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Refiguring a Dynamax 8" Schmidt Corrector

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#76 DAVIDG

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Posted 01 February 2017 - 09:56 AM

 I worked on the corrector again last night for about an hour. It is closer but still has a long way to go. I don't think this thread is going cause anyone to pull out a corrector from their SCT and just polish  away and make a mess of it. I have   been teaching mirror making for 35 years now and that doesn't happen with newtonian mirrors so I doubt it will happen with correctors.  As John Dobson's said "any errors  you  polish in, you can polish out"  So unless you drop the corrector you can fix it. 

   When it comes to double pass autocollimation the result you see  for the errors for the complete telescope are exactly what you see when your testing a spherical mirror. A telescope in theory takes parallel light  and brings it to perfect focus. The wave form it produces is spherical ie all the light comes to focus at the same point. It is same as testing a spherical mirror at the radius of curvature which will cause all the light to come to perfect focus. So if you look in any good book on making mirrors they will show you  what  these errors look like when testing a spherical mirror.  These errors will look the  same  and be the same when you test a complete telescope via double pass autocollimation. So a turned edge will look the same, or undercorrection or overcorrection will look the same. If you what the errors are then you understand what you need to do fix them.

     As for collimation and misunderstanding the results, that can't happen. Misalignment of the optics is very easy to detect because you don't get a symmetrical pattern left to right and up and down. So anyone that has any experience in making optics will know this and anyone that is trying this method out for the first time I hope would do their homework on how it works and what you should see. 

   Check out the Star Test thread in the ATM forum that is active right now. http://www.cloudynig...-test-question/ The author started out questioning  if he was doing the star test correctly and a number of us told him how to do double pass autocollimation using a pan of oil. He  went from believe that he had a good lens to determine very quickly that his lens had a major issue and was able to get a replacement . So within a short time he went from little understand to knowing how to do this test. 

 

                 - Dave 


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#77 highfnum

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Posted 01 February 2017 - 09:59 AM

I got a dy8 and c8 both from 70's period dx8 79 c8 a bit older
I wonder if I should do swap corrector thing for the hell of it
as experiment
its just I gotta re-collimate the whole thing several times ---eeech!
I will measure actual plate diameters and see if they will fit or if one
will simply fall thru!

#78 DAVIDG

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Posted 01 February 2017 - 10:08 AM

I got a dy8 and c8 both from 70's period dx8 79 c8 a bit older
I wonder if I should do swap corrector thing for the hell of it
as experiment
its just I gotta re-collimate the whole thing several times ---eeech!
I will measure actual plate diameters and see if they will fit or if one
will simply fall thru!

 Your wasting your time. The designs are different, meaning the corrector power is different, the spacing are different, it won't work. 

 

                    - Dave 


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#79 Darren Drake

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Posted 01 February 2017 - 10:33 AM

This thread is interesting because I don't believe any amateur has ever successfully figured a Schmidt corrector. And since it is being attempted without using the vacuum method as far as I know this could be a first if successful. Correct me if I'm wrong here. Thanks
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#80 highfnum

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Posted 01 February 2017 - 11:12 AM

ok Dave noted will not proceed

#81 highfnum

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Posted 01 February 2017 - 11:15 AM

This thread is interesting because I don't believe any amateur has ever successfully figured a Schmidt corrector. And since it is being attempted without using the vacuum method as far as I know this could be a first if successful. Correct me if I'm wrong here. Thanks

I believe Dave Daly up in stellafane made a SCT or SC in hartman museum.
not 100% sure
scsmall.jpg

Edited by highfnum, 01 February 2017 - 11:23 AM.

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#82 Geo31

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Posted 01 February 2017 - 11:27 AM

I'd be very surprised if there has never been an amateur who has successfully produced and figured a Schmidt system and I think Dave has already alluded to that.  My little monkey brain has recollections of reading about them in S&T in the past.

 

Related to Dave Daly above, is (was) he a pro or an amateur?


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#83 highfnum

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Posted 01 February 2017 - 12:12 PM

don't know what he did for living 

I meet in in 2012 in Vermont and it was his turn for museum run

I got personal tour with him

 

that museum is well worth it - you gotta set up invite time with them before hand to get in  

 

got to see dr henry paul's monochromater

 

and other cool 1 of a type stuff 



#84 ftwskies

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Posted 01 February 2017 - 12:13 PM

Here are some old S&T articles about SCTs made by amateurs:

 

https://archive.org/...ge/n45/mode/2up

https://archive.org/...ge/n37/mode/2up


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#85 DAVIDG

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Posted 01 February 2017 - 12:22 PM

 There have been many amateurs that have made Schmidt corrector both using the classic vacuum pan method and also by grinding the curve into a flat plate using specially shaped laps.  Sky and Telescope has published a number of article over the years them. The earliest article was one in the 30's and I believe Scientific American publish a few as well. 

   The small Schmidt camera in the Stellafane museum pictured in  Highfnum picture was made by my good friend and Schupmann expert Jim Daley. Jim ground the curve into a flat piece of glass by hand. He told me that he used a small, thin and round microscope cover slide as the tool. He worked the 86% zone of the plate which is the neutral zone using the cover slide on the end of his finger until it wore thru. When it did he knew he had removed the corrector amount of glass.  Jim has also made made many corrector using the vacuum method. So has Bob Pfaff and Gerry Logan http://www.considine...pfaff/pfaff.htm

 

        - Dave 


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#86 starman876

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Posted 01 February 2017 - 01:04 PM

I thought that was the case. I remember reading about those guys years ago.  Also, it would seem reasonable that amatures would have made the first corrector plates before any commercial production would be attempted. That is how all of our business starts in the beginning. 



#87 tim53

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Posted 01 February 2017 - 01:18 PM

This thread is interesting because I don't believe any amateur has ever successfully figured a Schmidt corrector. And since it is being attempted without using the vacuum method as far as I know this could be a first if successful. Correct me if I'm wrong here. Thanks

Gerry Logan and Bob Pfaff are regular attendees at RTMC every year.  One or both of them enters a telescope in the Merit Award judging pretty much ever year.  They've both made multiple SCTs.

 

-Tim.


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#88 deSitter

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Posted 01 February 2017 - 01:32 PM

Here are some old S&T articles about SCTs made by amateurs:

 

https://archive.org/...ge/n45/mode/2up

https://archive.org/...ge/n37/mode/2up

Holy cow the entire archive is there!! Thanks!

 

-drl


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#89 ftwskies

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Posted 01 February 2017 - 01:59 PM

 

Here are some old S&T articles about SCTs made by amateurs:

 

https://archive.org/...ge/n45/mode/2up

https://archive.org/...ge/n37/mode/2up

Holy cow the entire archive is there!! Thanks!

 

-drl

 

I know, right?  It's pretty cool.  My lunch breaks have never been less productive than they are since I found those!  :lol: I forget which CN thread someone posted a link to this archive on -- it was another recent thread about Schmidt correctors, though.  Probably from a few months back. 


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#90 highfnum

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Posted 01 February 2017 - 03:12 PM

davidg thx my memory ain/t that bad it was daley! got first name wrong!



#91 Garyth64

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Posted 01 February 2017 - 04:09 PM

Just a funny thought . . .  for "us" telescope makers, we really cannot figure in the cost per hour, when we grind a mirror. 

 

For the experts, their hours would be much lower.  For me, I would already have $300 in a 4-1/4" mirror, and I'm not done yet! :)

 

So part of the making is our own personal time/enjoyment.


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#92 orion61

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Posted 01 February 2017 - 05:54 PM

Is it worth it to take a good SCT and gut it to fix a DX8??   

I tried putting the optics from a good 2080 Meade into a Dynamax 8" tube. The components fit but it would not come to focus. You would need to cut the tube down about an inch or so.

I actually have a C8 Mirror and secondary with a cloudy Corrector that would be a perfect project for this..


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#93 Terra Nova

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Posted 01 February 2017 - 07:36 PM

If Emmett Lathrop Brown can transform a DeLorean into a time machine in his garage, then anything is possible!


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#94 kansas skies

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Posted 01 February 2017 - 07:41 PM

If Emmett Lathrop Brown can transform a DeLorean into a time machine in his garage, then anything is possible!

For some reason, when I read this I immediately thought of Robert and his Tinsley restoration...

 

Bill


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#95 DAVIDG

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Posted 01 February 2017 - 10:40 PM

 Things are starting to move in the right direction. When you polish a surface that your trying to make flat, it seems like not much happens for a good while  and the fringe pattern doesn't change by much. You have to  just keep polishing and then all of sudden the surface starts to drop in. That has started to  finally happen. So after about 4 hrs of polishing the fringes have all turned into arcs  and are all going in the same direction.  So the surface is starting to smooth out and become uniform. I have many of them but what  should happen is that  the fringes will move farther apart, become straighter and there will be fewer of them as the surface continues to flatten out and match the surface of the Master flat I'm testing the corrector against. I'm going for  5 fringes that are mild arcs. That will be flat enough to show that the surface is optically  smooth and close enough to optically flat that it won't add any error to the wave form  that is formed by the front surface that has the Schmidt curve.

   Here are two pictures, one is  a close up of the fringes and other shows the whole corrector on the flat.  

 

                     - Dave 

 

4hrfiguring.jpg

4.5hrsfiguring.jpg


Edited by DAVIDG, 02 February 2017 - 11:33 AM.

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#96 Mitrovarr

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Posted 01 February 2017 - 10:45 PM

Oh, wow! That looks drastically improved!


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#97 rolo

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Posted 02 February 2017 - 12:04 AM

Dave , that's impressive. Do you think the other side will require as much time to clean up or correct the issue?



#98 Gil V

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Posted 02 February 2017 - 06:07 AM

I think you are going to see a dramatic improvement just by polishing the flat side. That is the side that we didn't touch. The shaped side was polished at the plant.

That's my guess.
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#99 Bomber Bob

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Posted 02 February 2017 - 07:37 AM

Polishing the flat side sounds like a good mid-summer project for me, and I would think any improvement to the corrector would be worthwhile.  The torrid heat keeps us indoors, and I can minimize the mess by setting up in my man cave.  Thanks again, Dave!   You've given us Dynamax owners hope & inspiration -- and an illustrated how-to guide.


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#100 rolo

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Posted 02 February 2017 - 08:37 AM

I think you are going to see a dramatic improvement just by polishing the flat side. That is the side that we didn't touch. The shaped side was polished at the plant.

That's my guess.

I think it may be the opposite.




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