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Astigmatism- What happened here?

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#1 cloud_cover

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Posted 06 March 2013 - 09:33 PM

I finally got to first light the scope last night but to my horror even at 50x the mirror was severely astigmatic. Sirius would not cone to focus! To back up, I had disobeyed Don's advice and wedged the mirror into the rubber padding of the mirror clips figuring that if all the previous owners did it... So the astigmatism was all my fault. What floored me was that when I loosened EVERYTHING, the astigmatism stayed. After a lot of fiddling and not solving for it, I rotated the mirror to see if the astigmatism changed directions. I though it did so I gave the mirror a really good rotation in its cell and to my surprise,the astigmatism was severely diminished. Another few gentle shakes of the mirror and some rotation and the astigmatism was gone!
Apart from the relief and hoorays, can anyone tell me why it required rotating and gently agitating the mirror to remove the astigmatism instead of simply loosening everything?
The mirror is a plate glass substrate 14" f4.7 with good figure. Its the tropics so the difference between ambient and indoors temp is virtually nil since we use "natural ventilation"
Thanks!

#2 piaras

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Posted 06 March 2013 - 09:54 PM

I think, and others can correct me, the wedges had placed a strain on the mirror and it just took a bit of time to relax back to it original shape.
Pierre

#3 nirvanix

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Posted 06 March 2013 - 10:04 PM

Many mirrors have their performance ruined by improper mounting - both primary and secondary. Glad you got it sorted.

#4 Mirzam

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Posted 06 March 2013 - 10:40 PM

By rotating the primary you may have removed an uneven force on the mirror edge that caused the astigmatism.

Or the secondary has an opposite astigmatism and by rotating the mirror you managed to cancel both defects. (not likely).

JimC

#5 cloud_cover

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Posted 06 March 2013 - 10:42 PM

I think, and others can correct me, the wedges had placed a strain on the mirror and it just took a bit of time to relax back to it original shape.
Pierre

You mean to say my mirror needed a good massage to destress? :grin:

#6 GlennLeDrew

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Posted 06 March 2013 - 10:57 PM

My first thought was that the secondary is astigmatic, and the primary's astigmatism was nulled when rotated appropriately. In such a case, rotating the primary 90 degrees from its nominal position will result in astigmatism twice that introduced by each mirror alone, due to the mutual reinforcement.

#7 piaras

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Posted 06 March 2013 - 10:58 PM

Now for something completely different, the follow up. I would place a mark on the side of the mirror so that you can make sure to be always setup with that alignment and now you can rotate the mirror 90 degrees and recheck. Still gone or is the stig back again? Was it just a bad placement or the mirror really is stiggy? Further checks are required.

I am building a new cell for my dob, as I am waiting for a new mirror. Meanwhile the old cell was causing a lot of stig due to the hold down system. I changed to a cable sling as an experiment and things are so much better even with the old cell.
Pierre

#8 johnnyha

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Posted 07 March 2013 - 01:56 AM

It takes awhile for the mirror to cool down and as stated previously, to relax from pinching. The rubber pads and clips should not be anywhere NEAR the mirror, forget even the "business card width" garbage, adjust them WELL AWAY from the mirror, they should only be there to keep the mirror from falling out of the tube and they serve no other purpose - the mirror should be floating on the cell.






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