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Celestron sct secondaries, spheres or aspheres?

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#1 davidc135

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Posted 10 June 2022 - 07:54 AM

The question of whether Celestron scts were corrected for coma sometimes comes up for discussion. To do so the 2ndry would have to be figured to a prolate profile, the resulting under-correction being balanced by a more strongly aspheric corrector plate.

 

I've made the equivalent Primary conic constant of an 8'' Dall Kirkham in an F/2/10 arrangement with a back focus of 8'' of 0.712 the relative strength of the corrector when the 2ndry is spherical.

In contrast Rutten and van Venrooij give 0.834 as the relative strength of the corrector in their aplanat version having an aspherical 2ndry.

In both cases the remaining correction is achieved by the 2ndry.

 

The relative strengths of the corrector affect their positions in the null setup below.

A light source is placed at the focus S of the primary with the beam being transmitted through the plate before and after reflection from the primary and is null tested at the focus of 2nd scope.

 

The stronger plate of the aplanat will achieve a null closer to S than the plate of the all spherical system. For the 8'' scope I get 0.67 focal length for the aplanat against 0.8 for all spherical.

 

For the all spherical case the calculation is:

 

0.712 of total correction for corrector and 0.288 for 2ndry.

0.288/0.712 = 0.404.

The fourth route of 0.404 = 0.8 f.l from S  (Spherical aberration varying as the 4th power of the radius)

 

Assuming the same relative corrector plate position, a 5 fold amplification and the same relative back focus I think these numbers stay the same whatever the aperture. But the back focus gets shorter relatively as the aperture increases which slightly reduces both figures.

A corrector position between these extremes would suggest a part reduction of coma.

 

P6101829.JPG

 

David


Edited by davidc135, 10 June 2022 - 08:07 AM.


#2 davidc135

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Posted 10 June 2022 - 08:39 AM

I'd forgotten that the plate in the path of the strongly diverging beam will cause aberration which needs to be factored in. Perhaps the corrector can be assumed to be plane-parallel for this purpose.

 

David



#3 Bob Campbell

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Posted 10 June 2022 - 08:42 AM

The question of whether Celestron scts were corrected for coma sometimes comes up for discussion. To do so the 2ndry would have to be figured to a prolate profile, the resulting under-correction being balanced by a more strongly aspheric corrector plate.

 

I've made the equivalent Primary conic constant of an 8'' Dall Kirkham in an F/2/10 arrangement with a back focus of 8'' of 0.712 the relative strength of the corrector when the 2ndry is spherical.

In contrast Rutten and van Venrooij give 0.834 as the relative strength of the corrector in their aplanat version having an aspherical 2ndry.

In both cases the remaining correction is achieved by the 2ndry.

 

The relative strengths of the corrector affect their positions in the null setup below.

A light source is placed at the focus S of the primary with the beam being transmitted through the plate before and after reflection from the primary and is null tested at the focus of 2nd scope.

 

The stronger plate of the aplanat will achieve a null closer to S than the plate of the all spherical system. For the 8'' scope I get 0.67 focal length for the aplanat against 0.8 for all spherical.

 

For the all spherical case the calculation is:

 

0.712 of total correction for corrector and 0.288 for 2ndry.

0.288/0.712 = 0.404.

The fourth route of 0.404 = 0.8 f.l from S  (Spherical aberration varying as the 4th power of the radius)

 

Assuming the same relative corrector plate position, a 5 fold amplification and the same relative back focus I think these numbers stay the same whatever the aperture. But the back focus gets shorter relatively as the aperture increases which slightly reduces both figures.

A corrector position between these extremes would suggest a part reduction of coma.

 

attachicon.gifP6101829.JPG

 

David

Hi

 

I have had this same question.

 

Here is a thread from 2014 that delves into this issue in some detail.

 

https://www.cloudyni...on-secondaries/

 

My takeaway is:

1) the Meade ACF is clearly aspheric, as kind of expected

2) *some* celestron secondaries were mildly aspheric, but only corrected partially for coma, the deviations were in most cases for other reasons.

 

Hope this helps in some small way

 

Bob


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#4 davidc135

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Posted 10 June 2022 - 09:19 AM

Hi

 

I have had this same question.

 

Here is a thread from 2014 that delves into this issue in some detail.

 

https://www.cloudyni...on-secondaries/

 

My takeaway is:

1) the Meade ACF is clearly aspheric, as kind of expected

2) *some* celestron secondaries were mildly aspheric, but only corrected partially for coma, the deviations were in most cases for other reasons.

 

Hope this helps in some small way

 

Bob

Yes, I agree.  David



#5 DAVIDG

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Posted 10 June 2022 - 09:39 AM

 All the math for calculating a Schmidt Cass is in the back of Telescope Optics. The math shows what the power of the corrector needs to be for all spherical system that will not be corrected for coma and the position of the corrector and secondary. It then  shows that to correct for coma  that either the secondary or the primary is aspherized  and  calculates the  position of the secondary and corrector  for these designs.

     Having testing secondaries  out of the orange tube unit they have been aspherized  and Celestron own literature states that aspherized the secondary. 

 

   Here is snap shot of a  spread sheet  that a friend and I put together that calculates all the perimeters for many different versions of  an 8' f/10 Cass including a SCT with a spherical primary and aspheric secondary that corrects for coma using the math in the back of telescope optics.  The column for the SCT with the spherical primary shows the secondary needs an aspheric correction on -0.45  and the power of the Schmidt corrector 'g' needs to be 0.77. 

 

   The bottom line is Celestron knew that they needed  to correct for coma  to some level since their main selling point was for astrophotography. To correct for coma you can move the corrector to the radius of the  primary but that results in longer tube, aspherize the primary or aspherize the secondary. Since the secondary is  the smallest optical surface from a  production stand point that is the surface to work on. 

  

 

                         - Dave 

 

       Cass designs.JPG      


Edited by DAVIDG, 10 June 2022 - 12:23 PM.

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#6 davidc135

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Posted 10 June 2022 - 01:20 PM

 All the math for calculating a Schmidt Cass is in the back of Telescope Optics. The math shows what the power of the corrector needs to be for all spherical system that will not be corrected for coma and the position of the corrector and secondary. It then  shows that to correct for coma  that either the secondary or the primary is aspherized  and  calculates the  position of the secondary and corrector  for these designs.

     Having testing secondaries  out of the orange tube unit they have been aspherized  and Celestron own literature states that aspherized the secondary. 

 

   Here is snap shot of a  spread sheet  that a friend and I put together that calculates all the perimeters for many different versions of  an 8' f/10 Cass including a SCT with a spherical primary and aspheric secondary that corrects for coma using the math in the back of telescope optics.  The column for the SCT with the spherical primary shows the secondary needs an aspheric correction on -0.45  and the power of the Schmidt corrector 'g' needs to be 0.77. 

 

   The bottom line is Celestron knew that they needed  to correct for coma  to some level since their main selling point was for astrophotography. To correct for coma you can move the corrector to the radius of the  primary but that results in longer tube, aspherize the primary or aspherize the secondary. Since the secondary is  the smallest optical surface from a  production stand point that is the surface to work on. 

  

 

                         - Dave 

 

       attachicon.gifCass designs.JPG      

You've sent me to the back of Telescope Optics to find the very interesting graph fig.21.4 on page 274 which shows the inter-relationships between mirror deformations, corrector positions and corrector plate g.

 

The graph shows that your figures of 2ndry cc of -o.45 and corrector g of 0.77 are only aplanatic in a non compact form of sct. But coma would be usefully reduced in the commercial scope. And a test that could show that would be a fun thing. I'm still a doubting Thomas.

 

David


Edited by davidc135, 10 June 2022 - 01:28 PM.


#7 DAVIDG

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Posted 10 June 2022 - 01:56 PM

 David,

    I'll check the graph tonight but my calculation for 8" f/10 SCT with an aspheric secondary has the secondary to primary spacing at 12.47"  so the  length of the OTA would be in range of 14" or so. That is close to what a commercial SCT would be.  The equations we used are in  the back of Telescope Optics in the  chapter of how to design systems and  are stated to determine the conic for the secondary, corrector power G  and the spacing to correct from coma.

 

"  I'm still a doubting Thomas."  What do mean by this ?

 

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#8 GlennLeDrew

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Posted 10 June 2022 - 03:15 PM

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.


Edited by GlennLeDrew, 10 June 2022 - 03:17 PM.

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#9 davidc135

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Posted 10 June 2022 - 03:35 PM

 David,

    I'll check the graph tonight but my calculation for 8" f/10 SCT with an aspheric secondary has the secondary to primary spacing at 12.47"  so the  length of the OTA would be in range of 14" or so. That is close to what a commercial SCT would be.  The equations we used are in  the back of Telescope Optics in the  chapter of how to design systems and  are stated to determine the conic for the secondary, corrector power G  and the spacing to correct from coma.

 

"  I'm still a doubting Thomas."  What do mean by this ?

 

                  - Dave 

I'm doubting the asphericity of the 2ndries but certainly I could be wrong. At any rate they are worth an extra check.

 

It's the corrector to primary/ secondary to primary ratio (D/R) that looks to be aound 1.35 as against less than 1.1 in the typical sct. (For your figures to satisfy the aplanat condition, IMO)

 

The diverging beam impacting the corrector in the test adds some over-correction which would shorten the required separation from the focus in my test but I don't think it would make much difference.

 

David


Edited by davidc135, 10 June 2022 - 03:42 PM.


#10 DAVIDG

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Posted 10 June 2022 - 03:47 PM

 A  SCT design with spherical optics  has coma about equal to f/5 Newtonian of the same aperture.  If one just looks at the images that were taken at prime focus with a Celestron C-8 the coma is much less so they were doing something. Johnson stated that they were using a G factor great then one to flatten the field and aspherizing the secondary to reduce coma.  So you have to take this into account vs assuming that the design is what is in Telescope Optics and that is  what they were doing. These were smart guys and understood that the selling point was for astrophotography and needed  to design the optics to do that well and also make a profit. So what was the easiest to do ? Design the system to reduce coma by aspherizing the secondary and test and correct each system  as a set. The labor involved to touch up the secondary to remove spherical would take the same amount to also put the aspheric surface to correct for coma. 

 

                      - Dave  


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#11 davidc135

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Posted 10 June 2022 - 04:35 PM

Fair points but not definitive. It's taking a number of things on trust including the impressions that Johnson gave so another test that will give an answer is worthwhile.  David



#12 Jon Isaacs

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Posted 10 June 2022 - 08:01 PM

Meade builds coma free SCTs. Standard Celestron and Meade SCTs are not coma free. Starzona makes reducer correctors designed for standards SCTs.

 

Just how much coma do standard SCTs have? It seems that if Starzona can make a corrector, it must be consistent. How consistent is the aspherization of Celestron secondary's?

 

Jon


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#13 GlennLeDrew

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Posted 10 June 2022 - 11:57 PM

If Celestron did figure all their 'classic' SCTs so as to reduce coma in order to please the astrophotographers, it defies every precept of marketing that they should not have been trumpeting this fact to the heavens and beyond. Especially with competitors Criterion and Meade arriving on the scene soon enough, there would have been a drive to extoll every little difference. And an aspherized system would have been Item Number One, bar none. This would be given top billing and a big font in every ad.

 

As Jon highlights, the fact that the f/6.3 corrector, designed to cover the 35mm format and to reduce coma and flatten the field, performed pretty uniformly among the thousands of SCT specimens they were mated to over the decades is potent evidence of nominally spherical mirrors.


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#14 davidc135

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Posted 11 June 2022 - 02:07 AM

The weight of evidence to my mind is on the sphere. It would be an interesting experiment but not interesting enough to want to pull apart a C-8 unless it was already in bits.

 

An alternative bench test would be for the 2ndry to be placed at its RoC from the focus of an f/4 to f/5 paraboloid. Collimated light from a second scope would be reflected off the paraboloid, the 2ndry and again the paraboloid to return through the 1st scope to be tested at its focus. The aberrations from an aspherical 2ndry would be very obvious.

 

David


Edited by davidc135, 11 June 2022 - 02:10 AM.


#15 DAVIDG

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Posted 11 June 2022 - 09:00 AM

 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

 

 

 

 

 

                  

 


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#16 Jon Isaacs

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Posted 11 June 2022 - 09:16 AM

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 ?

 

 

The story I have heard time and time again was that Celestron hand figured the secondary to as final figuring to match the corrector-primary-secondary. Supposedly Meade did this by mixing and matching.

 

Here's the conundrum: if Celestron really did aspherize the secondary to correct for coma, why didn't they do a better job of it? And Meade would had to aspherize their secondary's because both Celestron and Meade SCTs have equivalent coma.

 

Jon


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#17 SandyHouTex

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Posted 11 June 2022 - 09:34 AM

The story I have heard time and time again was that Celestron hand figured the secondary to as final figuring to match the corrector-primary-secondary. Supposedly Meade did this by mixing and matching.

 

Here's the conundrum: if Celestron really did aspherize the secondary to correct for coma, why didn't they do a better job of it? And Meade would had to aspherize their secondary's because both Celestron and Meade SCTs have equivalent coma.

 

Jon

Do you have a link for your statement that "...both Celestron and Meade SCTs have equivalent coma."

 

I've never seen anyone do that comparison.



#18 DAVIDG

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Posted 11 June 2022 - 11:05 AM

The story I have heard time and time again was that Celestron hand figured the secondary to as final figuring to match the corrector-primary-secondary. Supposedly Meade did this by mixing and matching.

 

Here's the conundrum: if Celestron really did aspherize the secondary to correct for coma, why didn't they do a better job of it? And Meade would had to aspherize their secondary's because both Celestron and Meade SCTs have equivalent coma.

 

Jon

 For the original F/10 system used for 35mm photography  the coma correction was good for that application. Exposure times on 35mm film was many minutes so seeing , guiding issues and the swelling of the  star image on film reduced the visibility of coma. So coma didn't need to be perfectly corrected  but good for the size of 35mm frame but better then a design with a spherical secondary. Then people wanted to reduce exposure times so they designed the f 6.3 focal reducer. Which again was design for a 35mm frame.

    As time went on CCD showed  up. That greatly reduced exposure time so coma was now more of an  issue. At the same time  production cost went up and one of the major costs is the skilled manual labor to aspherize a surface. The "holy grail" in optical engineering is spherical surfaces since the production cost is less. So to reduce cost and also have a selling point both Meade and Celestron went with the "coma free designs".  Now they are using spherical optics and to correct for coma  and they added spherical lens to do that. The production cost is less for adding a couple of spherical lenses vs one hand figured aspheric surface.

  So they reduce costs  and have a selling point for astrophotography.  One has to look at  the design from  both the optical stand point and the business stand point.  You need to make a product that people want to buy and one that  can make a profit.  People are looking at this issue from just an optical stand point. 

     On the original Celestron design there is no advantage to having  a spherical secondary if your going to have someone manually figure it to null the system out. It is a no longer spherical.  The cost is the same if you design the system to aspherize  the secondary to  also reduce coma.  It makes sense optically and also from the cost bases since the time required is same hence the cost to produce is the same.

 

                  - Dave 



#19 davidc135

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Posted 11 June 2022 - 11:07 AM

 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 

attachicon.gifcelestron literature.jpg

Good stuff but did Johnson and Celestron actually do as they claimed? A test would reveal.

 

A simple, third test would be to use an independant (from scope) high power eyepiece on an off axis bright star. The star image would be in the ep's centre of field to avoid its own off axis aberrations and focusable to take out the effects of the scope's tight curvature of field.

 

David


Edited by davidc135, 11 June 2022 - 11:22 AM.


#20 davidc135

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Posted 11 June 2022 - 11:21 AM

In normal usage either visually or in taking photos not much difference would be seen between the all spherical and the aplanat forms as the faint outer fringe of the coma would mostly be imperceptible. Because of field curvature. It's only when a field flattener is used that the superiority of the aplanat is obvious. How many C8 customers also bought a field flattener?

 

David


Edited by davidc135, 11 June 2022 - 11:23 AM.


#21 MKV

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Posted 11 June 2022 - 11:57 AM

Good stuff but did Johnson and Celestron actually do as they claimed? A test would reveal.

There's a historical perspective to be taking into account here. The claim was made in 1962. Commercial manufacture of Celestron-Pacific SCTs didn't begin until 1964. And then it was introduced only in two size -- 20 and 22 inch models. Clearly, these were made for professional use, and chances are that they did "do as they say" for such customers. Soon thereafter, the production of these large SCTs stopped, and the Company dropped "Pacific" form tis name and continued only as "Clestron." Full-scale mass produced Celestrons as we know them didn't begin until 1970 when a new manager came onboard. Once they began making hundreds, or even thousands of OTA's it's very likely that the amount of aspherization adjusted not only to the volume demands but also to amateur clientele with more limited knowledge of optical quality and theory. The  emphasis clearly shifted to high volume production, and we know that time is money so it's a no-brainer it seems. Eventually, most amateur-oriented optics production adopted a more pragmatic warranty -- satisfaction (or your money back).


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#22 davidc135

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Posted 11 June 2022 - 12:23 PM

There's a historical perspective to be taking into account here. The claim was made in 1962. Commercial manufacture of Celestron-Pacific SCTs didn't begin until 1964. And then it was introduced only in two size -- 20 and 22 inch models. Clearly, these were made for professional use, and chances are that they did "do as they say" for such customers. Soon thereafter, the production of these large SCTs stopped, and the Company dropped "Pacific" form tis name and continued only as "Clestron." Full-scale mass produced Celestrons as we know them didn't begin until 1970 when a new manager came onboard. Once they began making hundreds, or even thousands of OTA's it's very likely that the amount of aspherization adjusted not only to the volume demands but also to amateur clientele with more limited knowledge of optical quality and theory. The  emphasis clearly shifted to high volume production, and we know that time is money so it's a no-brainer it seems. Eventually, most amateur-oriented optics production adopted a more pragmatic warranty -- satisfaction (or your money back).

That makes sense.  David



#23 GlennLeDrew

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Posted 11 June 2022 - 01:06 PM

DAVIDG,

That image of the text snippet you posted;  I've seen that in the magazine way back when. To me it reads exactly like marketing speak, with sufficient ambiguity to imply more than the truth, but to not cause legal repercussions. The description of the correction process implies the testing and figuring being done on axis. And the guarantee as stated can apply only on or near the axis in any event, for even a coma corrected system would fail in the outer field due to the residual aberrations.

 

A collimated SCT having two spherical mirrors and perfect optics will deliver an excellent null on axis. That is what is stated implicitly.

 

Aspherizing the secondary so as to combat coma is a significant process that requires no small care because the beam footprint upon it for any one image point occupies almost all the area. I've forgotten the amount of glass removal required, but seem to recall some number of wavelengths at the deepest. The small size helps here, in that it speeds the work. But the changing slope of the surface demands great attention to technique.

 

An aspheric secondary normally necessitates careful centration with respect to other aspheric elements and the optical and/or mechanical axes of the system. A Ritchey-Cretien illustrates this via its numerous adjustments for tilt and translation. Of course, the separation of the R-C's two aspheric elements introduces more stringent demands than does the SCT having an aspheric secondary almost co-planar with the corrector plate. Nonetheless, the SCTs crudity in corrector centration via cork or card shims, and the 'slop' in the secondary cell fit in the corrector's central hole, argue against a properly aspherized secondary. Indeed, it supports the spherical secondary due to its lack of a unique axis of symmetry. Leading to the ease with which collimation is achieved; an important feature for even an experienced telescopist.


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#24 GlennLeDrew

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Posted 11 June 2022 - 01:15 PM

To augment my previous... As Markus points out, that old blurb in S&T likely applied specifically to the big guns as would have been employed mostly by institutions. The mass market costs would have been more prohibitive. And again, a coma-corrected system would have been blasted forth in the ads. Instead, we had to await the introduction of explicit coma-free variants. Where the virtues are most definitely extolled in the literature.



#25 DAVIDG

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Posted 11 June 2022 - 06:01 PM

There's a historical perspective to be taking into account here. The claim was made in 1962. Commercial manufacture of Celestron-Pacific SCTs didn't begin until 1964. And then it was introduced only in two size -- 20 and 22 inch models. Clearly, these were made for professional use, and chances are that they did "do as they say" for such customers. Soon thereafter, the production of these large SCTs stopped, and the Company dropped "Pacific" form tis name and continued only as "Clestron." Full-scale mass produced Celestrons as we know them didn't begin until 1970 when a new manager came onboard. Once they began making hundreds, or even thousands of OTA's it's very likely that the amount of aspherization adjusted not only to the volume demands but also to amateur clientele with more limited knowledge of optical quality and theory. The  emphasis clearly shifted to high volume production, and we know that time is money so it's a no-brainer it seems. Eventually, most amateur-oriented optics production adopted a more pragmatic warranty -- satisfaction (or your money back).

 The literature I point too that stated they hyperbolized  secondary  was published in 1972 when Celestron started selling the orange tube version of C-5 C-8 and C-14 not from the 60's  So this was when they started sell   "mass produced  Celestrons"  Attached is a picture of the front cover and similar to ads Celestron ran in Sky and Telescope for years. 

   

Robert Piekiel is consider an expert on  the history of Celestron and how the optics were made. He wrote the book Celestron the Early Years. In that the aspheric secondary is discussed.  

 

   He also wrote a book " Making Schmidt Cassegrain Telescope  Optics".  In that book there is a chapter on making secondary mirrors. He gives three examples,  making a replacement for a commercial  C-8 SCT,  one for 12 f/10 that he designed and one  for C-22. In all three cases he goes into the need to aspherize the secondary. So you have an expert on the company and the  optics showing the need to aspherize the secondary. In the 12" f/10 that he designed he states that he chose the secondary to aspherize like Celestron did.

 

  Piekeil also points out that the correctors were made to over correct the wavefront from the spherical primary.  Tom Johnson also stated this as well. Why do this ? It is because now you have a  wavefront  similar to a classic RC were the primary is hyperbolic and the secondary is hyperbolic to eliminate the coma. By using a G power factor great then 1 the wavefront is acting similar to RC and with the aspheric secondary you get coma correction.

 

  As I said there are very good reasons to design the optics to reduce coma and that  requires designing the corrector to have the right power and adjusting the spacing , radius and aspheric surface on the secondary. It cost no more to make the corrector  with a G factor greater then 1 then less then 1 which is required for an all spherical system   From a production stand point if your going  to have skilled labor figure the secondary, there is no cost saving to design the system to be all spherical and then have someone work on the secondary  to touch up the figure vs designing to have better coma correction and also work on figuring the secondary. 

     

  Everything I have read from multiple sources states that they aspherized the secondaries and as I said Tom Johnson knew what he was doing. I find no evidence that the secondaries were spherical, except  from  people just repeating what has been said by others. 

 

                               - Dave 

 

celestron cover.jpg


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