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Star-testing MTS-SN6

catadioptric Meade
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#26 DAVIDG

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Posted 08 August 2020 - 10:08 AM

 The mirror is  a pretty good sphere, a bit rough from the machine polishing but nothing that is causing the problems in the star test.  So, that leaves the secondary and the corrector for causing the overcorrection. A  optical flat has no optical power so it can't add spherical aberration. If the surface isn't flat it will add astigmastism.  So that leaves the corrector like you said that is the problem.

   Tim's  sugguestion to do double pass autocollimation is good one. That tests the complete system. There is another test  you can do to check the optical smoothness of the corrector. You can place it a few inches in front of the spherical primary and Foucault test the assembly. It doesn't matter which side faces the mirror so you can flip it around so the diagonal is facing away from the mirror  If the corrector was perfect you would see smooth surface that tests like a parabolic mirror ie showing over correction but again you'll get an idea of how smooth the corrector is. It is best to test it with Ronchi screen since the bowing of the lines is easier to understand what the shape is and if there are problems.

   A  note on your Foucault test set up.  It is better to have the light source and return image as close as possible. If not your introducing astigmatism into the results because you are  off the optical axis. So you should raise up your frame so the LED is close to the center of the camera lens. Also I can't tell from the picture but the knife edge should cover about 1/2 the LED. Again this puts the light source and the return image closer to each other and closer to the optical axis.

 

    Here is a picture of corrector I tested this way. It shows astigmatism from "S" shape of the ronchi band in the middle 

 

 

                   - Dave 

scttest1.jpg

 


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#27 davidc135

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Posted 08 August 2020 - 01:34 PM

It's always likely to be the corrector. Plenty of over-correction, maybe 1/2 wave. If you were into optical work the mirror could be under-corrected to compensate but the off axis images may suffer. Otherwise one side of the corrector could be refigured which is a bit trickier but either way there isn't much work.

 

Still, it seems to work well as it is so long as the mag isn't too great.

 

David


Edited by davidc135, 08 August 2020 - 01:34 PM.

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#28 AlMuz

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Posted 08 August 2020 - 03:14 PM

 

   A  note on your Foucault test set up.  It is better to have the light source and return image as close as possible. If not your introducing astigmatism into the results because you are  off the optical axis. So you should raise up your frame so the LED is close to the center of the camera lens. Also I can't tell from the picture but the knife edge should cover about 1/2 the LED. Again this puts the light source and the return image closer to each other and closer to the optical axis.

Yeah, I understand what problems can bring some off-axis measurement.

But wanted to leave some aperture space to compfortably insert Ronchi-grating printed out with InkJet on clear plastic film.

The large aperture is not needed for a test itself, but rather not to bent plastic film in the area of beam section when pinched on the sides by mounting screws.

However this task my InkJet printer miserably failed - tried to do 5 lines/mm but got just a gray square without distinctive lines (guess it is more of a plastic film issue).

So ordered some cheapo 50 lines/mm diffraction grating from eBay - quite a bit tight, but maybe it will work.

 

 

It's always likely to be the corrector. Plenty of over-correction, maybe 1/2 wave. If you were into optical work the mirror could be under-corrected to compensate but the off axis images may suffer. Otherwise one side of the corrector could be refigured which is a bit trickier but either way there isn't much work.

 

Still, it seems to work well as it is so long as the mag isn't too great.

 

Unfortunately, my current arrangements do not allow me any kind of optical manufacturing/re-figuring setup. 

The scope is good enough for majority of non-sophisticated observers. Buying it was more of a "restoration project" for me.

So after cleaning up and testing I will probably put it back on Cragslist or give-away to a friend.


Edited by AlMuz, 08 August 2020 - 03:22 PM.

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