316 replies to this topic

[Bob Campbell]

 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 

Really interesting back and forth here. I have no dog in this fight, but I do have a simple question.

 

Why would Meade introduce the ACF line when they (and celestron, apparently) already correct for coma by figuring the secondary?

 

Inquiring minds (at least mine) want to know.

 

Peace to all,

 

Bob

[Jon Isaacs]

For the original F/10 system used for 35mm photography  the coma correction was good for that application.

 

 

If I recall correctly, Celestron made the F/6.3 flattener reducer/ corrector that was meant for correcting the coma as well as the curvature.

 

In the beginning, people did not have 31 Naglers and 41Panoptics to really see the curvature and coma.  I just wonder..  I think Glenn stated the traditional view which I seconded. Glenn has a lot more experience in optics than I do as he was an optical guy with Ceravolo until Peter moved west.

 

Jon

[MKV]

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. 

My impression as well, Glenn. 

 

Mladen

[SandyHouTex]

The fact is, unless someone does some testing, we will never know.  I tend to believe that when Tom Johnson said they were aspherizing the secondaries, they probably were.  Back then people had integrity and wouldn't just lie about something like that.

Edited by SandyHouTex, 11 June 2022 - 11:11 PM.

[Bob Campbell]

The fact is, unless someone does some testing, we will never know.  I tend to believe that when Tom Johnson said they were aspherizing the secondaries, they probably were.  Back then people had integrity and wouldn't just lie about something like that.

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.

[davidc135]

Does anyone know of an interferogram of the entire C8 corrector's Schmidt surface in contact with a plane? I've googled it in and have found very little.

 

You should be able to count or estimate the number of fringes visible and estimate the degree of correction provided by the corrector. The 8'' F/2 spherical mirror will be under-corrected by 22.5 waves green light so a corrector with g = 1 would require a (0.707r) neutral zone 90 fringes deep if ref. index = 1.50. (I think float or plate is actually 1.523). The missing centre adds a complication.

 

That reduces by 0.712 in the all spherical design if reasonable assumptions regarding corrector plate position and back focus are made giving 64 fringes or a little less after ref. ind. adjustment.

 

The ACF should show low 70s.

 

There are no end of tests available if one has the time. 

 

David

[Steve Dodds]

The fact is, unless someone does some testing, we will never know.  I tend to believe that when Tom Johnson said they were aspherizing the secondaries, they probably were.  Back then people had integrity and wouldn't just lie about something like that.

The standard Celestron SCT has a spherical sec, but when manufactured they set up all 3 elements then a technician hand figures the sec until it meets spec.  Meade just mixes and matches elements until they get acceptable results.

The result Celestron's are better corrected, but a little rougher.  Meades are smoother, but the correction is usually off a little. 

[SandyHouTex]

So late last night, I took out my "Telescope Optics" by Rutten and Venvrooij.  If you look at Fig. 9.3, it has spot plots of an 8 inch all spherical SCT.  I own all of the Celestron SCTs, no EdgeHDs, and a 10 inch Meade LX50 prior to the ACFs, and I find it hard to believe that the comma is as bad in any of them as the spot plots in Fig. 9.3.  If it was, you'd need to use a Parracor at low power, and I've used low power on all of them.  Also, if you look at Fig. 2 in the Celestron EdgeHD White Paper:

 

https://s3.amazonaws...paper_final.pdf

 

The comatic spot plots for a "Classic SCT" don't look near as bad as in the "Telescope Optics" Fig..

 

Also in "Telescope Optics" they give the prescription for an asperized secondary which has a conic constant of -0.8857.  Easily done with an appropriately shaped pitch lap.

 

It's also telling what they say in paragraph 3 under section 9.3, "It is clear that a compact (the current Celestron design with the secondary attached to the back of the corrector) Schmidt-Cassegrain with two spherical mirrors gives inadequate off axis performance."  They then go into the options for correcting the coma, the simplest being aspherizing the secondary.

[MKV]

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

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

In the 8: and 14" only OTA description, "primary mirror" -- spherical, and under "secondary mirror" -- "Spherical (final hand figuring yields a slight asphere)..."

[Bob Campbell]

In the 8: and 14" only OTA description, "primary mirror" -- spherical, and under "secondary mirror" -- "Spherical (final hand figuring yields a slight asphere)..."

that's the point! And who knows what the Chinese are doing now. That was the era of made in USA.

Edited by Bob Campbell, 12 June 2022 - 05:16 PM.

[KBHornblower]

It has always been my impression that both mirrors in the original consumer Celestrons from the '70s were spherical by design, for ease of figuring, and that touchup figuring of the secondary was nothing more than correction for minor errors in the figures of the three components.  This is the first time I have ever heard of assertions otherwise.  I thought the Meade ACF design was a new innovation at the time.

[hamishbarker]

which telescope optics book, the willman bell one? if so, I am looking forward to when the full WB inventory gets released by Sky and Telescope shop!

 

 

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

[davidc135]

which telescope optics book, the willman bell one? if so, I am looking forward to when the full WB inventory gets released by Sky and Telescope shop!

Yes, by Harrie Rutten and Martin van Venrooij. A great book.  David

[SandyHouTex]

that's the point! And who knows what the Chinese are doing now. That was the era of made in USA.

If Celestron has it in their contract with Synta to aspherize the SCT secondaries, I'm sure Synta would do it.  Do you think Celestron wouldn't know what is being delivered to them?

[SandyHouTex]

In the 8: and 14" only OTA description, "primary mirror" -- spherical, and under "secondary mirror" -- "Spherical (final hand figuring yields a slight asphere)..."

Frankly, I doubt if Company 7 has a clue.  They would have to disassemble them and somehow test the convex secondary.  Certainly if they did that, they would say they did.

[SandyHouTex]

It has always been my impression that both mirrors in the original consumer Celestrons from the '70s were spherical by design, for ease of figuring, and that touchup figuring of the secondary was nothing more than correction for minor errors in the figures of the three components.  This is the first time I have ever heard of assertions otherwise.  I thought the Meade ACF design was a new innovation at the time.

That's something else I've never understood.  How do you correct the errors of the three components by modifying the figure on the secondary?  I've read in places that people say that, I just don't understand how you could do it.

[Bob Campbell]

If Celestron has it in their contract with Synta to aspherize the SCT secondaries, I'm sure Synta would do it.  Do you think Celestron wouldn't know what is being delivered to them?

Here is an interesting link:

 

https://www.chuckhaw..._connection.htm

 

and two excerpts:

 

"They actually removed the production machinery and equipment from Celestron's Torrance, California factory and shipped it to China, where it is now used to build the telescopes formerly made in the USA. Celestron technicians and opticians traveled to China to teach the Synta technicians how to use this equipment and they still serve as consultants, frequently visiting China to ensure that the Synta built telescopes are being made to Celestron's specifications."

 

and, to your point:

 

"Synta made Celestron SCT's are still built with matched optics and the secondary mirrors are still hand figured, but the work is done in China. Celestron telescopes are no longer advertised as meeting a +/- 1/10th light wave standard, but they are advertised as having diffraction limited optics."

 

so there you go.

 

Bob

Edited by Bob Campbell, 12 June 2022 - 10:20 PM.

[KBHornblower]

That's something else I've never understood.  How do you correct the errors of the three components by modifying the figure on the secondary?  I've read in places that people say that, I just don't understand how you could do it.

 I am envisioning smooth surfaces with each one very close to the design specifications, but out just enough to result in residual spherical aberration of perhaps 1/2 wave.  This could be reduced to well under 1/4 wave by touchup figuring of any surface.  Apparently the secondary mirror is the surface of choice with a suitable pitch lap.  I don't think anyone in his right mind would do it on the Schmidt corrector.

[MKV]

Frankly, I doubt if Company 7 has a clue.  They would have to disassemble them and somehow test the convex secondary.  Certainly if they did that, they would say they did.

That's true, but neither does anybody else. It's all about he sad this and the other said that. With all the people who have access to Zygos, no one bothered to test the components. Why? Testing convex secondaries is pretty much out of ATM's hands. However, some have Hindle spheres, and they could have done it, but didn't. I find that odd, given the Celestrons' popularity, and the fact that some have even made Cassegrains of their own using that method. That's how it is in the amateur world -- mostly hearsay, and little if any explanations.

[MKV]

Here is an interesting link:

 

https://www.chuckhaw..._connection.htm

 

"Synta made Celestron SCT's are still built with matched optics and the secondary mirrors are still hand figured, but the work is done in China. Celestron telescopes are no longer advertised as meeting a +/- 1/10th light wave standard, but they are advertised as having diffraction limited optics."

 

Thank you, Bob! Good post. All optics are diffraction limited one way or another. That doesn't mean any specific wavelength accuracy. So, that's pretty much an empty phrase that sounds good. Also, most companies today guarantee only satisfaction. If you're not satisfied with your product, return it undamaged for a refund. No questions asked. They will repackage the item and sell it to someone else who may be happy with it even it the images are not perfect simply because the person doesn't know what a good image is supposed to look like. Some people don't notice coma or simply think that's "normal," etc.

 

Mladen

Edited by MKV, 13 June 2022 - 02:29 AM.

[davidc135]

As the 2ndry asphericity needed to correct coma in a Meade 8'' F/2/10 sct is large, a matter of waves wf rather than 1/10ths, it eventually proved quite easy to test. All you need is a simple, decent quality F/4 binocular objective of reasonable size. I used a 50mm aperture doublet as in the diagram below.

 

The Cassegrain calculator told me that the 2ndry RoC was 10ins so, in the setup below, all I had to do was adjust the testing lens' focii to take account of that. The longer focus turned out to be 28ins. Light from the precision pinhole travels through the doublet and is reflected off the convex mirror to retrace its steps to be tested by knife edge at A. The errors seen at A will be twice those of the doublet plus any contribution by the 2ndry. A spherical convex surface will not add any error.

 

In the viewed F/14 system 23mm of ke travel was observed in the severely under-corrected wave-front. This corresponds to 3.3 waves SA in an F/14 cone.

 

The 2ndry was removed and the pinhole placed now at the 2nd focus B, the beam again being tested and the error measured at A. The error of the single pass through the doublet was now half of that earlier showing that the secondary was a sphere within the  bounds of accuracy of the experiment. A green filter could be used and the details of measurement etc tightened up but there was nothing to indicate any asphericty.

Although I lost a little of the 2ndry margin it was nothing like indicated in the drawing and didn't affect the test significantly. The 2ndry could be offset to include the margin

 

 

Whilst this experiment featured a Meade secondary I doubt if Celestron's version would show anything different. It's easy to do so why not?

 

David

 

PS What's this test called?

Edited by davidc135, 13 June 2022 - 03:55 PM.

[duck]

Tom Johnson said they had a test which could detect 1/100 wave error in the wavefront?  Nobody questioned that?

[Steve Dodds]

Tom Johnson said they had a test which could detect 1/100 wave error in the wavefront?  Nobody questioned that?

really, a top of the line Zygo can't do that.

[MKV]

Tom Johnson said they had a test which could detect 1/100 wave error in the wavefront?  Nobody questioned that?

 

really, a top of the line Zygo can't do that.

Well, true, Zygo will go as far as 1/50 wave, etc. But the knife-edge is also one of those capable of detecting 1/100 wave claimed by some authors. Clearly, these are theoretical constructs, but the high-ended science will not shy away form such numbers.  Don't forget that Zygo, as much as it represents an unattainable fantasy of many an ATM is considered a "general-use" interferometer. Years ago, you could buy a Shack Cube Fizeau IF for some $12,000! It was certified only to 1/10 wave PV. A certified Zygo  with all the attachments can cost as much as a small house. Maybe ATMs shouldn't enter the interferometry field because these devices will, even at that rating, show them that the scopes they have are really not even close to being as good as they were led to believe...

 

https://www.alcor-sy...cope40x40.html 

Edited by MKV, 14 June 2022 - 04:34 AM.

[Bob Campbell]

Well, true, Zygo will go as far as 1/50 wave, etc. But the knife-edge is also one of those capable of detecting 1/100 wave claimed by some authors. Clearly, these are theoretical constructs, but the high-ended science will not shy away form such numbers.  Don't forget that Zygo, as much as it represents an unattainable fantasy of many an ATM is considered a "general-use" interferometer. Years ago, you could buy a Shack Cube Fizeau IF for some $12,000! It was certified only to 1/10 wave PV. A certified Zygo  with all the attachments can cost as much as a small house. Maybe ATMs shouldn't enter the interferometry field because these devices will, even at that rating, show them that the scopes they have are really not even close to being as good as they were led to believe...

 

https://www.alcor-sy...cope40x40.html 

I think Suiter touches on this, but to the group:

 

What would the seeing have to be in order for optics better than diffraction limited be apparent? My recollection is that Suiter makes the point that it would have to be darn good, and people hand-wring far too much about their optics not being good enough. Unless you have adaptive optics corrections in your imaging train, none of this may matter. Likely the distinction between 1/5 wave and 1/10 wave is undetectable, and for sure 1/10 wave and 1/50-100 wave is not unless tested in the lab.

 

Bob

Edited by Bob Campbell, 14 June 2022 - 09:06 AM.