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Are SCT corrections still made on secondaries?

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#51 wh48gs

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Posted 22 August 2014 - 05:10 PM

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

 
Confuse it with the facts? Anything specific you could support this accusation with?
 

IIn "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.

 
The blur at 10mmm off axis in Fig. 9.3 measures 17mm, and the Airy disc is little over 2.5mm, or about 6.5 times smaller, not 10 times. Since the coma blur is 1.56 times the Airy disc at its "diffraction limited" level of 0.074 wave RMS (0.42 wave p-v),, this implies the p-v error of about 1.75 wave p-v. Just about identical to what raytrace indicates. That makes it 3.5 waves p-v 20mm off axis,
 
If, as you say, the SCT blur is "slightly worse" than the f/6 paraboloid blur, that doesn't make it comparable to f/5 paraboloid, whose blur at 10mm off axis would've been 11 times the Airy disc. It makes it comparable to f/6 paraboloid, since its blur is also about 6.5 times the Airy disc. Like I said, Rutten and Venroij either erred, or are (more likely) talking about the linear blur size, which cannot be a measure of the aberration magnitude in systems with different Airy disc diameters (i.e. of different focal ratio).
 

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.

There is nothing "easily visible" about the blur three times the Airy disc diameter at 20mm off axis. It corresponds to coma in f/9.6 paraboloid. Beside, 2% error in the corrector power is a lousy piece of work even by the commercial SCT standards. It equals 1.75 wave corrector profile error, and requires nearly 1/2 wave of spherical aberration correction by aspherizing the secondary.

 

Vla



#52 Dan McConaughy

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Posted 23 August 2014 - 09:59 AM

Keep in mind that both Celestron and Meade have extensive 'trade secrets and know-how'.  They have people who do nothing but correct and match optical systems day after day.  What might be difficult for someone correcting a secondary the first of fifth time, is not so difficult if it's your hundreth or thousandth time and someone taught you the 'tricks of the trade' when you started.

 

For example:  check out the interferograms of Paul Jones' (Star Instruments) R-Cs.  They are amazingly good, before considering that both the primary and secondary are hyperboloids, which are difficult to make.  He's done it for decades.  Also, you get what you pay for in a competitive market place.  His optics are more expensive.  Considering the relative mass production at Meade and Celestron, one has to respect their processes to produce 'pretty good' optics at low prices.



#53 Gil V

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Posted 24 August 2014 - 02:28 PM

To Rod's point about 40 year old advertising, I remember our Dynamax specs from the catalog. "Type tube military fires rockets from".

I mean, really? Bottle rockets, maybe.

#54 DAVIDG

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Posted 24 August 2014 - 03:30 PM

The purpose of Celestron Schmidt Cass design was and is  for use in astrophotography were teh off axis performance is critical.  In  the Celestron documents I posted a link to, it contains a spot diagram showing the off axis performance of a number of designs including a Schmidt cass is in which the off axis performance at 1/3 degree off axis is diffraction limited. The Celestron documentation also reference an April 1962 Sky and Telescope article.  That article  was one of the pieces of information that Johnson  used in deciding on using the Schmidt Cass design. In that article it states on page 193 of the April 1962 Sky and Tel article  "the secondary mirror and corrector are aspheric with their figuring adjusted to eliminate coma. Astigmatism is undetectable. The secondary is a conic section - a convex ellipsoid that must be tested by zones or by interference using a concave ellpsoid figured on the tool" So Celestron knew they had to come up with way to test the secondary and this matches what has been said by employees that each secondary was figured until the system nulled. By using the method of figuring the secondary along the primary and corrector as a unit they were able to achieve the need apsheric surface and also corrector for the errors in the primary and corrector. This method is also referenced to Johnson in the Optics Cooke Book in the article in figuring commerical Schmidt optics.

   Why would a company that wanted to make a telescope designed for photography go back to a system that had poor off axis  coma correction using a spherical secondary mirror ?  You can say what you want about a 40 year old ad, but what people have said that work at Celestron fits what is in the 1962 Sky and Tel article and why they used an aspheric secondary.

 

                             - Dave



#55 Jon Isaacs

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Posted 24 August 2014 - 04:34 PM

 

The purpose of Celestron Schmidt Cass design was and is  for use in astrophotography were teh off axis performance is critical.  In  the Celestron documents I posted a link to, it contains a spot diagram showing the off axis performance of a number of designs including a Schmidt cass is in which the off axis performance at 1/3 degree off axis is diffraction limited....  Why would a company that wanted to make a telescope designed for photography go back to a system that had poor off axis  coma correction using a spherical secondary mirror ?  You can say what you want about a 40 year old ad, but what people have said that work at Celestron fits what is in the 1962 Sky and Tel article and why they used an aspheric secondary.

 

David:

 

Something here doesn't quite add up. Prior to the ACFs and Edges, the standard Meade and Celestron SCTs show significant coma, that is the reason  both Meade and Celestron went to the effort of designing coma free optics.  It does seem that indeed Celestron did use a basic design for many years that was not well corrected for coma nor did it have a flat field.  

 

For astrophotography, Celestron provided the F/6.3 corrector/reducer that reputedly flattened the field, corrected the coma and provided a more useful F/6.3 focal ratio. 1/3 of a degree in a C-8 corresponds to a 12mm radius which corresponds to a 24mm illuminated circle which just happens to be the short side of a 35mm film frame.  The C-8 has a 1.5 inch rear baffle/rear port which used with the F/6.3 reducer/corrector provides a 24.0mm illuminated circle.  It seems to me that Celestron took that approach to correcting the coma for photographic use. 

 

Jon Isaacs



#56 Ed Holland

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Posted 24 August 2014 - 11:14 PM

Perhaps there was a period early in production where the more sophisticated optical prescription was used? Just a thought... not based in fact.



#57 wh48gs

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Posted 25 August 2014 - 07:28 PM

The purpose of Celestron Schmidt Cass design was and is  for use in astrophotography were teh off axis performance is critical.  In  the Celestron documents I posted a link to, it contains a spot diagram showing the off axis performance of a number of designs including a Schmidt cass is in which the off axis performance at 1/3 degree off axis is diffraction limited. The Celestron documentation also reference an April 1962 Sky and Telescope article.  That article  was one of the pieces of information that Johnson  used in deciding on using the Schmidt Cass design. In that article it states on page 193 of the April 1962 Sky and Tel article  "the secondary mirror and corrector are aspheric with their figuring adjusted to eliminate coma. Astigmatism is undetectable.

 

I would say that you are putting too much weight on what was published half a century ago, and too little (apparently, none) on the fact that such claim could be nowhere found ever since, until the Edge and Meade's coma free "RC" SCT variety hit the market. They may have planned on producing coma free arrangement in the beginning, maybe even had a few runs at the time - or it was deliberate marketing move to help move the sales - but if the freedom from coma is so critical, why would they drop that line altogether for the decades to come? Wouldn't make much sense, would it?

 

Smith/Ceragioli/Berry clearly state:"The classic 8-inch (203mm) f/10 SCT embodied in the Celestron C8 and Meade LX200 defined the word "telescope: for an entire generation of amateur astronomers. The design consists of a corrector plate and two spherical mirrors." (p236). Evidence of it can be easily found online; just google, say, "celestron c8 coma". Why should we disregard all that just because of what was published in a Celestron's advertising brochure half a century ago?

 

Vla


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