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Refiguring a Dynamax 8" Schmidt Corrector

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#26 highfnum

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Posted 30 January 2017 - 03:21 PM

DaveG after your done with corrector do you plan to put it all back together and test to see if "that's good enough"
or to see what that did in terms of noise removal(my term for roughness like Db noise in a way optically)

or will you move directly to refigure secondary

#27 Mitrovarr

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Posted 30 January 2017 - 03:34 PM

I would also be really curious how much improvement was gained by cleaning up the flat side of the corrector. That sounds like a much, much easier fix than fixing the curved side.

#28 DAVIDG

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Posted 30 January 2017 - 03:53 PM

Much of the roughness I'm seeing are  circular zones and astigmatism. Most of that is on  the Schmidt curve side of the corrector.  You can see some of these major defects in the interference pattern at the 12, 3, 6, 9 o'clock positions.  So fixing the backside isn't going to totally fix the problem. Both surfaces need to be cleaned up if I want a  really good system. 

    I plan to reassemble and test after the flat side is fixed, then the Schmidt curve and finally to refigure the secondary. 

 

     

                        - Dave 


Edited by DAVIDG, 30 January 2017 - 03:55 PM.

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#29 Mitrovarr

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Posted 30 January 2017 - 04:17 PM

I think it'd be great if there's a way to fix these things. Even if it's out of reach for me, I'd feel much better about transferring my Dynamax to someone who planned to fix it, rather than either dumping it on some poor newbie or throwing it away.
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#30 highfnum

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Posted 30 January 2017 - 04:19 PM

Ok so your doing in 3 steps then reporting results do you think it's possible you may be able skip secondary refigure?



#31 DAVIDG

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Posted 30 January 2017 - 04:39 PM

Ok so your doing in 3 steps then reporting results do you think it's possible you may be able skip secondary refigure?

 I don't think so for two reasons.  Besides the optical roughness and astigmatism that is partially or fully coming from the corrector I'm also see a turned edge and over correction when I tested the complete system via double pass. Those errors are most likely coming from the secondary since when I tested the primary I saw nothing wrong with it.  Also when I clean up the corrector, even when both sides are optically smooth that doesn't mean that it is  fully correcting the spherical aberration from the primary the amount called for in the design.  So the corrector can be still be  a little under or over corrected. So by figuring the secondary I can take that  out as well. Like I keep saying a multi-surface optical system is just like an electronic circuit. In the circuit all the parts have some level of error in them and you make one or more variable to tune out the errors from all the rest. The same is true in optics, you are going to have slight errors in all the surfaces and also in all radii,  so you pick one and change it's figure slightly from the ideal design specs  it to correct for all the rest of the errors. 

 

             - Dave 


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#32 SteveNH

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Posted 30 January 2017 - 04:57 PM

Dave,

 

I am very curious as to how you push the thin corrector across the pitch lap as you do your strokes. With a conventional mirror or tool, you have the thickness to insulate the heat from your hands away from the work surfaces, and to prevent warpage from uneven hand pressure on the back. Do you use any insulating pads or friction pads or gloves when you push the corrector across the lap, or just bare hands directly on the corrector plate (and where do you place your hands on the back of the plate?)?

 

Thanks for such an informative post.

 

Steve



#33 starman876

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Posted 30 January 2017 - 05:02 PM

would it not be nice that like in electronic circuits you could just adjust and go with a turn of a pot or an adjustable coil.  If only you did not need to re apply the coating on the secondary.  Do you need to completely strip the coating before you test. I guess you would have to because the coating could introduce errors.  


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#34 DAVIDG

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Posted 30 January 2017 - 05:12 PM

Dave,

 

I am very curious as to how you push the thin corrector across the pitch lap as you do your strokes. With a conventional mirror or tool, you have the thickness to insulate the heat from your hands away from the work surfaces, and to prevent warpage from uneven hand pressure on the back. Do you use any insulating pads or friction pads or gloves when you push the corrector across the lap, or just bare hands directly on the corrector plate (and where do you place your hands on the back of the plate?)?

 

Thanks for such an informative post.

 

Steve

  I just use  my thumb and index finger on each the side of the plate and left up slightly. Been making award winning lens this way for years. 

            

                                   - Dave 


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#35 SteveNH

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Posted 30 January 2017 - 06:58 PM

Interesting! Thank you!

 

Steve



#36 rolo

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Posted 30 January 2017 - 09:54 PM

Much of the roughness I'm seeing are  circular zones and astigmatism. Most of that is on  the Schmidt curve side of the corrector.  You can see some of these major defects in the interference pattern at the 12, 3, 6, 9 o'clock positions.  So fixing the backside isn't going to totally fix the problem. Both surfaces need to be cleaned up if I want a  really good system. 

    I plan to reassemble and test after the flat side is fixed, then the Schmidt curve and finally to refigure the secondary. 

 

     

                        - Dave 

Looks like the correctors can easily be tested with relatively inexpensive tools. How Criterion could have missed this issue is unexplainable.


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#37 Bomber Bob

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Posted 30 January 2017 - 10:05 PM

Criterion may have tested the correctors, and made a corporate / sales decision to ship the scopes out anyway.  Lots of companies ship products with known issues (that should be recalled), and find it cheaper to address the few complainers -- and / or lawsuits.


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#38 Mitrovarr

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Posted 30 January 2017 - 10:25 PM

Yeah, there's no way they didn't know the scopes weren't good. Five minutes with an artificial star will show it, at least on mine.
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#39 TG

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Posted 30 January 2017 - 10:40 PM

 Thanks everyone who "like" this thread. I have 70 "likes" so far !  Please note that I am posting images of the test results so everyone can see what I'm talking about vs saying that because I have been observing for 35 years I know good optics from bad and  because of this I can  stating the quality of the optics I'm using.

   The stroke I'm using  to polish the back surface against the pitch lap is the  classic "W" with about 1/2" of the corrector over hanging the pitch lap as I push the  corrector  over the lap in making a 'W". The green light panel is an UniLamp, Edmund sells them  for $425  dollars http://www.edmundopt...tic-lamps/1659/ but again Ebay to the  rescue.  I got a couple of them and paid under $30 each. You can make your own. All it is are two uncoated UV fluorescent bulbs. There is Mercury in the bulbs and it has a very strong emission at 546nm which is bright green. There is a  piece of green plexiglass to isolate the green emission from the other wavelengths  and a white piece of plexiglass to act as a diffuser. You can also use a cheap CFL bulb.  

 

              - Dave 

Dave,

 

Do you have a source for uncoated mercury fluorescent tubes? The ones sold as UV black lights have black coating to absorb pretty much all colors except violet and UV. While the violet can be used to see fringes, it's very hard to see due to the eye's low sensitivity to violet and perhaps weak emissions from the tube as well.

 

I remember looking at a mercury lamp via a spectroscope as an undergrad and the blue-green emission line was simply beautiful.

 

Tanveer.



#40 DAVIDG

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Posted 30 January 2017 - 10:43 PM

 I spent another evening polishing away. It is getting there but it is slow going. The flat surface is 50 wave or more from flat and a lot of glass needs to be removed. Here is a picture of the latest progress. There are fewer fringes and they are farther a part.

        

                          - Dave

 

3hoursfiguring.jpg


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#41 Tarzanrock

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Posted 30 January 2017 - 10:47 PM

They didn't miss it.  Modern failure analysis testing requires such.  That type of design failure analysis has been well known and it is in the scientific design literature for more than 100 years before Criterion even began manufacturing its DX8.  Every college educated engineering student studied it in first year collegiate engineering classes since at least the 1920's.  They just made a deliberate corporate financial decision to ignore the deficiencies and market the defective instrument.  Even after the Pinto litigation, major American automobile manufacturers still choose to do the very same thing -- market easily correctable dangerously designed defective products -- it has become a business practice in modern American manufacturing.


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#42 starman876

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Posted 30 January 2017 - 10:49 PM

Dave I put a post in the Iphone colimation section about my AC experiment.  Not sure if I am doing this properly. Please take a look at the post.

 

Thanks

 

Johann



#43 DAVIDG

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Posted 30 January 2017 - 10:54 PM

 

 Thanks everyone who "like" this thread. I have 70 "likes" so far !  Please note that I am posting images of the test results so everyone can see what I'm talking about vs saying that because I have been observing for 35 years I know good optics from bad and  because of this I can  stating the quality of the optics I'm using.

   The stroke I'm using  to polish the back surface against the pitch lap is the  classic "W" with about 1/2" of the corrector over hanging the pitch lap as I push the  corrector  over the lap in making a 'W". The green light panel is an UniLamp, Edmund sells them  for $425  dollars http://www.edmundopt...tic-lamps/1659/ but again Ebay to the  rescue.  I got a couple of them and paid under $30 each. You can make your own. All it is are two uncoated UV fluorescent bulbs. There is Mercury in the bulbs and it has a very strong emission at 546nm which is bright green. There is a  piece of green plexiglass to isolate the green emission from the other wavelengths  and a white piece of plexiglass to act as a diffuser. You can also use a cheap CFL bulb.  

 

              - Dave  

Dave,

 

Do you have a source for uncoated mercury fluorescent tubes? The ones sold as UV black lights have black coating to absorb pretty much all colors except violet and UV. While the violet can be used to see fringes, it's very hard to see due to the eye's low sensitivity to violet and perhaps weak emissions from the tube as well.

 

I remember looking at a mercury lamp via a spectroscope as an undergrad and the blue-green emission line was simply beautiful.

 

Tanveer.

 

 You want to look for BL blacklight  fluorescent tubes  vs the BLB ones. 

 

                             - Dave 



#44 terraclarke

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Posted 31 January 2017 - 07:15 AM

 I spent another evening polishing away. It is getting there but it is slow going. The flat surface is 50 wave or more from flat and a lot of glass needs to be removed. Here is a picture of the latest progress. There are fewer fringes and they are farther a part.

        

                          - Dave

 

attachicon.gif3hoursfiguring.jpg

I bet they used just crummy, cheap window glass! And it almost looks like it was cut with a hardware store glass-cutter!


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#45 DAVIDG

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Posted 31 January 2017 - 10:03 AM

 

 I spent another evening polishing away. It is getting there but it is slow going. The flat surface is 50 wave or more from flat and a lot of glass needs to be removed. Here is a picture of the latest progress. There are fewer fringes and they are farther a part.

        

                          - Dave

 

attachicon.gif3hoursfiguring.jpg

I bet they used just crummy, cheap window glass! And it almost looks like it was cut with a hardware store glass-cutter!

 

 The key is that it the glass sheets need to be "float glass" which is  made by  floating the glass sheet on molten tin. You'll get areas that can be very optically flat. So you need too scan a large sheet  and use the areas that are good. If not  grind and  polish one side flat or  put  the Schmidt curve on both side but with the 1/2 the strength. 

   Gil said in one of his posts that they would uses a diamond wheel to scribe a circle and break the glass away then clean up the edges against a grinder. 

   When you make a corrector via the Master Block method, the glass disk that will become the corrector is pulled down against the Master Blocks surface. So in theory the  surface of the glass corrector blank that is against the Master surface needs to be optically smooth. If not any defects in the bottom  surface are transferred to the top surface. So say you  have a high spot of the surface that is against the Master, that will show up as high spot on the top surface that will be ground flat. When the vacuum is released and  the corrector springs back, on the Schmidt corrector side you  will now have a low spot. 

  If you compare the pictures of the "flat" side on my corrector and the top side with the Schmidt curve, you'll see 4   low areas on the Schmidt curve at 12, 3, 6 , 9 o'clock and on the flat side surface there are four independent areas as well. So these defects transferred from the back surface to the front surface when the plate was pulled against the Master. 

 

                  - Dave 


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#46 orion61

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Posted 31 January 2017 - 12:33 PM

It would be kind of annoying if it wasn't the shaped side of the corrector that causes the problems, but the flat side. I can imagine it being difficult to make the curved side, but messing up the flat side seems like pure carelessness.

Criterion did not touch the flat side of the corrector, only the Curved side. The flat side came from the mfgr that way. and it seems is 1/2 of the problem.

But this is an excellent post. It is informative and may answer a lot of questions about what went wrong in the execution of these telescopes.

It may also open the door for someone to correct these for people like me that like other features of the system. I would be happy to pay for such a service.


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#47 Bomber Bob

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Posted 31 January 2017 - 12:46 PM

I would be happy to pay for such a service.

 

Same here.  There are lots of things about the DX8 to like.  IF the solution involved just the flat side, I'd probably give it a go myself.  But working curves on an old piece of thin glass -- I don't think there's enough Scotch in Scotland to keep my nerves steady...


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#48 tim53

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Posted 31 January 2017 - 12:58 PM

This is heartening.  I've never owned a Dynamax, but do like that they're more reminiscent, estheticallly, of the deluxe dynascopes than they are of the RV-6.  And while it's too bad the optical quality wasn't good, the time may be right for someone interested in preserving them for future generations to make some of them worth seeking out (with test results to confirm optical quality).

 

 

-Tim.


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#49 Mitrovarr

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Posted 31 January 2017 - 01:26 PM

I wonder if you could just replace the Dynamax corrector with a Meade or Celestron one? I mean, they seem like the same optical design.
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#50 rayden68

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Posted 31 January 2017 - 01:36 PM

Outstanding work. This post could be a PBS special. I have learned a lot. Its good to see so many posts now where contributors are actually showing the picture of the optical test so you can see the problems with the optical system.
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