pinchedoptics Do I have Pinched Optics?

[kgunessee]

Hello,

I recently purchased a Skywatcher 130PDS and go to use it for the fist time last night, however, all of my stars are triangular. Am I looking at pinched optics here? If so, I can adjust the primary mounting plates to relieve some tension but I want to be sure in case there is anything else at play.

Any help would be appreciated, cheers.

 

[Dave scutt]

When I done a star test I found out I had pinched optics,  instead of concentric rings it was more of a triangle 

[AstroKeith]

Are the mirror clips touching the mirror?

[Peter Drew]

I would say definitely pinched, the mirror is too small to give that appearance due to weight on the support.  Most mass produced reflectors seem to have mirrors overtightened by the manufacturers, possibly to mitigate damage in transit.   🙂

[Adam J]

Yes that is a classic case of badly pinched optics. Your mirror clips are too tight. 

[kgunessee]

Hi guys,

Thanks for the info.

The clips were indeed pretty tight to the mirror. I've loosened them so will test it as soon as it's clear out next!

[powerlord]

yup - they should be loose. i.e. actually allowing the mirror to move. I know that means, if you shoogle it all about you will need to realign it, but even a slight pressure on the mirror will cause pinching.

When I was but toddler in the world of astro (ie. 18 months ago..!), I didn't know better, took my old 200p apart to clean it all, and put it all back together making sure the mirror was screwed in, not too tight..just so it wouldn't move.. turns out.. not the way to do it at all.

stu

[SthBohemia]

On 11/09/2022 at 12:53, Adam J said:

Your mirror clips are too tight. 

^ New one on me! Having made several mirrors over the years and mounted them within home made cells, all of which secured the mirrors quite firmly, the term pinched mirror is new to me... Maybe the Asian mirrors are 2 thin? My latest 8" mirror (nearly finished) is 1.5" annealed pyrex thick and will be very firmly 'bolted' into its cell.

[Skyline]

Those mirror clips can touch the mirror, but without pressure. They just need to be loose on the mirror.

Their usually tight to make sure the mirror does not swing about while the scope is being shipped.

[markse68]

Check the cell isn’t tight on the sides also. Not sure how the 130pds cell works but on my Tal 150p the mirror is shimmed so it can just rotate in its cell. Last night i noticed triangular stars after a couple of hours in the cold and sure enough the cell had shrunk enough to lock the mirror tight (I could no longer rotate it) so i’ll be shaving the 3 shims down a bit tonight!

Mark