316 replies to this topic

[davidc135]

The question of whether the secondaries of non ACF scts have always been left spherical or had their figures tweaked or were aspherised to remove coma hasn't been settled and is still of interest if recent posts are anything to go by.

 

Their figures could be tested by interference with a test plate of matching curvature or by Hindle sphere but these are complicated to make and there are almost no accounts with photos that I've seen. Robert Piekel has a few that are ambiguous.

A non null Gaviola type test can be easily set with the help of an f4 objective and a beamsplitter cube (eg mine from Surplus Shed cost $25).

 

This simple test sets out to detect significant asphericity in the secondary, not to measure it accurately. I should think this could be done with a Bath interferometer in monochromatic light and DFTFringe. Perhaps also with Maciek's Ronchi software.

 

I recently came by a 1980s C8 and a decent Ronchi grating and so decided to compare its secondary's figure with those from a Dynamax DX-8 and 2003 8'' Meade.

 

The testing set up is reproduced in post 73 above, this time using a good quality Dynamax finder objective of 2'' aperture and 7.85'' f.l. Its longer conjugate is reduced from infinity to around 28'' such that its shorter conjugate is coincident with the centre of curvature of the secondary being tested.

 

The rays from the light source are returned after reflection off the secondary or, in the alternative configuration with the secondary removed, after reflection off the plane placed at the secondary's CoC.

 

In the setup, the testing objective shows considerable under-correction that is common to both configurations and so any difference detected must be down to asphericty of the 2ndry.

 

The background under-correction will vary with the curvature of the 2ndry being tested.

 

First up the DX-8. RoC is 265mm. Long conjugate=711mm

 

objective alone

 

with secondary

 

Looks the same to me other than the grating position being marginally different.

 

David

Edited by davidc135, 16 September 2023 - 07:12 PM.

[davidc135]

2003 Meade.  RoC 255mm. Long conjugate=762mm

 

With secondary

 

 

Without secondary

 

 

The secondary shows a zone out towards the edge which may just be down to so so quality. The Meade's finder objective similarly wasn't the best, not even being centered. Ironically and judging by only one sample, Criterion sourced high quality components from Japan although the OTA as a whole is a let down.

Meanwhile the Meade is a fine scope.

 

David

Edited by davidc135, 16 September 2023 - 07:10 PM.

[SandyHouTex]

Over the decades, and from a variety of sources, I have got the strong impression that the 'classic' Celestron SCT has spherical mirrors, the design accepting the presence of coma. Any asphericity applied to the secondary, if required at all, was minor, and only to locally correct for zonal errors imparted mostly by the corrector. Any such small departures from the nominally spherical surface would have essentially no effect on coma.

 

After all, and as I understand it, the corrective action was taken to deliver a desired degree of spherical correction in the AXIAL CONDITION ONLY. And, again, upon a design accepting of coma.

With all the evidence presented in this thread, I'm surprised you would still hold to this obviously incorrect assumption.

[davidc135]

With all the evidence presented in this thread, I'm surprised you would still hold to this obviously incorrect assumption.

Do you think that GlenLeDrew's opinion is incorrect? The two test results above are in line with his conclusion.

 

David

[davidc135]

Tomorrow I'll put the C8 secondary on the bench and we'll see how it looks.  David

[davidc135]

The last of the trio, the C8, RoC 245mm, long conjugate = 863mm

 

Without 2ndry

 

 

With 2ndry

 

 

Here the 2ndry seems to add some under-correction but is it enough to affect coma?

 

David

Edited by davidc135, 16 September 2023 - 07:30 PM.

[davidc135]

Another measure of SA is the knife edge travel from centre to edge. Instead of judging zones as in the Foucault tesst I used the poor man's caustic which measures spots, either dark, emerging ones on one side or light, about to be extinguished ones on the opposite side.

 

The set up isn't very accurate especially as it's hard to judge the exact point of emergence just within the edge but the same limitations apply to both configurations. Again, I'm looking for large differences caused by the secondary.

 

The k.e.t can be converted into waves of SA as the focal ratio is known. For the C8 it is f17 and the k.e.t is 80mm. To convert to Foucault zone figures that is divided by 3 to get the double pass aberration with a quick formula:

 

80/((17/10)² x 0.87 x 3 x 4) = 2.65 waves under-correction or half of that per pass.

 

I only found around 5mm difference between the two configurations corresponding to less than 1/4 wave contribution from the C8 secondary, very likely due to a bit of tweaking from a Celestron optician.

 

Below are images of the poor man's caustic test at edge and centre for the DX-8. The edge oval would be 1/4 of that size when first seen. The centre image is the same as in the Foucault

 

 

 

David

Edited by davidc135, 17 September 2023 - 05:49 AM.

[davidc135]

Test limitations and errors. 

 

It's not a null test. Background under-correction increases rapidly as the secondary's RoC increases. However, the slow speed from f14 to f17 of these tests enable small differences to stand out. I expect a movable component such as a meniscus could be inserted into the train to cancel (much) of the objective's under-correction but that would defeat the simplicity.

 

The strong chromatic error could be eliminated using a monochromatic source.

 

Tip/tilt of the plane mirror caused distortion.

 

The longer conjugate was probably under estimated by around 25mm+.

 

Difficulty in precisely measuring k.e.t as mentioned above.

 

Which all would matter if you needed to be precise. But if the 2ndry conic needed to correct coma is say -0.88, that's over 2.5 waves of under-correction being added, on top of what the objective causes. The C8 2ndry seems to manage only 1/10 of that.

 

There's no way any of the above three secondaries are part of a coma free sct.

 

But that doesn't prove that a C14, for example, would be the same. I suspect it is but it would need to be shown. IIRC, in his book Robert Piekel only shows one 2ndry interferogram with significant asphericity and that was from a C22, not a mass market product. Over 50 years and virtually no hard evidence regarding the secondary but correct me if I'm wrong.

 

David

Edited by davidc135, 17 September 2023 - 09:22 AM.

[SandyHouTex]

The last of the trio, the C8, RoC 245mm, long conjugate = 863mm

 

Without 2ndry

 

P9172338b.JPG

 

With 2ndry

 

P9172337b.JPG

 

Here the 2ndry seems to add some under-correction but is it enough to affect coma?

 

David

I have no idea what I'm looking at, nor what it shows.  Please explain.

 

I know it's difficult to analyze a convex optical surface.  I've always used a  identical concave spherical surface to analyze them.  I think you can also use a larger convex lens somehow, but since I was grinding and polishing the convex surface, I had a ready made test article.

Edited by SandyHouTex, 17 September 2023 - 11:20 AM.

[SandyHouTex]

the venerable company 7 has a few words to say about this:

 

http://www.company7....ts/schcass.html

 

It is their belief that the secondary was worked on to improve the figure, not so much coma per se.

 

Now all this was written before the move to Chinese optics, at which time C7 washed their hands of the brand altogether.

So here are some quotes from your link:

 

"Fine corrections are made on the secondary mirror that smooths the optical figure. Hand figuring of an optical system is a complex optical procedure done on a commercial basis soley by Celestron. The art of hand figuring takes time but the results are easily evidenced by the surprisingy good resultant astrophotographs and viewing obtained. Company Seven does test and inspect every system that we recieve, and not every telescope meets our requirements. However, one of the better C-8 systems we delivered tested on an interfereometer at 1/4.6 wave Peak to Valley (1/22 wave RMS) with a Strehl Ratio of 92.4; this was tested at 543.3 nanometers (more demanding than testing at common helium neon lines)."

 

I don't think you would be able to get "...surprisingly good resultant astrophotographs..." if there was significant coma equal to a f/4 Newtonian.

 

Here's one more quote, "Optical testing is performed on double-pass auto-collimators in a temperature controlled environment. This ensures reasonably high quality on a fairly consistent basis. Celestron is working towards providing the most stringent optical fabrication, testing and quality control in the mass production SCT industry."

 

Interestingly, the statement you make about coma is not in there.

[SandyHouTex]

 The weight of evidence is on the asphere. 

    1) Tom Johnson and stated this in a number of books

    2) In a number of a Celestron's own  literature it is states

    3) Each optical set was  hand figured on the secondary so why go through the trouble of figuring a spherical surface that would not correct for coma when you can design the system to correct for coma and aspherize the secondary ? There is no advantage to going through the process of figuring a spherical surface that doesn't reduce coma.  The cost is the same from a production stand point and the results is inferior from an optical stand point. 

    4)  Johnson was a smart guy, especially when it comes to figuring out  how to make the corrector plate. So he knew SCT optics very well. Both himself and Celestron literature reference a 1962 Sky and Telescope article that examined a number optical designs of the cassegrain family for astrophotography. That was the article that inspired him that a SCT was the best system for a commercial telescope aimed at astrophotography In that article it showed that a SCT with an aspheric secondary produced the best spot diagram. Why would he ignore this fact   if the goal was to produce a commercial telescope that one of the major selling point was for astrophotography ? 

    

      There is no evidence that the secondary is spherical that I can find from any statement from Celestron but  just the opposite. The spherical secondary comes from people just saying it over and over and making assumptions on the design and based on  a G power factor on the corrector of less than 1 when Johnson stated that  they made the correctors  power greater then 1. 

 

   Here is a section of Celestron's literature stating they "hyberbolize" the secondary.  Why use the term "hyberbolize" if they were just touching up an spherical  surface?  It also reference the 1962 Sky Tele article of why a SCT with a aspheric secondary produces the best spot diagram.  I can dig up more statements that  they aspherized the secondary and that they were doing it to reduce coma. 

 

      By the way Meade was doing the same thing but instead of hand figuring the secondaries to match primary and corrector  they had them aspherized in Japan and they would then swap them in and out until they found one that the correction would meet their spec.   They also understood the need for an aspheric secondary to reduce coma and to  compete with Celestron.  Meade had the aspheric secondaries made in Japan to reduce labor cost since from a production stand point that  is one the most costly factors in building the scope. 

 

 

                - Dave 

celestron literature.jpg

It appears that DAVIDG has provided plenty of evidence for an aspheric, hyperboloidal secondary.

[davidc135]

I have no idea what I'm looking at, nor what it shows.  Please explain.

 

I know it's difficult to analyze a convex optical surface.  I've always used a  identical concave spherical surface to analyze them.  I think you can also use a larger convex lens somehow, but since I was grinding and polishing the convex surface, I had a ready made test article.

Ah, Ok, there is an explanation back in the thread but buried some time ago on post46.

 

 

Here, the 50mm f4 objective C plus 2ndry D returns light from the source A to a focus also at A. C to A is the long conjugate. As this is no longer at infinity spherical under-correction shows in the Ronchigram together with whatever contribution from the 2ndry D. This shown in image ....37b.

 

The 2ndry is then removed allowing light to go to the shorter conjugate (or 2ndry RoC) where it is reflected off the plane mirror B. The position of B is adjusted so that the returning rays pass through the system and are focused again exactly at A.

 

A second Ronchigram is taken and shows the under-correction only of the objective as the plane can't contribute any errors. This is Ronchigram ....38b

 

Any differences between the two Ronchigrams are due to deviations of the 2ndry figure from spherical as a sphere also won't contribute any errors.

 

As any differences in these tests are so small it's not possible for any of the three 2ndries to correct for coma in the typical sct.

 

David

Edited by davidc135, 17 September 2023 - 12:20 PM.

[davidc135]

So here are some quotes from your link:

 

"Fine corrections are made on the secondary mirror that smooths the optical figure. Hand figuring of an optical system is a complex optical procedure done on a commercial basis soley by Celestron. The art of hand figuring takes time but the results are easily evidenced by the surprisingy good resultant astrophotographs and viewing obtained. Company Seven does test and inspect every system that we recieve, and not every telescope meets our requirements. However, one of the better C-8 systems we delivered tested on an interfereometer at 1/4.6 wave Peak to Valley (1/22 wave RMS) with a Strehl Ratio of 92.4; this was tested at 543.3 nanometers (more demanding than testing at common helium neon lines)."

 

I don't think you would be able to get "...surprisingly good resultant astrophotographs..." if there was significant coma equal to a f/4 Newtonian.

 

Here's one more quote, "Optical testing is performed on double-pass auto-collimators in a temperature controlled environment. This ensures reasonably high quality on a fairly consistent basis. Celestron is working towards providing the most stringent optical fabrication, testing and quality control in the mass production SCT industry."

 

Interestingly, the statement you make about coma is not in there.

Celestron have made statements that have been repeated by Piekel and Company 7. Celestron are going to say that they do everything necessary to make their optics as good as possible. They're not going to say that they do the bare minimum to get within spec. Steve Dodds mentioned that an ex Celestron optician spent seconds or a couple of minutes actually figuring which sounds like bare minimum.

 

If Celestron or others suggest that the 2ndries have been hand figured to produce a coma free scope and my tests show that they clearly haven't, isn't the matter shown to be a myth? Unless I've made a mistake of course.

 

I think the all spherical sct has the coma of an f5 Newt. Wallis and Provin show some fine photos taken with an f4.5 Newt.

 

David

Edited by davidc135, 17 September 2023 - 02:34 PM.

[davidc135]

It appears that DAVIDG has provided plenty of evidence for an aspheric, hyperboloidal secondary.

The evidence is weak and mostly goes back to claims from the manufacturer who were trying to sell telescopes.

It makes good business sense not to over engineer a fledgling design. It was tough enough to get high performance on axis let alone worrying about what's optimum at the edge of the field.

 

The photographic angle has been over stated. Most buyers wanted nice views of the night sky and wouldn't have the photographic skill to see the difference between the all spherical sct and the aplanat, after an hours exposure. Celestron might as well put out the hype as the customer wasn't able to tell anyway.

If they were all coma free, what was so advanced about Meade's ACF when it came out?

 

The secondary figure wouldn't be a hyperbola even if you are right. It's prolate conic is less extreme than -1. People used the term hyperbolising casually to imply a strong correction even if there was only some light touching up.

 

David

Edited by davidc135, 17 September 2023 - 01:22 PM.

[Starman1]

1) last I heard, f/10 SCTs have about the same coma as an f/6 newtonian.  Seems about right to my eye after 11 years with an 8" SCT.

And, they have curved focal planes--a field flattener helped a LOT in my 8" SCT.

2) I think Celestron used to hand figure the secondaries on SCTs back before they were owned by Synta.

After that, I think they simply used the "rotate the secondary to get the best result" method.  That was explained by Celestron to me when I worked at Scope City.

A lot of descriptions of their processes relate to how it was done pre-1995, and they haven't been done that way since.

3) The ACF was the first commercial coma free SCT.  The Edge went one better with field flattening accompanying the coma correction.

[SandyHouTex]

Ah, Ok, there is an explanation back in the thread but buried some time ago on post46.

 

P6131857 (2).JPG

 

Here, the 50mm f4 objective C plus 2ndry D returns light from the source A to a focus also at A. C to A is the long conjugate. As this is no longer at infinity spherical under-correction shows in the Ronchigram together with whatever contribution from the 2ndry D. This shown in image ....37b.

 

The 2ndry is then removed allowing light to go to the shorter conjugate (or 2ndry RoC) where it is reflected off the plane mirror B. The position of B is adjusted so that the returning rays pass through the system and are focused again exactly at A.

 

A second Ronchigram is taken and shows the under-correction only of the objective as the plane can't contribute any errors. This is Ronchigram ....38b

 

Any differences between the two Ronchigrams are due to deviations of the 2ndry figure from spherical as a sphere also won't contribute any errors.

 

As any differences in these tests are so small it's not possible for any of the three 2ndries to correct for coma in the typical sct.

 

David

And yet there is physical evidence to the contrary.

[SandyHouTex]

Celestron have made statements that have been repeated by Piekel and Company 7. Celestron are going to say that they do everything necessary to make their optics as good as possible. They're not going to say that they do the bare minimum to get within spec. Steve Dodds mentioned that an ex Celestron optician spent seconds or a couple of minutes actually figuring which sounds like bare minimum.

 

If Celestron or others suggest that the 2ndries have been hand figured to produce a coma free scope and my tests show that they clearly haven't, isn't the matter shown to be a myth? Unless I've made a mistake of course.

 

I think the all spherical sct has the coma of an f5 Newt. Wallis and Provin show some fine photos taken with an f4.5 Newt.

 

David

As I said, there is physical evidence that Celestron said the secondaries are hyperboloidal.  The founder of Celestron said it.  That's called prima facia evidence.

 

You of course, are free to believe whatever you want.

[SandyHouTex]

1) last I heard, f/10 SCTs have about the same coma as an f/6 newtonian.  Seems about right to my eye after 11 years with an 8" SCT.

And, they have curved focal planes--a field flattener helped a LOT in my 8" SCT.

2) I think Celestron used to hand figure the secondaries on SCTs back before they were owned by Synta.

After that, I think they simply used the "rotate the secondary to get the best result" method.  That was explained by Celestron to me when I worked at Scope City.

A lot of descriptions of their processes relate to how it was done pre-1995, and they haven't been done that way since.

3) The ACF was the first commercial coma free SCT.  The Edge went one better with field flattening accompanying the coma correction.

And the ACFs had bad astigmatism and field curvature.

 

We've also neglected the discussion of the Edge SCTs.  They are absolutely corrected for coma, astigmatism, and field curvature.  With the correcting lenses in them, it's immaterial whether the secondary is aspherized.

Edited by SandyHouTex, 17 September 2023 - 03:33 PM.

[davidc135]

As I said, there is physical evidence that Celestron said the secondaries are hyperboloidal.  The founder of Celestron said it.  That's called prima facia evidence.

 

You of course, are free to believe whatever you want.

That's not physical evidence. That's Celestron and Tom Johnson with his salesman's hat on putting a good gloss on the products that they want to sell.

 

If you accept that then you are saying that my C8 secondary has a hyperboloid figure. My test directly contradicts that. Some may feel that I've made a mistake and these scts are coma free after all. Fair enough but if they don't come up with a practical demonstration to support their case or show how my test got it wrong how convincing is their argument?

 

It seems a good idea to put all claims to the test, including mine.

 

And the ACFs had bad astigmatism and field curvature.

 

We've also neglected the discussion of the Edge SCTs.  They are absolutely corrected for coma, astigmatism, and field curvature.  With the correcting lenses in them, it's immaterial whether the secondary is aspherized.

How are the ACFs different from the pre Edge Celestrons as you see them? If you vary the secondary figure from a sphere to strongly prolate and adjust g of the corrector plate then stronger field curvature etc inevitably follows. 

 

David

Edited by davidc135, 17 September 2023 - 05:12 PM.

[jimhoward999]

In the broadly distributed Celestron Edge HD white paper the  "Celestron engineering team" (the author of the paper) writes the following:

 

"Our classic SCT has three components : a spherical primary mirror, and spherical secondary mirror and a corrector plate with a polynomial curve"

 

They also write "in the EdgeHD, the primary and secondary retain smooth spherical surfaces, and corrector plate remains unchanged"

 

So the Celestron design engineering team says they are all spheres and has committed to that statement to writing.   

[davidc135]

In the broadly distributed Celestron Edge HD white paper the  "Celestron engineering team" (the author of the paper) writes the following:

 

"Our classic SCT has three components : a spherical primary mirror, and spherical secondary mirror and a corrector plate with a polynomial curve"

 

They also write "in the EdgeHD, the primary and secondary retain smooth spherical surfaces, and corrector plate remains unchanged"

 

So the Celestron design engineering team says they are all spheres and has committed to that statement to writing.   

That's settles it    David

Edited by davidc135, 18 September 2023 - 01:10 AM.

[Starman1]

How are the ACFs different from the pre Edge Celestrons as you see them? If you vary the secondary figure from a sphere to strongly prolate and adjust g of the corrector plate then stronger field curvature etc inevitably follows. 

 

David

The ACF has a different curve on the corrector plate.  The mirrors were unchanged.

Edited by Starman1, 17 September 2023 - 06:36 PM.

[starspangled]

FYI    Theres a history of Celestron CD with many PDF chapters , some containing long phone interviews with ex Celestron optician from the early 1970's  and a very long one with Tom Johnston .

 

 Before the use of interferometers at Celestron , SCT's were null tested against giant 40"aperture collimator with a laser illuminated steel ball at focus  . There were a number of stations in front where optical sets could be simultaneously  aimed into the collimator. Up to seven opticians would be aspherising the  secondaries to matching corrector and primaries all sharing the testing work station .

 

A ronchi grating was used at focus and each optician had a library of cut out  paper masks which were hot pressed into the secondary polishers to deal with whatever ronchi shapes were apparent  , Work proceeded until the lines were straight .    At the peak of popularity of C14's  circa 1974 from memory they were fulfilling orders for 50 C14's per month .

 

The astro shop I used to work for built a collimator for testing and I still have a typed explanation and hand drawings by Tom Johnston explaining the construction and use of a large collimator , complete with Ronchi sketches , I believe it was in response to user enquiries about the C14 quality , which I believe really stemmed from unreasonable expectations of what a 14" scope could do in a day when a 10"scope was considered very large .

Edited by starspangled, 18 September 2023 - 08:20 PM.

[davidc135]

The ACF has a different curve on the corrector plate.  The mirrors were unchanged.

The ACF secondary figure has to be changed to strongly aspheric. 

 

Something like this. But interesting to check it out.

 

 

David

Edited by davidc135, 18 September 2023 - 01:34 AM.

[SandyHouTex]

That's not physical evidence. That's Celestron and Tom Johnson with his salesman's hat on putting a good gloss on the products that they want to sell.

 

If you accept that then you are saying that my C8 secondary has a hyperboloid figure. My test directly contradicts that. Some may feel that I've made a mistake and these scts are coma free after all. Fair enough but if they don't come up with a practical demonstration to support their case or show how my test got it wrong how convincing is their argument?

 

It seems a good idea to put all claims to the test, including mine.

 

How are the ACFs different from the pre Edge Celestrons as you see them? If you vary the secondary figure from a sphere to strongly prolate and adjust g of the corrector plate then stronger field curvature etc inevitably follows. 

 

David

So clearly there is nothing that will convince you that the Celestron SCT secondaries were aspherized, even the designer  of the scope and owner of the company that produced them, so there is no point in continuing this discussion.

 

ACFs are supposed to mimic the performance of a Ritchey-Chretien telescope.  Ritchey-Chretien scopes have bad astigmatism and field curvature,  Celestrons are completely different design.  A compact Schmidt-Cassegrain.  If you can get a copy of "Telescope Optics" by Rutten and ven Venrooij.  It's all explained in there.

 

I'm out.

Edited by SandyHouTex, 18 September 2023 - 05:30 PM.