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Are SCT corrections still made on secondaries?

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#26 DAVIDG

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Posted 18 August 2014 - 09:15 AM

 I've refigured a number of Schmidt Cass optics over the years and all the ones I've worked on had aspherical secondaries when they were tested agianst a concave test plate.  By the way aspheric is any conic from an ellipse to a hyberbola. In the Schmidts that I have worked on the primaries are also not perfect spheres but close but what is important is that they are optically smooth. The same goes for the corrector plate.  In  the book "Optics Cooke Book" by Frank Cooke who wrote a monthly article in Applied Optics there is chapter on figuring commerical Schmidt optics that quotes Tom Johnson founder of Celestron and explains what they were doing at Celestron.

 There are couple of reason why you aspherize the secondary.  When your making mass produced optics, they all will have some level of error in them so to tune out these errors you select one surface and figure it so the system nulls. The secondary was  selected because it requires the least amount of glass removal and Johnson  stated this in his article. The other reason for aspherizing the secondary is that it allows for better correction of coma. If you don't what to aspherize the secondary to remove coma but leave it spherical, you can move the corrector plate forward ie away from the primary and place it at the radius of curvature of the primary. The reason why this is not done is that it make for a longer tube and reduces the portablity of the telescope. The other method is to add additional optics and that is what is being done now, with a two element corrector placed in the main buffle. The corrector in theory corrects for coma and also spherical aberration of the spherical secondary but reality, you still need to touch up a surface to get the system to null out, so again the secondary is figured to do this.

 

 

                           - Dave


Edited by DAVIDG, 18 August 2014 - 09:21 AM.


#27 GlennLeDrew

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Posted 18 August 2014 - 01:32 PM

Dave,

The slight degree of correction on the secondary is unlikely to be sufficient to have any impact on coma. Most certainly if it's only zonal work to clean up a largely corrected wavefront to begin with. The overall primary spherical aberration would have to be rather bad--and of the right sign--in order that secondary aspherizing go some way to reducing coma, would it not?



#28 DAVIDG

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Posted 18 August 2014 - 05:24 PM

Glenn,

   Bob Johnson and Celestron's owe literature states that they aspherized the secondary to reduce coma. One of the main selling points of the Schmidt Cass design was/is it use for astrophotography and because of that you need  to have a field of view that has the coma controlled. You have three choices then it come to reducing coma  you either move the corrector plate forward, aspherize the primary or aspherize the secondary.  Johnson stated they choose the secondary because it requires the least amount of figuring and the article in "The Optical Cooke Book" talks about peddle shapes laps and the amount of correction needed to aspherize the secondary and the problem of getting a turned up edge that is  required to make a convex hyberbolic figure on the secondary. Here is a link to some of Celestron's  literature were they state they hyberbolize the secondary http://geogdata.csun...erature1972.pdf. One needs to understand that in a short radius convex surface that it doesn't take much correction before you are technically at a hyberbola. 

  In the Edge design the purpose of the two element corrector is to reduce the coma and  if all the surface were perfect have a spherical secondary but because you still have a  multi-surface system, and they still have to touch up the secondary but by a lesser amount then "classic" SCT design to null the system.

 All I can say is the ones that I refigured all had a good amount  of aspherizing on the secondary so it wasn't a small amount to just correct residue spherical.   As you stated, if you place the optics up on a test bench and test them via double pass, it is pretty easy the see the problem and if you started out with spherical secondary "blank" and you are  were doing this day in a day out, it wouldn't  take long to aspherize  the secondary to get the system to null even if the correction required was hyberbolic.

 

                      - Dave



#29 PowellAstro

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Posted 18 August 2014 - 06:51 PM

The C8 that I rebuilt had a touched up secondary like this because any off center error produced very noticeable astigmatism.



#30 GlennLeDrew

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Posted 18 August 2014 - 06:51 PM

Dave,

Between a sphere and the required hyperbola on the secondary (assuming a nominal primary and corrector plate), how many fringes of deformation differentiates?



#31 DAVIDG

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Posted 18 August 2014 - 08:17 PM

 Glenn,

    Against a spherical test plate I was getting around two fringes  of aspheric correction the last time I worked on one.  

 

                             - Dave



#32 RodgerHouTex

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Posted 18 August 2014 - 10:20 PM

Actually if you check "Telescope Optics" by Rutten and VanVerooj you will see that aspherizing the secondary significantly reduces coma in an SCT versus leaving it spherical.



#33 GlennLeDrew

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Posted 18 August 2014 - 10:42 PM

Dave,

Did that degree of asphericity eliminate coma? If so, that's a pretty mild departure from spherical to do the job. And again if affirmative, it makes one wonder why this has not been SOP (standard operating procedure) for all SCTs from year one. Then with a basic/simple field flattener incorporated....



#34 freestar8n

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Posted 19 August 2014 - 01:28 AM

I have read a few sources that some sct's over the years were designed to have aspheric secondary to reduce coma - but the key is that the target design would need to factor that in to the whole system.  You can't just choose at the last minute to aspherize or not - because you always need to eliminate spherical aberration.  If the final retouching is done in an autocollimator, and if the overall design is aplanatic, with zero coma and spherical, then by zeroing the spherical in the autocollimator - by retouching the secondary - you aim at the same time to eliminate coma.  But the radii and the schmidt corrector would need to be different if you intend to eliminate coma, otherwise aspherizing the secondary would introduce spherical aberration.

 

As I recall from Sigler's paper, the asphere on the secondary would be either elliptical or hyperboloid - depending on the sct layout.  But the degree of asphere needed for an aplanat would be a greater departure from a sphere than the simple retouching of a spherical system to correct small errors in the radii and zonal errors that introduce spherical aberration in an autocollimator.

 

Thanks David for the background info and references - particularly Cooke's book.

 

Frank



#35 wh48gs

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Posted 19 August 2014 - 07:43 AM

deleted


Edited by wh48gs, 19 August 2014 - 07:47 AM.


#36 wh48gs

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Posted 19 August 2014 - 07:48 AM

Glenn,

 Here is a link to some of Celestron's  literature were they state they hyberbolize the secondary http://geogdata.csun...erature1972.pdf. One needs to understand that in a short radius convex surface that it doesn't take much correction before you are technically at a hyberbola.

 

 

To state the obvious, this Celestron's brochure is a heavy weight advertising. Gave mi a real good laugh, but the way they mislead people is not funny.

 

They say they had each of the three elements down to 1/100 wave, but no, that's not good enough, they "hyperbolize" the secondary to get their SCTs to a perfect null. The problem is, if you'd hyperbolize the secondary at such a low level of correction error, you'd not only induce at least a couple of waves of undercorrection, which would make necessary to entirely rework the corrector.With the typical f/2/10 system It would also induce too much of negative coma, so the system would end up with negative coma, that would be 20% or more - depending on the degree of hyperbolization - of the system with spherical secondary.

 

"No other system can match SCT in the size of corrected field",  "refractors suffer from severe coma", "Airy disc brilliance factor gives an extra contrast/resolution advantage to larger apertures", etc.

 

It takes nearly -0.8 conic (prolate ellipsoid) on the secondary to make the typical f/2/10 SCT coma-free. With an 8-inch that amounts to about 4.5 waves depth difference vs. sphere. Te lower magnification, the more aspherization needed; at f/8 (i.e. secondary magnification 4), the coma-free secondary is just slightly hyperboloidal. The degree of coma correction is proportional to the conic: a 10% of the conic needed for a full correction will take out 10% of the system coma.

 

Vla



#37 rmollise

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Posted 19 August 2014 - 09:19 AM

Well, the above derided brochure is kinda silly. But seeing as it's from over 40 years and _four_ Celestrons ago, it's more the character of a Straw Man than anything else. :lol:



#38 DAVIDG

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Posted 19 August 2014 - 01:15 PM

  Look at the amount of coma correction in classic C-8 either in images or at the eyepiece and it is less then what you would have if the secondary was spherical or close to a sphere with some minor touch up. If you raytrace the design with a spherical secondary, the amount of coma is about what 6" f/5 newtonian  as stated in "Telescope Optics"  So all I can say is that I have tested and  refigure a handful of these and there is aspheric correction on the secondary. For what ever it is worth Johnson stated in the Celestron literature that they used aspheric secondary and was referenced to the same in Applied Optics. You have employees that work at Celestron saying that they corrected the secondary to clean up the system and from what I was told that each secondary started as spherical "blank" and each was one was figured until the system met spec  so it sounds to me that each one was aspherized for some reason. Celestron's goal may not have been to have fully coma free field but one that was corrected well enough for astrophotography using a 35mm film so the amount of correction would be less.

   The next time I pull one apart I'll  post pictures of the secondary being  tested against a test plate and everyone can judge from themselves what is going on.  If someone else would like to provide actual data of the secondary figure that would be great as well since we'll have some real data vs speculation.

 

                 All the Best 

                     - Dave



#39 GlennLeDrew

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Posted 19 August 2014 - 01:39 PM

Vla,

Good info! I did wonder if a mere couple fringes of asphericity could eliminate coma. Your figure of 4.5 waves (9 fringes) accords more with my intuitive feeling.

 

If Celestron was indeed achieving even close to nil coma, surely they would have trumpeted that to high heaven.

 

I should read that brochure. It sounds like a real hoot! The bald-faced 'exaggeration' is breathtaking in its audacity! Today's community would roast them over the coals if anything remotely so bold was attempted.



#40 freestar8n

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Posted 19 August 2014 - 04:57 PM

Yes Glenn - that is my point.  You can't just take a spherical sct design - with coma - and aspherize the secondary slightly with a small amount of retouching to end up with no coma and no spherical.  The overall design is different and the amount of asphere needed is greater.  That is why it isn't "just done" as you asked.  But that is entirely different from correcting errors in the as-made system by custom retouching of the secondary.

 

I don't know how the ACF secondary is made and if it is retouched in an autocollimator.  I assume it is made to a sphere and then directly aspherized by automated means - but whether it is then retouched in an autocollimator I don't know - and they don't have an equivalent of the celestron whitepaper to go by.  But the principle would be the same - the overall design is intended to be aplanatic, and retouching in an autocollimator would allow correcting any residual spherical aberration in the system - due to any combination of errors in the element radii, schmidt corrector, or the secondary conic.

 

Frank



#41 RodgerHouTex

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Posted 20 August 2014 - 09:21 AM

Again, if you refer to "Telescope Optics", Rutten and VanVerooj show ray traces for an all spherical compact SCT and for one where the secondary has been aspherized. Aspherizing the secondary does not eliminate coma but significantly reduces it.  



#42 freestar8n

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Posted 20 August 2014 - 09:40 AM

They didn't just aspherize the secondary - the adjusted other parameters to make an overall improved design.  The resulting design is consistent with the aplanatic sct form that Sigler had described earlier.  There are many ways to change the design to make it aplanatic - but you can't just change the secondary by itself.  Changing only the secondary would introduce spherical - so you need to adjust all the parameters together to eliminate spherical and coma at the same time.  The conic value on the secondary provides an extra degree of freedom to make those overall adjustments.

 

Frank



#43 RodgerHouTex

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Posted 20 August 2014 - 10:17 AM

The "other parameter" they adjusted was the power of the corrector which for the optimized design is 83.4% of the all spherical SCT design.  There are no spacing changes and it is not aplanatic.  There is no Sigler compact SCT designs in the book.  He is mentioned in relation to Mak-Casses only.

 

It's clear to me that the optimized design is the one that Celestron has built and is building.  I own 5 Celestron SCTs and none of them have the coma of an f/5 Newt.  I also own an 8 and 10 inch f/3.9 Newts, a 16" f/4.25, and a 20" f/4.5 dobs.  So I do have some Newts that I can use for comparitive purposes.



#44 wh48gs

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Posted 20 August 2014 - 11:22 AM

It's clear to me that the optimized design is the one that Celestron has built and is building.  I own 5 Celestron SCTs and none of them have the coma of an f/5 Newt.  I also own an 8 and 10 inch f/3.9 Newts, a 16" f/4.25, and a 20" f/4.5 dobs.  So I do have some Newts that I can use for comparitive purposes.

 

I see no evidence of them having the coma-corrected design before the Edge, As someone already said it, if they had, they would have made it very clear. And BTW an f/2/10 SCT w/spherical mirror has linear field coma comparable to an f/6 paraboloid. Not much to see there.

 

Also, SCT with aspherized secondary is aplanatic in the primary aberration domain. What's left after the primary coma is cancelled is the higher-order coma term, which is less than 5% of the primary coma in magnitude. It can be reduced by 79% by adding a bit more of primary coma, by slightly strengthening the secondary conic. So if we have some 3.5 wave p-v 20mm off axis in 8-inch aperture with spherical secondary, it is reduced to less than 1/6 wave p-v of higher-order comal with zero primary coma, and to about 1/30 wave p-v of balanced secondary coma. That far off, it is only a small fraction of the system's astigmatism, but enough to give an asymmetric (comatic) shape to the astigmatic blur.

 

Vla


Edited by wh48gs, 20 August 2014 - 06:14 PM.


#45 RodgerHouTex

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Posted 21 August 2014 - 08:29 AM

I did not say that Celestron had a coma corrected design prior to the Edge scopes.  Please reread my posts.  The "optimized" SCT that is analyzed in "Telescope Optics" and which has an aspheric secondary is NOT aplanatic.  Please refer to the ray trace diagrams.

 

I also said I believe Celestron has used and is using the "optimized" design to reduce coma because in the 5 Celestron SCTs I own (all non Edge scopes) the coma visible is less than that in a f/5 Newt.

 

To eliminate coma you need to go the Ritchey-Chretien route and then you get large amounts of astigmatism.



#46 gnowellsct

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Posted 21 August 2014 - 05:06 PM

WE had test results for posted for a C14 secondary on the C14 group that indicated it was 1/8th wave spherical.  GN



#47 RodgerHouTex

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Posted 21 August 2014 - 08:52 PM

All I can say is there are people who used to work at Celestron that say the secondaries were aspherized, and Celestron literature has said for years that they do it.  So I don't know how to respond to your comment.

 

How was the test performed?  Was the primary a sphere?  Maybe the one sample you tested was missed somehow.  What kind of images did it produce?  Was there coma equal to an f/5 Newt?

 

I don't think a single data point invalidates what Celestron and the people who worked there have said.  In addition to the fact that the 5 Celestron SCT OTAs I have do not have coma equal to an f/5 Newt.


Edited by RodgerHouTex, 21 August 2014 - 10:22 PM.


#48 wh48gs

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Posted 22 August 2014 - 07:21 AM

I did not say that Celestron had a coma corrected design prior to the Edge scopes.  Please reread my posts.  The "optimized" SCT that is analyzed in "Telescope Optics" and which has an aspheric secondary is NOT aplanatic.  Please refer to the ray trace diagrams.

 

I also said I believe Celestron has used and is using the "optimized" design to reduce coma because in the 5 Celestron SCTs I own (all non Edge scopes) the coma visible is less than that in a f/5 Newt.

 

To eliminate coma you need to go the Ritchey-Chretien route and then you get large amounts of astigmatism.

 

It would be helpful if you'd qote what you are responding to.

 

Perhaps, it is you who should read more carefully. You did say - as I already quoted - that you don't doubt that Celestrons units were and are so called (by Rutten and Venrooij) "optimized" SCT. What you are calling "optimized" SCT *is* corrected for coma for all practical purposes. The 1/30 p-v 20mm off axis design coma in an 8-inch SCT corresponds to 1/170 wave RMS, or 0.999 Strehl. What is there left to correct?

 

The blur shown in the Rutten/Venrooij at 20mm off axis is barely larger than the Airy disc, and it is - as I already stated - mostly astigmatism. Specifically, it is 1/3 wave p-v or 1/15 wave RMS - more than 10 times the RMS error of coma. But even a slight amount of coma readily affects the shape of astigmatic blur, giving to it "comatic" form.

 

The statement in the Rutten/Venrooij's book that the linear-field coma of the spherical-mirrors SCT "approximately" compares to that of f/5 paraboloid is either not accurate, or referring to the nominal blur size, not the wavefront error. Raytrace gives 3.4 waves p-v at 20mm off axis in an 8-inch SCT. (it includes about 1/2 wave p-v of astigmatism - which is about 2/3 higher with spherical secondary -  but its effect on the nominal compounded error is negligible, since it is proportional to the error squared). That corresponds to the coma wavefront error in a paraboloid slightly slower than f/6 (from W=h/48F^3, where "h" is the point height in the image plane and F the mirror focal ratio).

 

In the very book: at 20mm off axis the blur length is around 18mm in the SCT, and about 10mm in a paraboloid, at the same linear scale (both blurs are affected by astigmatism, but coma dominates). Since the Airy disc at f/6 is 60% of the size at f/10, the corresponding wavefront errors are within less than 10% difference. Since the linear coma blur scales with the third power of focal ratio, an f/5 paraboloid would have it around 17mm. So, while the don't say it specifically - which may be misleading - their comparison is probably in the context of the linear blur size, not the wavefront error (it is the latter that matters for visual observing).

 

Vla


Edited by wh48gs, 22 August 2014 - 07:23 AM.


#49 RodgerHouTex

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Posted 22 August 2014 - 09:53 AM

I was responding to the post directly above mine so I didn't feel the need to "quote".

 

I disagree with most of what you say and think you are trying to confuse the issue.

 

In "Telescope Optics" by Rutten and VanVerooj, Fig. 9.3 in the book shows the terrible coma for the all spherical design which anyone could easily see if they looked.  The comatic images just 10mm off axis are 10 times the Airy disk diameter for both the curved focal surface and the flat focal surface.  They are slightly worse than the spot plots for a f/6 parabolic mirror in Fig. 5.6.  So the statement that the all spherical SCT design has coma equivalent to a f/5 Newt. is accurate.

 

For the spot diagrams of the "optimized" SCT design with an aspherized secondary in Fig. 9.5, if the manufacturer is off just plus or minus 2% on the power of the corrector plate the coma is easily visible at 20 mm off axis as the spot plot is 4 times the Airy disk.  If the manufacturer makes a "perfect" optic(good luck with that) the spot plots are much better, but only on the "curved" focal plane.  The flat focal plane will of course be worse.

 

And in response to the OPs question, I believe they are.


Edited by RodgerHouTex, 22 August 2014 - 09:54 AM.


#50 GlennLeDrew

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Posted 22 August 2014 - 01:34 PM

My feeling, after all's considered, is that the touching up is restricted to cleaning up an already largely corrected wavefront, and will come nowhere near to being sufficient to lessen coma. The design as is suffers coma, and this is accepted by the maker.








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