Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Classic Telescopes

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | (show all)
Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5570873 - 12/14/12 01:36 PM

Upon removing and studying the mirrors, Larry finds that they are pitted. There must have been some weird, corrosive reaction between the aluminum and the salt. One would think the Pyrex substrate would be unaffected, but apparently that's not the case. The scope will be receiving new StarBright mirrors from... my history here is slipping... an era that Larry says should have good optics. An earlier black tube, with an orange cosmetic cover over the secondary. The original mirrors may be okay for a determined home-builder to polish and use again.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Masvingo
scholastic sledgehammer
*****

Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5570876 - 12/14/12 01:39 PM

Hi Joe, that's a shame about the mirrors - good that you have a replacement source though. Will you be swopping the corrector as well?

James


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: Masvingo]
      #5570946 - 12/14/12 02:34 PM

This will be a Frankenscope. The StarBright mirrors are available because their corrector was shot. We'll be using the original orange C8's corrector with the StarBright mirrors. Tests will show how well they match. It will likely be just fine, at least good enough -- maybe great!

Post Extras: Print Post   Remind Me!   Notify Moderator  
Masvingo
scholastic sledgehammer
*****

Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5570973 - 12/14/12 02:53 PM

Fingers crossed! It's good to hear that Margarita will carry on!

James


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: Masvingo]
      #5571284 - 12/14/12 06:23 PM

On these old Celestron scopes, the primary and secondary are matched, because errors in the primary were corrected by tweaking the secondary. This was a convenience of manufacturing. The 5x convex mirror could be slightly flattened here and there, so that 1/5 the grinding would induce the necessary corrections. This was much easier than reshaping the entire bowl of the concave primary, or than trying to tweak just a concave section.

The correctors were mass produced, and could vary slightly. If one did not test well, it was rotated; if that did not fix the problem, a different corrector was substituted. By far, most of the fuss was in matching the mirrors. It's not that the corrector was unimportant, but that they were remarkably consistent. Margarita will likely be just fine.

If you look back at the pictures and read his description, Larry did a remarkable job in refurbishing Margarita's original corrector. After about two hours with Novus Three Step Polish and a few special additives of his own, it cleaned up beautifully.


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5571469 - 12/14/12 08:12 PM Attachment (43 downloads)

Actually Jon hasn't seen a pic of the corrector after I hand rubbed the oxidation from the multicoatings, I was surprised that they cleaned up as much as they did,
I use the Novus system and I add different percentages of Cerium Oxides, Cerium NANO Oxide, and fine powdered Aluminum Oxide. mixed in as my own "slurry recipe" tried and perfected through the years from my first days working in an Optical lab, grinding and polishing lenses.
here is the "after" result of the the carefull hand polishing, I actually use a converted lensometer to measure the aspheric gradient power so not to change it. I marked the "Plano" side in different colored dry erase marker showing the gradient steps of power
There is some dust as I sat it onto the empty OTA.
The lighter and darker uneven area isn't the coarings but the Ultra Black paint drying in thicker areas.. You can see the difference from the pictures on the top of the page showing it as a nearly Charcoal powder look.
I'm very happy with it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Masvingo
scholastic sledgehammer
*****

Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: orion61]
      #5571947 - 12/15/12 02:30 AM

Gosh, that has cleaned up amazingly well, a big change from the salt encrusted shots a couple of days ago.

Looking forward to seeing the rest of your work over the coming days.

James


Post Extras: Print Post   Remind Me!   Notify Moderator  
Masvingo
scholastic sledgehammer
*****

Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5571961 - 12/15/12 03:01 AM

That's interesting, I hadn't realised that Celestron had managed to get that level of consistency in making the correctors. Tom Johnson's master block process really opened up SCT mass production.

James


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Restoring a C8 After Purchase -- Update new [Re: Masvingo]
      #5572294 - 12/15/12 10:20 AM

It wasn't that difficult once he got the basic shape of the mould down,
They put a flat blank on a mould of the aspheric corrector shape, then they placed a very thin cloth over the bottom of the mold and set a very high vacuum. (*This simple step is the basis of the Lawsuit between Celestron and Criterion which due to the overwhelming legal costs helped Criterion to its demise).They then polished the top of the blank which had formed to the mould flat.
when the vacuum was removed the glass had the aspheric curve.
an amazingly simple but brilliant process.
*I suggest reading the minutes from the Lawsuit on Google, it is very interesting reading, But basically ended pretty much in a tie but Criterion made some consessions to end the litigation.

http://www.leagle.com/xmlResult.aspx?xmldoc=19781064461FSupp603_1970.xml&...


Post Extras: Print Post   Remind Me!   Notify Moderator  
Masvingo
scholastic sledgehammer
*****

Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: orion61]
      #5572666 - 12/15/12 02:10 PM

Hi Larry,

Thanks for the link to the Celestron - Criterion court case, made for some very interesting reading and it has filled in quite a few gaps in my understanding of what happened. I see there was a further report a few years later in 1982

http://www.leagle.com/xmlResult.aspx?xmldoc=19821164552FSupp612_11075.xml&....

I've also found the patents issued to Johnson and O'Rourke of Celestron and John Krewalk Sr of Criterion to be interesting reading.

James


Post Extras: Print Post   Remind Me!   Notify Moderator  
DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5573099 - 12/15/12 08:02 PM

Quote:

The correctors were mass produced, and could vary slightly. If one did not test well, it was rotated; if that did not fix the problem, a different corrector was substituted. By far, most of the fuss was in matching the mirrors. It's not that the corrector was unimportant, but that they were remarkably consistent. Margarita will likely be just fine.






Joe,
I've refigured a number of SCT optics and designed and fabricated optics for over 30 years now. Both the primary and corrector have residue errors. They were placed in a test fixture and the secondary aspherized. The exact amount of aspherizing needed was variable since it has two functions. One was to remove the errors of all the elements and the second function was to fully correct the system to remove the spherical aberration. So replacing one corrector with another requires that the secondary again be refigured to remove the errors in the system. The correctors had the same amount of variablity as the primaries. The reason was that each master block that was used to make the correctors had some variablity in it's exact surface and they would also wear from use. The other source of variablity was the "flat" surface of the corrector. In theory these should be optically flat but in reality they almost always never are and can vary by a number of wave concave or convex. This results in the corrector either under or over correcting the wavefront from the primary. The reason why it is not a problem is, because all three elements are corrected together by figuring the secondary. The image in your scope won't be a fuzzy mess but the odds are low that it will be anything more then around 1/2 wave at best. If you want better your going to have to refiguring the secondary to correct the complete system.

- Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5573176 - 12/15/12 08:40 PM

From what I havve seen Celestrons Correctors were more consistant than the other commercial offerings in that era.
Meades and Criterions were all over the place.
I got the system together and have first light through it,
It will be at least defraction limited..
This is a very fun project, I feel honored Joe trusted me to do the work for him.
Larry


Post Extras: Print Post   Remind Me!   Notify Moderator  
DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Restoring a C8 After Purchase -- Update new [Re: orion61]
      #5573224 - 12/15/12 09:06 PM Attachment (26 downloads)

What method did you use the test the variablity of the correctors that you worked on ? What method are you going to use to test the overall optical correction when the scope it is assembled to varify the results ?
Here is a picture of vintage C-10 corrector plate that I'm testing against a Master 12" optical flat by interference. This test showed the corrector was astigmatic. Like I said I have actually tested and refigured these type of optics, using the correct optical tests to determine the optical correction. The odds will be low that any mix and matched SCT optical set will be diffraction limited without refiguring at least the secondary.
Celestron understands that their optics have variablity and need to be figured as a system. This is why they won't sell individual elements.

- Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Restoring a C8 After Purchase -- Update *DELETED* new [Re: DAVIDG]
      #5573282 - 12/15/12 09:42 PM

Post deleted by orion61

Post Extras: Print Post   Remind Me!   Notify Moderator  
DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Restoring a C8 After Purchase -- Update new [Re: orion61]
      #5573386 - 12/15/12 11:02 PM

A Foucault tester can only be used to test the primary not the complete system of a Schmidt cassegrain. To test the overall correction one needs to do a test like double pass autocollimation.
I'm just trying to make sure that false information about these system doesn't keep getting spread that one can just swap a corrector in and the optics will be automatically diffraction limited. That simply is not the case. You can rotate the elements all you want and that doesn't do anything to the spherical correction. So if the corrector doesn't match with the primary and secondary that isn't going to fix the problem.
You made the statements that the Celestron correctors you have seen have less variablity then others and the system will be at least diffraction limited after you installed the optics. I just asked how you determined this. If your statement has any validity, then you should have no problem providing the methods you used to collect this data so everyone can understand the strength or weakness of your claims.
Narcissism, no just someone who has actually worked on Schmidt cassegrain systems, designed and fabricated precision optics for many years, won a number awards as proof of my expertise in the subject, as been teaching mirror making for about 20 years now, being a consultant on optics like my most recent input on "Telescope , Eyepieces and Astrographs" for the authors and Willmann Bell Publishing, another first place in the optical judging at Stellafane this past year and just had an article in Nov issue of Sky and Telescope about telescope making.

Happy Holidays,

- David Groski


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5573504 - 12/16/12 01:12 AM

DavidG, thank you for your input. I think you're saying that the scope will work with the mix n' match parts, it will show an image at the eyepiece, but it's not guaranteed to be as good as a C8 should be.

Larry, I don't believe DavidG was insulting your eyes. He was only warning that, in mixing and matching parts, we may be less lucky than we had hoped. Why don't we skip to Plan B, and ship the original mirrors for evaluation for a cleaning and recoating? You said earlier today that the mirrors were not pitted as badly as you had thought at first. Perhaps they could be used, keeping everything original. If not, we continue with the mix n' match parts.

Let me guess: Polishing the mirrors would alter their surfaces, requiring refiguring of the secondary to match the system's new abberations?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5573751 - 12/16/12 08:57 AM

DavidG, looking at the picture of your 10" corrector, an arc of stacked, squiggly lines runs through it, and there is a puddle of splotchy light off to the right. (Highly technical description!) Are those the aberrations that, for this particular corrector, would be countered by specially aspherizing the scope's secondary? I understand that, actually, the fix only works when the primary is also in the optical train.

Suppose my scope, if assembled to current plan with mix n' match optics, were only 1/2 wave (which, itself, may not be guaranteed). What would be the significance? Is, for example, the difference between 1/2 wave and 1/8 wave the difference between recognizing Jupiter in the eyepiece, and being wowwed at the detail in its cloud bands?

I'm interested in everyone's thoughts, and do not presume opposing views as being attacks on each others competence. I see this as a practical exercise. It's not surprising that a perfectly matched set of optics would be ideal, so the question becomes picking the best solution among five:

(1) All new optics (expensive -- beyond budget for now!)

(2) Mix n' match

(3) Refurbish original optics (if possible; maybe also expensive?)

(4) Anyone know how to refigure the secondary?!

(5) Quit (why? we've come this far, and at least will have working mechanicals with the potential for improved optics later)


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5573791 - 12/16/12 09:29 AM

I have deleted the above reply to get back to the Nature this Post was intended which is to get a frozen up unuseable scope back under the stars with as little expence as possible as a pet project.
All I can say is that I am very encouraged with the results so far by viewing the in focus airy disc and intra and extra defraction patterns. The final product will speak for itself.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: orion61]
      #5573865 - 12/16/12 10:12 AM

Years ago, when I was a student in London, I was advised to visit the viewing galleries at the law courts. It's a fascinating spectacle. The barristers are required, by law and custom, always to address each other's comments as plausible; and, upon disagreeing, to refer to each other as "our learned friend." Let's keep our discussion at that level.

Given how close my scope is to assembly with the original corrector and StarBright mirrors, would there be a way to post pictures of its star tests? Larry, could you replace the eyepiece with a camera, or digiscope a decent image?

I'm grateful to both of you, to Larry for his passionate commitment to a workable, practical, affordable rehab for this grand old scope, and to Dave, for the considerable time invested in offering his technical input to our endeavors.

Thank you,

Joe


Post Extras: Print Post   Remind Me!   Notify Moderator  
DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5575516 - 12/17/12 10:52 AM Attachment (16 downloads)

Quote:

DavidG, looking at the picture of your 10" corrector, an arc of stacked, squiggly lines runs through it, and there is a puddle of splotchy light off to the right. (Highly technical description!) Are those the aberrations that, for this particular corrector, would be countered by specially aspherizing the scope's secondary? I understand that, actually, the fix only works when the primary is also in the optical train.

Suppose my scope, if assembled to current plan with mix n' match optics, were only 1/2 wave (which, itself, may not be guaranteed). What would be the significance? Is, for example, the difference between 1/2 wave and 1/8 wave the difference between recognizing Jupiter in the eyepiece, and being wowwed at the detail in its cloud bands?

I'm interested in everyone's thoughts, and do not presume opposing views as being attacks on each others competence. I see this as a practical exercise. It's not surprising that a perfectly matched set of optics would be ideal, so the question becomes picking the best solution among five:

(1) All new optics (expensive -- beyond budget for now!)

(2) Mix n' match

(3) Refurbish original optics (if possible; maybe also expensive?)

(4) Anyone know how to refigure the secondary?!

(5) Quit (why? we've come this far, and at least will have working mechanicals with the potential for improved optics later)




Joe,
I would strip the coatings off the original mirrors, chemically. I have seen mirrors with very bad coatings look like they were hopeless, yet when the coatings were removed the glass was perfect and when recoated, looked perfect as well. In my opinion the original optics has the best chance of giving the best image, but I would check the new ones as well.
If it was me I would sets the optics up in double pass autocollimation to test them. This is one of most sensitive test one can do and it is done on the complete system. Also there is nothing to measure. If the Ronchi lines are straight and/or the system nulls like a perfectly spherical mirror, the correction of the complete system is dead perfect. If not one can see exactly were the problems are.
Knowing that most people don't have a large optical flat to do double pass testing, then I would at least Foucault test the primary mirror. This will give one some insight to overall optical quality of the system and if can be optical corrected, well. In theory the primary should be a perfect sphere so it is a straight foward null test with a Focuault tester to do. In most cases the primary won't be a perfect sphere but will be a very weak ellipse. This is just a fact from rapid machine polishing used to make them but what is critical is that the figure is optically smooth. So no turned down edge or rough surface. A common error is what is known as "record grooving". The optical surface is rough will and looks like a grooves under the Foucualt test. Again this is caused by the high speed polishing of the surface. Luckily all this can be fixed by repolishing the surface on a pitch lap by hand. If the mirror has these errors then no amount of correcting the secondary is going to fix these problems until the primary is refigured. So the set of optics with the better primary and with it's matching secondary has a better chance of giving the better image.
As for the picture of 10" corrector being tested via interference against a 12" optical flat, the pattern of wiggly lines is were the Schmidt curve is it's deepest. What one wants to see is that the pattern forms a concentric ring around the corrector. If it is oval shaped or has any bumps then the corrector has astigmatism. If that is case then it's time for a new corrector. I've attached a close up picture. The pattern on the right side is different then the left from the astigmatism.
If you want to refigure the secondary, first you need to be sure that the primary is optically smooth and close to spherical and the corrector doesn't have any astigmatism. Then you jig up the optics just like how they are going be installed in the scope. You need to test the system by double pass autocollimation using a large optical flat. The light passes thru the system twice so any errors are doubled making for a very sensitive test. By the way this was the same method that Sky and Telescope used in 1989 article that looked at the optical quality of commerical SCT sold at that time. The secondary is not spherical as has been stated in the past. It is aspherical and depending on the residue errors in the corrector and primary, it is figured somewhere by an ellipse to a hyberbola to fully correct the system. You use a pitch lap with a peddle shaped pressed into it and slowing polish the secondary while testing every few minutes until the system shows straight ronchi lines and/or null like a spherical mirror. When that condition is meet, the system is perfect.

Happy Holidays,
- Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | (show all)


Extra information
8 registered and 9 anonymous users are browsing this forum.

Moderator:  Rich (RLTYS), Brian Risley, Chuck Hards 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 9812

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics