Posted 26 August 2009 - 01:28 PM

Vla
I haven't followed your math closely, but the curves support what I see as slight improvement with Barlow lens
and the prism diagonal with my 102mm F7 Achro. Combo is
better than prism alone.
And, I get a wider exit pupil to view through.

Wes

#63 Sarkikos

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Posted 26 August 2009 - 09:58 PM

For BK7 n=1.51432, the in-glass path for 1.25" prism t~25mm, and with F=10 the right side is 0.0116mm. That corresponds to best focus PV wavefront error of 0.116/64F^2=0.00000182mm or 1/361 wave @ 656nm. Since the longitudinal aberration LA increases with F^2 and the wavefront error is proportional to LA/F^2, the latter changes in proportion to F^4 for given plate thickness. Even at f/5, the induced overcorrection in red is still less than 1/20 wave. At f/4 it is little over 1/10 wave.

0.1 wave PV of induced spherical can significantly affect performance. For instance, if a telescope is 0.25 wave overcorected, it deteriorates to 0.35 wave of overcorrection. And the other way around, if it is 0.25 wave udercorected, it improves to 0.15 wave undercorrection.
At f/5, 1.25" prism induces less than 1/20 wave of overcorrection, which is still not quite negligible. A 2" prism would induce twice as much, so it has comparable effect at f/6.

The left side of the above relation implies that plane-parallel plate causes color foci shift according to it/n^2 relative to the green focus, with "n" being the green (usually e-line) index, and "i" the index differential relative to the green. Taking BK7 gives index differential of -0.0044, 0.00366 and 0.00797 for the red (C-line), blue (F-line) and violet (g-line). Substituting these values for t=25mm (1.25" barrel prism) gives that focus separation between green and red is reduced by 0.07mm, and increased by 0.06mm and 0.13mm for the blue and violet lines, respectively.

The separation due to secondary spectrum in a standard 80mm f/5 achromat is about 0.2mm for the blue and red, and about 0.5mm for the violet g-line. With the prism added, focus separation decreases to 0.13mm in the red, while increasing to 0.26mm and 0.63mm in the blue and violet, respectively.

If a Barlow is added after the prism, the linear foci separation quadruple, and color defocus error remains nearly unchanged. Barlow lens itself, being of negative power, probably also tends to extend blue, while reducing red focus separation, but the effect is generaly minor, as the plots from OSLO show. The combined effect of a prism and Barlow will tend to add up, but still no significant change in chromatic correction: it is somewhat better in the red, but worse in blue/violet. Barlow lens with less than perfect achromatism will also improve one end of the spectrum, while worsening it on the other - no free lunch here. Vla


:tonofbricks:

So .... do you think a BK7 1.25" prism diagonal might sharpen the planetary images in my 4" f/10 achromat? Or some other type of prism diagonal?

Yes, no or maybe would be sufficient... :rainbow:

Mike

#64 Sarkikos

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Posted 26 August 2009 - 10:05 PM

Does anyone else beside me think this thread is getting a little humorous? :question: :lol:

#65 wh48gs

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Posted 26 August 2009 - 10:34 PM

So .... do you think a BK7 1.25" prism diagonal might sharpen the planetary images in my 4" f/10 achromat? Or some other type of prism diagonal?

Yes, no or maybe would be sufficient...



The answer to your question is before your eyes - I encourage you to come to it yourself. Or you can opt for 1.25" prism inducing 1/4 wave PV of overcorrection to your scope. Your call.

Vla

#66 astroducky

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Posted 27 August 2009 - 06:25 AM

Hi,

Sorry made a mistake in the value. I re-did it with Zemax with a 4" f/10 achromat that I 'rough design' and got about 1/16 Wave P-V difference for 656nm. Did Oslo output such small numbers as 1/361 P-V waves at 656nm? I do not know the reason for the discrepancy in the numbers..

Regardless of the numbers, I do see Jupiter being more brown and the belts resolved sharper and 'appeared' when I use the Bk7 prism with my 88mm ED refractor instead of the dielectric diagonal.Also, there was violet CA and less red CA, showing that it has shifted in colour; I can't quantify 'how much' is the shift but to my eyes it has shifted in colour. It was easy to notice. Using a 25mm optical thickness bk7 with a pre designed lens, I do get more than 10nm shift in colour in Zemax, basing on the design that I have done and adding the prism.

#67 deSitter

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Posted 27 August 2009 - 06:39 AM

Again I would like to point out that much of the difference can be attributed to the diffuse scatter caused by a reflecting surface. The difference is dramatic in a standard aluminum mirror vs. a prism, perhaps less so with a dielectric. The virtue of the enhanced mirror is maximum light transmission for faint objects.

-drl

#68 Sarkikos

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Posted 27 August 2009 - 07:10 AM

Vla,

The answer to your question is before your eyes - I encourage you to come to it yourself. Or you can opt for 1.25" prism inducing 1/4 wave PV of overcorrection to your scope. Your call.


Literally before my eyes, in the sense that I need to take a chance, buy the prism diagonal, and see how it works in my scopes? Or before my eyes, in that I need to read and comprehend the technical explanations? Because, sorry to say, I cannot follow the high-level, optical jargon and mathematically-based technical language. I have not had those classes. (For example, what is "PV?") I have a good working knowledge of telescope optics, but any theory beyond a simple, layman's understanding of aberrations is beyond me at this point. You might as well be speaking Chinese. Just give me your expert opinion. Or not as you see fit.

Mike

#69 Scott Beith

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Posted 27 August 2009 - 07:50 AM

Mike P-V is peak to valley. Top of a wave pattern measured to the bottom. :waytogo:

#70 Sarkikos

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Posted 27 August 2009 - 08:04 AM

Scott,

Gotcha. :bow: Now should I buy the Celestron prism diagonal? :lol:

Mike

#71 Scott Beith

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Posted 27 August 2009 - 09:33 AM

:lol: That I can't answer for you as I don't have the knowledge base/experience with the prisms to add anything useful to this conversation. :)

It has been a good read so far.

#72 Sarkikos

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Posted 27 August 2009 - 10:18 AM

"It has been a good read so far."

Agreed. :rainbow:

#73 HandyAndy

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Posted 27 August 2009 - 05:38 PM

Hi,

The Meade/ES 127mm Triplet is corrected towards the red end for visual use. Would a prism move the correction towards the blue end for imaging?

Cheers. Andrew.

#74 Sarkikos

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Posted 27 August 2009 - 05:43 PM

Andrew,

Don't ask me... I'm still wanting for a direct answer to my question. :grin:

Clear Skies and Correct Colors,
Mike

:rainbow:

#75 wh48gs

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Posted 27 August 2009 - 10:50 PM

I re-did it with Zemax with a 4" f/10 achromat that I 'rough design' and got about 1/16 Wave P-V difference for 656nm. Did Oslo output such small numbers as 1/361 P-V waves at 656nm?



Yes, it gives 0.002883, or 1/347 PV wavefront error @656nm, which is slightly more than the theoretical result from PV=[(n^2)-1]t/512(F^4)n^3, which for the same index value n=1.514322, t=25mm and F=10 gives the PV=0.0000018183mm, or 1/361 in units of 0.000656mm wavelength. The differential is most likely due to the raytrace not being 100% accurate at such a low error level.

Since the error changes inversely to the fourth power of F number, 1/16 wave at f/10 would imply 1 wave at f/5. That would make an f/5 scope w/1.25" prism pretty much useless, which evidently isn't the case.

But in small fast ED refractors even 1.25" prism can exert noticeable, although not dramatic change in color correction. Their foci are packed tight enough that the shift in focus position, which is greater for shorter wavelengths, effectively bringing the red closer to the optimized wavelength, while shifting the blue/violet farther away, can be noticed. But that is only a general scenario; in reality, anything is possible since the effect depends on the position of best foci of non-optimized wavelength vs. best focus of the optimized wavelength.

As illustration, here's an 88mm f/5.6 FPL51/K4 doublet with and without 1.25" prism. Change in the RMS wavefront error in the red and blue is large enough to be noticed.

Vla

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