56 replies to this topic

[Nightfall S Africa]

Were the C'stron and Meade Schmidt secondary mirrors up till the Edge & R-C era figured to simple convex spherical sections? I understand that in earlier times zonal corrections to the optical assembly were made on the secondaries; hence the need to align to their original configuration after disassembly. Some users have mentioned that q/c in mass production has become good enough that secondary figuring is no longer needed. Could anyone elaborate on this issue?

[rmollise]

At LEAST the Edge secondaries are still touched up as required, I am told.

[Geo.]

Meade ACF's use an aspheric secondary. Celestron's Edge uses a spherial secondary. The correction is provided by a lens mounted in the baffle tube. Hard to say which is more difficult/expensive to make.  Aspheric lenses are quite common in camera lenses, so the production kinks are probably well worked out. I'd guess the Edge is a bit more difficult to make. Four components to match is almost twice as hard as three.

[rmollise]

Meade ACF's use an aspheric secondary. Celestron's Edge uses a spherial secondary. The correction is provided by a lens mounted in the baffle tube. Hard to say which is more difficult/expensive to make.  Aspheric lenses are quite common in camera lenses, so the production kinks are probably well worked out. I'd guess the Edge is a bit more difficult to make. Four components to match is almost twice as hard as three.

 

 

"Aspheric" being "parabolic" in the case of Meade's telescopes. That's sort of a side issue, though, as are the correctors in the Edges. The matching vice touch-up procedures of Meade and Celestron have been going on LONG before ACFs or Edges.

 

From the beginning, Celestron has applied small corrections to the secondary to get well corrected optical sets. Meade, on the other hand, has always matched by trying different combinations of optics off the line till a well corrected set is found.

Edited by rmollise, 15 August 2014 - 12:46 PM.

[TG]

From the beginning, Celestron has applied small corrections to the secondary to get well corrected optical sets. Meade, on the other hand, has always matched by trying different combinations of optics off the line till a well corrected set is found.

 

 

In the EdgeHD whitepaper Celestron says:

 

If the combined optics set shows any slight residual under-or
over-correction, zones, astigmatism, upturned or downturned
edges, holes, or bulges, the optician marks the Foucault test
shadow transitions on the secondary mirror, then removes the
secondary mirror from the test fixture and translates these
markings into a paper pattern. The pattern is pressed against
a pitch polishing tool, and the optician applies corrective polishing
to the secondary mirror—as we show in Figure 11—until the
optical system as a whole displays a perfectly uniform illumination
(no unwanted zones or shadows) under the double-pass
Foucault test and smooth and straight fringes under the double-
pass Ronchi test.

 

So, still the case, at least for EdgeHD scopes. I think the generally still do it for all scopes that need it.

 

Tanveer.

[jhayes_tucson]

I know that folks have long believed believed in this story but I'm left scratching my head over it.  First, the primary is a concave sphere.  That is the easiest possible surface shape o produce and to test.  It should be easy to make all of these parts within tolerance.  I believe that they make the corrector plate using the vacuum plate method.  Again, this process should produce consistently good, rotationally symmetric parts.  Still I have to believe that most residual errors come from the corrector plate.  The secondary is a "high magnification part" which means that small slope errors have a big effect.  I've made convex, flat and concave surfaces but I'm not an optician so I may be all wet; but it seems like it would be very hard to accurately make small local corrections by rubbing on the secondary.  Correcting the wavefront on axis wouldn't necessary help fix off-axis errors either.  Get it even a little wrong and you could end up doubling the errors.  Maybe they actually use this procedure, but it sounds difficult.  Still, a good optician is way better at this stuff than I can imagine.

John

[Gil V]

I am of the opinion that Celestron stating they touch up their secondaries holds as much truth as their statement that Dynamax tubes were made of cardboard.

An out and out lie.

[John Boudreau]

I am of the opinion that Celestron stating they touch up their secondaries holds as much truth as their statement that Dynamax tubes were made of cardboard.

An out and out lie.

 

Going back about to around 2000, I was active on a web group called the Astro Photography Mailing List (APML) which was a film-based group, sadly now defunct for close to 10 years or so. One of the members was ATM Dave Rowe, who would later design the Celestron CDK (now Planewave CDK) and had a hand in the Edge HD design and now of course the new Celestron Rowe-Ackermann Astrograph. Rowe had made himself a beautiful medium format film Concentric Schmidt astrograph, and had made a few more for sale. At the time he was not affiliated with Celestron, but he was imaging from a club site that had a couple of the Celestron guys (who would later start Planewave) in regular attendance. They invited him to see the production facilities, and he posted a few things about that tour on the APML. His description of the secondary matching process was definitely consistent with what Tanveer has posted here.

Edited by John Boudreau, 16 August 2014 - 08:19 AM.

[rmollise]

 

From the beginning, Celestron has applied small corrections to the secondary to get well corrected optical sets. Meade, on the other hand, has always matched by trying different combinations of optics off the line till a well corrected set is found.

 

 

In the EdgeHD whitepaper Celestron says:

 

If the combined optics set shows any slight residual under-or
over-correction, zones, astigmatism, upturned or downturned
edges, holes, or bulges, the optician marks the Foucault test
shadow transitions on the secondary mirror, then removes the
secondary mirror from the test fixture and translates these
markings into a paper pattern. The pattern is pressed against
a pitch polishing tool, and the optician applies corrective polishing
to the secondary mirror—as we show in Figure 11—until the
optical system as a whole displays a perfectly uniform illumination
(no unwanted zones or shadows) under the double-pass
Foucault test and smooth and straight fringes under the double-
pass Ronchi test.

 

So, still the case, at least for EdgeHD scopes. I think the generally still do it for all scopes that need it.

 

Tanveer.

 

 

HI Tanveer...yep...that's my guess too.

[rmollise]

I know that folks have long believed believed in this story but I'm left scratching my head over it.  First, the primary is a concave sphere.  That is the easiest possible surface shape o produce and to test.  It should be easy to make all of these parts within tolerance.  I believe that they make the corrector plate using the vacuum plate method.  Again, this process should produce consistently good, rotationally symmetric parts.  Still I have to believe that most residual errors come from the corrector plate.  The secondary is a "high magnification part" which means that small slope errors have a big effect.  I've made convex, flat and concave surfaces but I'm not an optician so I may be all wet; but it seems like it would be very hard to accurately make small local corrections by rubbing on the secondary.  Correcting the wavefront on axis wouldn't necessary help fix off-axis errors either.  Get it even a little wrong and you could end up doubling the errors.  Maybe they actually use this procedure, but it sounds difficult.  Still, a good optician is way better at this stuff than I can imagine.

John

 

Celestron uses and since the beginning has used Tom Johnson's Master Block method of corrector fabrication. There is no question at all that Celestron used to do "touch-ups" to the secondaries (see Bob Piekiel's Celestron: the Early Years for documentation of that). The question is...do they still do it? They say they do on the Edges...

[rmollise]

I am of the opinion that Celestron stating they touch up their secondaries holds as much truth as their statement that Dynamax tubes were made of cardboard.

An out and out lie.

 

 

The latter? You are correct on...more or less. The former? You are completely wrong about. They've never made a secret about that part of their process. All they've ever been closed mouthed about to any extent is the Master Block fabrication.

Edited by rmollise, 16 August 2014 - 08:23 AM.

[Starhawk]

The reason you would touch up secondaries is they are the last element you can get to.  After the primary is in place with the corrector, the combined figure is different than each of them showed separately.

 

as for mix and match, that will, of course, include further afield parts which don't want to work with anything.  And if your manufacturing process isn't centered on the nominal figure (and they often are not), then only a part of the population of parts may be able to go together to produce a good figure.  In that case, you either eat low yield and high manufacturing expense, or you try to make least offensive optics to ship, but none of them are very good.

 

-Rich

[Geo.]

I've heard that TeleVue tosses more of it's Asian sourced glass than it sells.

[GlennLeDrew]

That the secondary is the 'high magnification' component poses no difficulty from that standpoint. It's main advantage for touching up the system figure derives from its small size; very little glass need be removed and so the work goes quickly. If one overdid it, a spherical surface to start over from could be established in a few minutes.

 

And no selective damage to off-axis performance is possible if the axial correction is good. The one follows the other in the system's design.

[freestar8n]

I don't really understand all this distrust and suspicion when for EdgeHD the manufacturing process has been documented in such detail.  They even show a polishing stand where you can see paper cutouts and pitch scraped away - in the setup where the secondary is retouched.  And they include interesting details such as the fact that the autocollimation setup uses a fixed set of the corrective lenses, because the production specs on them are very tight.  But the other three items - primary, secondary, and schmidt corrector - are assembled on the autocollimator test stand and tested as a complete system.

 

If people agree the quality of the final result is high - then either they spend a bit of time with a final stage of correction - or they have to make each element perfectly so the system needs no additional correction.  To me it makes perfect sense that the approach they describe is not only believable - but it is most cost effective if you want to get high yield with minimum time and labor.

 

I think many people regard any individual touch up as requiring a master optician days in a lab or something - when instead it might just take a quick check and then a brief, directed polish to improve the wavefront greatly.  A slight directed change to an existing nearly ideal figure is very different from taking a steep sphere to a specific conic with a lot of material removed.  In both cases you are making an "asphere" - but in one case you remove a little extra material from a certain zone - and in the other you are carving out a specific and deep curve from a sphere.

 

Another thing that's missed is that even if the schmidt corrector is made perfectly, and even if the primary and secondary are perfect spheres - if the radii are slightly off, the complete system will have spherical aberration - and it will show in the autocollimation test.  The only way to get a high Strehl result is to get the radii exact - or to introduce a slight asphere in one of the elements to compensate for the other system errors.

 

I took the 8" spherical sct design from SCB in Oslo and it has a Strehl of 0.99.  But if I change the primary radius from -812.8 to -806mm, which is less than 1% error, the Strehl drops to 0.75 after adjusting the primary spacing and refocusing.  But if I then impart a small amount of asphere to the secondary, the Strehl returns to 0.97.  The person doing the final figuring wouldn't know where exactly the problem is - but they would know that a slight targeted polishing on the secondary would greatly reduce the zone that shows in the test - and improve performance.  The system started with a perfect corrector and perfect spheres - and it was improved by adding slight asphere to the secondary.

 

And not only would these small corrections not require a lot of labor - but even in the case of mass producing RC's - which are notorious for hard to make curves - you can get a complete 8" RC from astronomics for only $895 nowadays.  And that is a real RC with hard to make curves on both the primaries and secondaries.  I don't know how those curves are produced, but I assume it is a mixture of automated and manual work with testing involved.

 

So - I don't see any reason to doubt that retouching is done, since it makes sense to do it that way, and I think it would represent an overall savings in terms of meeting a particular performance spec. with minimum time, labor, and skill.

 

We know that the elements have markings on them indicating an optimal orientation - and that also makes sense in terms of making any asymmetries in the elements optimally cancel in the overall system.  I would find that orientation - of the primary and secondary relative to the schmidt corrector - and then clean up anything remaining.  Presumably some systems would meet the spec. directly and wouldn't need retouching - but many would - or else they wasted time and money making the individual elements to too high a spec.

Frank

[GlennLeDrew]

When making the optics for a certain field-corrected Cassegrain, which employed an elliptical primary, spherical secondary and three lenses at the back end, we simply made all radii on all elements except the primary to within three fringes of nominal via reference surfaces used for contact testing. The primary was finished--prior to aspherizing--to a sphere whose radius was correct to within 1mm, or 0.08%. And so all elements were completely interchangeable--no matching nor rotational considerations ever applied.

 

This maintenance of tightness on tolerance permitted to set the primary-secondary separation by locating the focus at the designed-for distance from the field corrector's rear surface.

 

It's not particularly difficult to fabricate optics so as to achieve nominal curvatures for good overall spherical correction, and to eliminate the need for element matching. Then a wee bit of localized work can handily fix zonal errors.

[freestar8n]

When making the optics for a certain field-corrected Cassegrain

 

The key word there is 'a' - singular.  And how much did you charge?  Compare that with mass production at a small profit margin - with many workers and many different telescope sizes.

 

I don't doubt that the main sct makers can make 'a' sample that doesn't require aspherization - but things are different in mass production, where it makes less sense to make each element to a high spec.  It is all about specs in the completed system, yield, and profit margin.

 

I don't know how accurately they can make the radii on the primaries, but I know that as you tighten the spec, you add time and labor to the process for each element.  It may be ok to use 0.08% in a boutique situation - but not mass production.

 

And anyway - by definition if your as-made system had zonal errors - you are doing retouching of a single element to correct errors in the complete system - which is the whole question of this thread - and that Celestron describes in detail for EdgeHD scopes.

 

Most ATM's have only made a Newtonian, which has no need for testing in an autocollimator as long as the secondary is high quality.  But for any system with more than one curved surface - testing the complete system makes perfect sense, as long as you have a large enough flat for an autocollimator.  And in mass production the cost for such items, and interferometers, is amortized.

 

Finally - we know the sct elements are marked for optimal orientation.  That is a direct indication the tolerances on each surface are relaxed so the combination can optimally cancel.  That is a way to achieve higher performance of the system by spending less time on the elements - and pass the savings on to the customer.

 

Frank

[vahe]

I made a number of compound scopes in my early ATM years, based on my experience with hands on optics convex secondaries are the most difficult to make, in fact in order to avoid zonal errors and turned down edges one safe method is to stat with much larger blank and cut the smaller secondary from the center once figuring is complete, doing any aspherics or touch-up to the secondary would be a no-no in my books.

Perhaps M & C have developed a secret method to touch up the secondary safely without messing up the overall figure, highly doubtful, but anything is possible, I just do not know of a method that is practical.

The only safe method to aspherize a surface and not end up with disaster is to slightly deform the mirror blank under pressure, polish it and let it pop back to its original shape, this is the method used by Meade for their 7” Mak primary.

Doing old school aspherics by hand method requires high degree of optical skill and is not commercially practical for assembly line telescopes.

 

Vahe

[GlennLeDrew]

Frank,

By "a" I meant a certain model on the market, of which I worked on a bunch, at an hourly rate of pay you might find shockingly low.

 

My point was that it's not difficult to achieve the required tolerances on radii to realize complete part interchangeability. If the SCT corrections were to be applied to the corrector only, the same could obtain for them. Assuming no astigmatic elements, which for spherical surfaces should be de rigeur.

[GlennLeDrew]

Vahe,

What was the very first telescope optic I ever made? A 12.5" f/2 ellipse for a field corrected D-K. Done old school with a variety of tool sizes and pitch hardness. The departure from spherical, with respect to the 0.707 radius, is 22 fringes down at the center and edge. The rate of change in slope toward the edge is frightful, and is the trickiest region to work by far!

 

Why did I jump into the deep end? The boss decided to see what I could do, after a couple of years working on 9mm diameter, cylindrical, highly hyperbolic laser line generator lenses. He said I "took to it like a duck to water." I think that's because I visual the surface the way a sculptor might.

 

And I would have no fear whatsoever of tweaking a convex surface. (I did aspherize a pair of f/3.3 bino objectives, but that's not nearly so demanding, naturally.) It might first take a while getting the feel of things. of course.

[freestar8n]

The fact that you assume no astigmatic elements misses one of my main points.  We *know* there are astigmatic elements - because they are marked for a preferred orientation.  That is a direct indication they do not over-spec the elements - and instead are taking advantage of degrees of freedom in the system assembly.

 

If you are saying the labor cost is low - then even more reason to loosen tolerances on the elements and use a little labor on the finished product.

 

By "mass production" of an "SCT" - I mean they have a large stack of elements waiting to be tested - as shown in the picture from the whitepaper.  And by "sphere" I mean a large, f/2 sphere - not f/3 or something.

 

And I'm not sure why people are arguing what is best for celestron to do - and that they should apply corrections to the primary or the corrector.  They are not starting with a sphere or a flat - and then imparting a specific complex asphere into it.  They are providing a slight correction to the secondary to improve the overall figure.  I feel like I'm arguing about the moon hoax - where people are just denying the evidence and saying how things really work.  The products are optically good performers and have low cost - and they have elements marked for orientation - and they describe in detail how they do final corrections on the secondary - with pictures.  Unless people have direct experience with mass produced sct's with f/2 primaries - I don't see any reason to doubt that is how they are made.

 

But I don't deny that for a fresh ATM to make a cassegrain and then try to "improve" the secondary - disaster might unfold.  But that is not mass production with experienced people doing this stuff all the time.

 

Frank

[SandyHouTex]

I made a number of compound scopes in my early ATM years, based on my experience with hands on optics convex secondaries are the most difficult to make, in fact in order to avoid zonal errors and turned down edges one safe method is to stat with much larger blank and cut the smaller secondary from the center once figuring is complete, doing any aspherics or touch-up to the secondary would be a no-no in my books.

Perhaps M & C have developed a secret method to touch up the secondary safely without messing up the overall figure, highly doubtful, but anything is possible, I just do not know of a method that is practical.

The only safe method to aspherize a surface and not end up with disaster is to slightly deform the mirror blank under pressure, polish it and let it pop back to its original shape, this is the method used by Meade for their 7” Mak primary.

Doing old school aspherics by hand method requires high degree of optical skill and is not commercially practical for assembly line telescopes.

 

Vahe

 

Actually only Celestron touches up their secondaries and it's not that hard if you use petal laps which can be made to remove material only from certain zones.

[GlennLeDrew]

Frank,

I don't doubt for a millisecond that Celestron touches up the secondaries. Indeed, this approach has much to recommend it, as I noted earlier when pointing out the fact of very small amounts of material removal and hence rapid progress. My (barely) peripherally related comments from my own experience in telescope optics fabrication (including 12.5" f/2 aspheres) are intended to illustrate what is readily feasible, and to dispell the notion of surpassing difficulty in localized figuring of a convex surface. The feedback loop provided by double-pass testing of the system tells the optician exactly what is required to do. The deformations are on full display, and the zones from which to remove material are easily visualized. In my case all figuring was applied to the aspheric primary, but the principle is precisely the same for any surface (aside from the differences for refractive vs reflective elements.)

[Gil V]

Never mind.

 

[Herr Ointment]

Nice description, Glenn!