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Refiguring a Dynamax 8" Schmidt Corrector

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916 replies to this topic

#901 DAVIDG

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Posted 14 February 2020 - 12:16 PM

Star tools for something like that on a mirror grinding machine, I believe is the alternate way of doing this as opposed to a master block or vacuum pan.

 If I had the prescription for the corrector, the fastest way to make a new one would be the vacuum pan method.  Again I'll say the reason why I'm taking the approach I am is 1)  to find a method that someone that has a made of mirror two could do, 2) I don't have the prescription to make a new one from scratch. 3) I find this stuff fun, yes I know I'm not normal just ask my better half Heather. 

 

             - Dave 


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#902 markb

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Posted 15 February 2020 - 10:11 PM

Dave, so interesting to see this project move along again. I can't wait to hear the results once you finish and reassemble the DX8.

 

This is only an intellectual excercise for me, but wonder- if you successfully get the DX8 working at a high standard, could you reverse engineer or measure the corrector curve, and describe or list the optical prescription here for the handful of folks that want to try to make a fresh corrector using a vacuum pan?

 

Also, do you expect to find any change in the original prescription due to your refiguring efforts?

 

Also, again from curiosity, could a Mak mniscus element be ground to convert the SCT to a Mak design? From my extremely limited understanding of lens grinding, that might be a project capable folks could try, or does the 8 in aperture make it unrealistic outside a commercial setting? Or are there manifold reasons it couldn't work?  


Edited by markb, 15 February 2020 - 10:17 PM.

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#903 G-Tower

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Posted 15 February 2020 - 11:10 PM

I noticed this topic started over three years ago...



#904 markb

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Posted 15 February 2020 - 11:42 PM

Good optics take time, lol.

 

Seriously though, I believe Dave always indicated this was a low priority, presumably fun, exercise. I'm very happy to get the update, personally. This thread has been so helpful in understanding SCTs and the methods used to manufacture them.

 

And I'm happy the thread has remained 'open'.

 

A topic for another thread, i'd expect some lens or mirror project to stretch far longer.


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#905 G-Tower

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Posted 16 February 2020 - 12:59 AM

To me, it would be a waste of time...I'll take a good C8 which are a dime a dozen now...



#906 DAVIDG

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Posted 16 February 2020 - 10:37 AM

Good optics take time, lol.

 

Seriously though, I believe Dave always indicated this was a low priority, presumably fun, exercise. I'm very happy to get the update, personally. This thread has been so helpful in understanding SCTs and the methods used to manufacture them.

 

And I'm happy the thread has remained 'open'.

 

A topic for another thread, i'd expect some lens or mirror project to stretch far longer.

 

Good optics take time, lol.

 

Seriously though, I believe Dave always indicated this was a low priority, presumably fun, exercise. I'm very happy to get the update, personally. This thread has been so helpful in understanding SCTs and the methods used to manufacture them.

 

And I'm happy the thread has remained 'open'.

 

A topic for another thread, i'd expect some lens or mirror project to stretch far longer.

 As I have said it is hobby project. I have too many of them ! I'm working on two binoculars, another antique radio , building a GPS  module to automatically set the time and Date on my Autostar controller and a folded 3" refractor to name a few.

    I have taught mirror making and other forms of optical fabrication for over 35+ years now. I have seen ATMs show up to my class with mirrors they have worked on for years and not been able to finish them for many reasons. Many times within a few hours we have it  finished to an excellent figures. The difference is having the right tools and the understanding of how to do the job.

  Refiguring of  the Criterion corrector as I said many times is an experiment. Once I understand how to do it most likely will take a few evening to repeat the process. To me that is not a waste of time and the end result will be a telescope that has better optics then a "good C8" By the way there are many C-8s with optics that are not much better then these Criterions. There is just a thread up in the Cats and  Cass forum about Double Pass testing on C5. The optics in it aren't so great. https://www.cloudyni.../#entry9980253 

   As I said many times you can't assume optical quality, you need to confirm with it an unbiased test method.  I have taken many mechanically excellent but optically poor telescopes and turn them into both. I was out last night observing with my old 4.25" Edmund Palomar Jr. A few weeks back I refigured the zoney primary to an excellent parabola. https://www.cloudyni...cellany/page-30

The scope was giving a perfect star test and excellent images. I'll put up against a Tak, Zeiss, AP or any other one of the same aperture and I know it will equal or exceed the  views from  them. Too me that is not bad for few hours it took to refigure it and  the cost the to have it recoated.  My hope is the same for my DX-8.

 

 

             - Dave 


Edited by DAVIDG, 16 February 2020 - 04:17 PM.

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#907 Gil V

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Posted 19 February 2020 - 09:48 PM

David - if memory serves, the mounting block with vacuum-drawn corrector was ground first, then polished. You can see both in one of my article’s photos.
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#908 Chuck Hards

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Posted 19 February 2020 - 10:29 PM

Here is a link to Gil's article:  

 

https://www.cloudyni...rion-days-r2753


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#909 DAVIDG

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Posted 20 February 2020 - 10:16 AM

David - if memory serves, the mounting block with vacuum-drawn corrector was ground first, then polished. You can see both in one of my article’s photos.

 Gil,

  Any possibility that you post the picture here and upload at it maximum size so we can get a better look ?

 

                         - Dave 


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#910 Gil V

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Posted 20 February 2020 - 10:37 AM

It's in the articles. "Criterion Days". In the view of the optics department, the corrector grinding laps are behind the two operators, and the polishing lap is to their left.



#911 DAVIDG

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Posted 20 February 2020 - 10:47 AM

It's in the articles. "Criterion Days". In the view of the optics department, the corrector grinding laps are behind the two operators, and the polishing lap is to their left.

 Hi Gil,

   I saw the picture  in your  but it is embedded in your article so I can't increase the size to get a better look at what was going on. If possible if you could post it in this thread at it's maximum size we could all get a better look at the process. Many thanks.

  On a  different note, in your article you discuss that you cored the hole in the center of the correctors. Were they done one at a time or was a stack of them at once ? 

 

             - Dave 



#912 Ben H

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Posted 20 February 2020 - 11:49 AM

Gil, how did you ensure the optical center was the same as the mechanical center when coring the blanks correctors?


Edited by Ben H, 20 February 2020 - 06:08 PM.


#913 Chuck Hards

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Posted 20 February 2020 - 12:00 PM

Blanks have no optical center.

 

I don't think it's difficult on a finished corrector though.  A simple positioning jig.


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#914 DAVIDG

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Posted 20 February 2020 - 02:48 PM

   Blanks have no optical center  but you put the hole in the center of the corrector after it has been ground and polished into the Schmidt curve profile. The blank needs to be continues so you can pull a vacuum on it and pull it down against the Master surface.

 

                - Dave 


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#915 Ben H

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Posted 20 February 2020 - 06:09 PM

Sorry, should have said corrector, not blank. 



#916 Gil V

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Posted 20 February 2020 - 08:40 PM

Dave, my original photos were scanned for the article. I have the scans - somewhere. Give me a few days to dig those up.

Ben - the setup of the workholder in our water-cooled coring machine.

#917 AstroKerr

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Posted 22 February 2020 - 07:24 PM

gramps.gifI vote 'don't rush'. Much discussion and thought saves much frustration. Ain't like we're shippin' out any time soon.popcorn.gif


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