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SN10 took a fall and secondary glass shattered

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#1 Redpog01

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Posted 16 September 2020 - 10:39 PM

My SN10 took a fall and the glass for the secondary mirror shattered. After some searching I found a shop that wa able to make replacement glass for me. I have gotten it fully reassembled all the mirrors seem to be lined up, however I can get it to focus. I have to extend the eyepiece out much further than the focused extended and than its a nice clear picture. I have used http://www.weasner.c...t_Newtonian.pdf to try and get it lined up/collimated properly. Does anyone have any idea how I can get the focus back to the right area?


Edited by Redpog01, 16 September 2020 - 10:50 PM.


#2 TOMDEY

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Posted 16 September 2020 - 11:09 PM

My SN10 took a fall and the glass for the secondary mirror shattered. After some searching I found a shop that was able to make replacement glass for me. I have gotten it fully reassembled all the mirrors seem to be lined up, however I can get it to focus. I have to extend the eyepiece out much further than the focused extended and than its a nice clear picture. I have used http://www.weasner.c...t_Newtonian.pdf to try and get it lined up/collimated properly. Does anyone have any idea how I can get the focus back to the right area?

Are you describing the round window thing up front? That's a Schmidt Corrector Plate, not a flat window. If you had a window cut out of glass --- that's not going to work. Please clarify!    Tom


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#3 Redpog01

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Posted 17 September 2020 - 04:25 PM

Tom,

Yes it was the glass that holds the secondary mirror that shattered, it was heart wrenching. How does the Schmidt correct plate function? I just assumed the glass was to remove the veins to support the secondary mirror. My problem seems to be that my point of focus is out about 1/2 an inch further than the focuser allows. I have pulled the mirror as far towards the back of the scope as possible and re-collimated.  



#4 Redpog01

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Posted 17 September 2020 - 04:27 PM

Answered my own question- https://en.wikipedia...corrector_plate



#5 KBHornblower

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Posted 17 September 2020 - 05:42 PM

I would be surprised if substituting flat glass for a Schmidt corrector moves the focal point that much.  It will give you a mushy image, because the Schmidt corrector corrects the aberration of a spherical mirror.


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#6 Jon Isaacs

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Posted 17 September 2020 - 05:53 PM

Answered my own question- https://en.wikipedia...corrector_plate

 

:ubetcha:

 

I'm sad that  your corrector plate was broken. You could contact Meade to see if the can help but I'm doubtful.

 

Jon



#7 osbourne one-nil

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Posted 17 September 2020 - 06:26 PM

That's bad news - I feel your pain. I know Ed Ting has one as he reviewed it on his new YouTube channel recently. Perhaps if you ask nicely....



#8 TOMDEY

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Posted 17 September 2020 - 08:57 PM

I would be surprised if substituting flat glass for a Schmidt corrector moves the focal point that much.  It will give you a mushy image, because the Schmidt corrector corrects the aberration of a spherical mirror.

There is a whole menu of acceptable Schmidt Corrector Plates. The only one with zero focus shift is what the literature typically shows, cartoon where the quartic and quadratic terms cross over. In particular, the first one in the list below. But here are some of the other acceptable preferences, in various ways optimum:

 

>pure Zernike Primary Spherical (zero focus shift solution)

 

>minimum removal solution (from initial plano-plano true window)

>minimax slope (which minimizes some chromatics)

>minimum residual design wavefront PV

>minimum wavefront RMS

>tightest geometric impulse response

>most vacuum mandrel compliant

>monotonically increasing slope solution(s)

>as above with one zone where the second moment vanishes

 

Most always, the designer finesses only the quadratic and quartic terms (aka blend of Zernike Power and Zernike Primary Spherical)... with the other radially-even Zernikes left out as confusing, insignificant, and difficult to execute. So, if that Zernike Power term is non-zero... the focus will indeed shift, by a readily computable amount, if the plate is removed, or replaced with a plano-plano window. This is clearly what happened when Red inadvertently replaced the Schmidt Plate with a plain window.    Tom




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