971 replies to this topic

[highfnum]

Rolo - why?

[tim53]

I'm sure Dave will comment, but IMHO it's likely to improve the performance, since the flat side was so unflat.  But the Schmidt curve still has its own problems.  The best thing about interference testing against a flat is that the errors of both surfaces can be assessed independently.  I would imagine that starting with polishing the flat side makes sense because he can test the whole optics set while he's working on the Schmidt curve, once the flat side has been dealt with.

[DAVIDG]

 The "flat" surface of the corrector was the one that was pulled against the Master Block  via vacuum which has the opposite Schmidt curve . The top surface now has the shape of the Master block ie the opposite of the Schmidt profile you want. Then this top surface is ground and polished flat. When the vacuum is released the  corrector springs back and the top surface that  was polish flat now has the Schmidt profile and the rear surface which was against the Master block returns to it original untouched stated.  It is this rear surface that I'm working on now.

   The flat surface needs to optically smooth from the start since when it is pulled against the Master Block's surface these defects will get transferred to the top surface. So when the top surface is ground and polished flat and when the vacuum is  release you will get the opposite shape of the these defects in the top surface along with the Schmidt curve. So to have the Master Block method work correctly the surface of the corrector that is brought in contact with it must start out as being optically smooth and one must also unsure that the two surfaces are in perfect contact. 

   I still have a couple of hours work on the back surface to get it were I want . I'm guessing that I'll have 8  to 10 in the back surface so the front surface with the Schmidt profile will most likely take at least the same amount.

 

                  - Dave 

[DAVIDG]

I'm sure Dave will comment, but IMHO it's likely to improve the performance, since the flat side was so unflat.  But the Schmidt curve still has its own problems.  The best thing about interference testing against a flat is that the errors of both surfaces can be assessed independently.  I would imagine that starting with polishing the flat side makes sense because he can test the whole optics set while he's working on the Schmidt curve, once the flat side has been dealt with.

  Correct. When I tested the telescope via Double Pass Autcollimation  what I saw was  the total  errors at the focal plane. Those errors can come from one or all the optical surfaces. So you have to test a couple surfaces independently to determine what surfaces need work. I tested the primary and it was fine. I then tested the corrector and saw problems on the back and the front. So I need to clean up the back surface to take it out  of the equation. Once the front side of the corrector is smooth I'll reassemble the telescope and test it again using double pass. I'm sure I'll see problems but the hope is  that I'll see either smooth under correction or smooth over correction. If that is the case then I can refigure the secondary to remove those errors and the system will null, meaning all the light will be coming to perfect focus. If I can achieve a null the wave front at the eyepiece will be good to  a 1/10 wave or better and my Dynamax will be a very excellent telescope.

 

                 - Dave  

[Bomber Bob]

Even though it's months away, if you don't mind, Dave, I'll post my corrector corrections here, too.

 

Between now & then, I'll crack open some books, and get a handle on the polishing hardware & techniques.

[Geo31]

Dave, while I'm not trying to suggest that Criterion's process didn't induce errors in the surface, now that we know the side of the glass that was pulled against the master block wasn't optically flat, is it possible that much of the problem stems from this?  I guess we'll know once you get the Schmidt curve smooth., right?

[Jeff B]

So not to get off topic, but it seems to me what Dave is doing can be adapted to making optical windows too, like for a newtonian.

 

Jeff

[DAVIDG]

So not to get off topic, but it seems to me what Dave is doing can be adapted to making optical windows too, like for a newtonian.

 

Jeff

  I had a "student"  last year at the Delmarva Mirror Making class that I help teach  make a 6" optical window using this polishing technique. All the stuff is in books and been known for many many years. 

 

               - Dave 

[DAVIDG]

Dave, while I'm not trying to suggest that Criterion's process didn't induce errors in the surface, now that we know the side of the glass that was pulled against the master block wasn't optically flat, is it possible that much of the problem stems from this?  I guess we'll know once you get the Schmidt curve smooth., right?

 Some of the problem was the quality of the glass surface but not all.  In the test images I posted of my  scope being tested via double pass along with others, they all show ring zones. I have seen them on Celestrons as well. You get these  ring defects  from how the top surface was polished. So they are manufacturing errors as well. 

  Even thou the quality of the glass was a problem it was easy to check. All they had to do is  place the glass against an optical flat and find areas were the glass was flat enough and use that section.  Since one side was going to be ground and polished they could have checked  both sides as well and again found areas on one side or the other that was flat enough.  That would be the least inexpensive way to go or they could have ground and polished one side close to flat on their machine and then used those blanks to make the corrector. 

  This not a Haters comment but what keeps puzzling me is that Criterion was in business of making telescopes and the optics in them since the late 50's so why didn't they check their process of making the correctors to determine what the quality was ? As I'm showing this stuff isn't hard. I'm doing it on my kitchen table and these techniques are "Optics 101" stuff. 

    

                         - Dave 

[deSitter]

I was looking through some old issue of S&T from the mid-late 70s and had forgotten what a blistering pace was being set in optics and gear. Side by side full page ads full of glowing terms for both the Celestron and Dynamax. I believed it all and would have bought a Dynamax based on brand loyalty had I not been just a poor soon-to-be-in-college kid. It went on for months and years. All the kids who grew up in the 50s and 60s had jobs and could buy things, and did. Criterion failed to keep up. It's like a classic case of being penny-wise and pound-foolish. They should have taken a hit on profits, hired more people, instituted strict quality control and let the word spread about the excellence of the product. Instead they just continued to lose ground. Word spreads in these small tight worlds about what is good and what is bluster. Meade picked up the ball that Criterion dropped.

 

-drl

[Bomber Bob]

Dave, I don't consider your honest criticisms of the product & processes as hating on the Dynamax -- ditto for the specific issues posted by other owners, including myself.  Lots of good things we can say about the DX8, but it's only fair to point out where it went wrong - and try to understand why.

 

I had a "student"  last year at the Delmarva Mirror Making class that I help teach  make a 6" optical window using this polishing technique.

 

Too bad we can't all get together for a Dynamax 8 Corrector-Fixing Class!

[Geo31]

 

Dave, while I'm not trying to suggest that Criterion's process didn't induce errors in the surface, now that we know the side of the glass that was pulled against the master block wasn't optically flat, is it possible that much of the problem stems from this?  I guess we'll know once you get the Schmidt curve smooth., right?

 Some of the problem was the quality of the glass surface but not all.  In the test images I posted of my  scope being tested via double pass along with others, they all show ring zones. I have seen them on Celestrons as well. You get these  ring defects  from how the top surface was polished. So they are manufacturing errors as well. 

  Even thou the quality of the glass was a problem it was easy to check. All they had to do is  place the glass against an optical flat and find areas were the glass was flat enough and use that section.  Since one side was going to be ground and polished they could have checked  both sides as well and again found areas on one side or the other that was flat enough.  That would be the least inexpensive way to go or they could have ground and polished one side close to flat on their machine and then used those blanks to make the corrector. 

  This not a Haters comment but what keeps puzzling me is that Criterion was in business of making telescopes and the optics in them since the late 50's so why didn't they check their process of making the correctors to determine what the quality was ? As I'm showing this stuff isn't hard. I'm doing it on my kitchen table and these techniques are "Optics 101" stuff. 

    

                         - Dave 

 

I know you're not hating, and in some measure of fairness, given what you've been telling us about other optical houses and what gets out, I wonder what basics are still being ignored in some shops.

Edited by Geo31, 02 February 2017 - 12:44 PM.

[Bomber Bob]

Here's a thread for discussing optical issues with ALL our Classic SCTs --> http://www.cloudynig...optical-issues/

[ftwskies]

Dave, can you give me a sense of how much glass you're actually removing as you do this?  I'm guessing that the changes are too small to be seen without the special light and grating, yes?  How thin will the corrector be at the middle of the Schmidt curve when you're done?

[deSitter]

Possibly of interest, Gleanings for ATM on  SCT from 1965

 

https://archive.org/...ge/n53/mode/2up

 

-drl

Edited by deSitter, 02 February 2017 - 01:28 PM.

[DAVIDG]

Dave, can you give me a sense of how much glass you're actually removing as you do this?  I'm guessing that the changes are too small to be seen without the special light and grating, yes?  How thin will the corrector be at the middle of the Schmidt curve when you're done?

 I'm taking off molecules of glass. That is why you need to test the flatness using the interference of light vs trying to measure it mechanically.

 

                    - Dave 

[tim53]

 

 

Dave, while I'm not trying to suggest that Criterion's process didn't induce errors in the surface, now that we know the side of the glass that was pulled against the master block wasn't optically flat, is it possible that much of the problem stems from this?  I guess we'll know once you get the Schmidt curve smooth., right?

 Some of the problem was the quality of the glass surface but not all.  In the test images I posted of my  scope being tested via double pass along with others, they all show ring zones. I have seen them on Celestrons as well. You get these  ring defects  from how the top surface was polished. So they are manufacturing errors as well. 

  Even thou the quality of the glass was a problem it was easy to check. All they had to do is  place the glass against an optical flat and find areas were the glass was flat enough and use that section.  Since one side was going to be ground and polished they could have checked  both sides as well and again found areas on one side or the other that was flat enough.  That would be the least inexpensive way to go or they could have ground and polished one side close to flat on their machine and then used those blanks to make the corrector. 

  This not a Haters comment but what keeps puzzling me is that Criterion was in business of making telescopes and the optics in them since the late 50's so why didn't they check their process of making the correctors to determine what the quality was ? As I'm showing this stuff isn't hard. I'm doing it on my kitchen table and these techniques are "Optics 101" stuff. 

    

                         - Dave 

 

I know you're not hating, and in some measure of fairness, given what you've been telling us about other optical houses and what gets out, I wonder what basics are still being ignored in some shops.

 

Remember the RCX400?  Most people don't!  

[orion61]

 

Meade picked up the ball that Criterion dropped.

 

-drl

I am sorry to say The SCT's that meade produced from 1980-about 1985 were no great performers. In fact I haven't looked through a Meade that was made before 1985 that was worth a darn. When Meade introduced the LX3 series is when I saw major optical improvements, but even then there were still dogs. I bought a new Meade 2120 GEM 10" that was beautiful to look "AT" but Stars looked like little puff balls. I sent it back to Meade and they said it was within their specs!  I returned it for a refund at the place where I bought it, minus about $125.00 spent on shipping.

 On the other hand I have always loved the meade form factor and about 6 years ago I found a 2120 LX3 being sold by a trusted friend here on CN. He said it was really good so I bought it. Optically it is not perfect but very good. That is more than I can say about their MCOG coatings. I started a poll and 50% replied thinning or spotty coatings.. I am glad they have their act together, and sad that B&L didn't continue making the Criterion line because their Pro series scopes were as good as any being made at the time.

[earlyriser]

If spending dozens of hours correcting the optics of an old telescope is hate, I'd like to see what love is.

 

I imagine it is very fulfilling to fix an old scope and make it perform to it's maximum capability.  I would also think that observing with it will be more meaningful than with an off-the-shelf scope.  

 

After you fix the curvature of the corrector plate, do you plan to have it optically coated?

[rolo]

 

 

Meade picked up the ball that Criterion dropped.

 

-drl

I am sorry to say The SCT's that meade produced from 1980-about 1985 were no great performers. In fact I haven't looked through a Meade that was made before 1985 that was worth a darn. When Meade introduced the LX3 series is when I saw major optical improvements, but even then there were still dogs. I bought a new Meade 2120 GEM 10" that was beautiful to look "AT" but Stars looked like little puff balls. I sent it back to Meade and they said it was within their specs!  I returned it for a refund at the place where I bought it, minus about $125.00 spent on shipping.

 On the other hand I have always loved the meade form factor and about 6 years ago I found a 2120 LX3 being sold by a trusted friend here on CN. He said it was really good so I bought it. Optically it is not perfect but very good. That is more than I can say about their MCOG coatings. I started a poll and 50% replied thinning or spotty coatings.. I am glad they have their act together, and sad that B&L didn't continue making the Criterion line because their Pro series scopes were as good as any being made at the time.

 

Please don't hate on Meade. There are folks here that love them enjoy them and can't afford anything better. Some even worked there. Please show the same respect that you ask for Criterion to these folks. Please lets not start a Meade SCT hate thread.  It's about the love not the hate     

[Tarzanrock]

Thanks, I wish I had a box of chocolates -- then I would feel even better. 

[tim53]

 

 

 

Meade picked up the ball that Criterion dropped.

 

-drl

I am sorry to say The SCT's that meade produced from 1980-about 1985 were no great performers. In fact I haven't looked through a Meade that was made before 1985 that was worth a darn. When Meade introduced the LX3 series is when I saw major optical improvements, but even then there were still dogs. I bought a new Meade 2120 GEM 10" that was beautiful to look "AT" but Stars looked like little puff balls. I sent it back to Meade and they said it was within their specs!  I returned it for a refund at the place where I bought it, minus about $125.00 spent on shipping.

 On the other hand I have always loved the meade form factor and about 6 years ago I found a 2120 LX3 being sold by a trusted friend here on CN. He said it was really good so I bought it. Optically it is not perfect but very good. That is more than I can say about their MCOG coatings. I started a poll and 50% replied thinning or spotty coatings.. I am glad they have their act together, and sad that B&L didn't continue making the Criterion line because their Pro series scopes were as good as any being made at the time.

 

Please don't hate on Meade. There are folks here that love them enjoy them and can't afford anything better. Some even worked there. Please show the same respect that you ask for Criterion to these folks. Please lets not start a Meade SCT hate thread.  It's about the love not the hate     

 

LOL.  Remember the Burger King jingle from the early 80s?  "Hold the pickle..."?

 

I wrote a Meade version while I was building 2080s there.  Wanna hear it?

[Terra Nova]

I do! Youtube it Tim!

[rolo]

We're gonna get in trouble!

https://m.youtube.co...h?v=KJXzkUH72cY

 

I would love to,hear the Meade version. It's SCT related so....

Edited by rolo, 02 February 2017 - 10:40 PM.

[Tarzanrock]

Well, I had a "Whopper" my way yesterday.  It wasn't any better than any which I have had before and I'm old enough to remember that television commercial.  Go ahead, Tim, sing us the song.