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Optics still pinched?

Smithfire

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Smithfire

Smithfire

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Hi guys

Got an opportunity to star test my Skywatcher 12" dob last night, having loosened the retaining lugs that were indicating a pinched optics pattern. I'd purchased a 5mm eyepiece specifically for this as the dob is an f5. So centered on a bright overhead star and it was still showing the flattened triangular sides??. I'd left the recommended fag paper thick gap between the reatainers and the mirror so I wonder if problem if the tension from the plates on which the mirror sits (one of the adjustment screws is at the end of its play and the other 2 are mid thread) can this pinch the optics?

Just wondered if anybody else has a similar experience?

Many thanks

Clive

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Peter Drew

Peter Drew

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The mirror support plates shouldn't cause deformation of the primary mirror, their prime purpose it to avoid it!. However, optics are finnicky things and all sorts of surprising effects can be produced. The next thing I would try is to rotate the mirror through 90 degrees and see if it has a noticeable effect on the out of focus star image.   :smiley:

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Smithfire

Smithfire

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Thanks Peter, if it was the same would that imply damage to the mirror or something else?

I suppose it cound be the secondary....to find i would have to slacken the 3 allen bolts back to their stops and re collimate.

Clive

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Peter Drew

Peter Drew

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Clive. The triangular star shape you describe is usually indicative of a primary mirror issue and usually caused by pinching or inadequate support, inherent astigmatism in either mirror would give elliptical star images that changed major axis shapes orientation either side of focus.   :smiley:  

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Astrobits

Astrobits

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Not necessarily, Peter. I once produced a mirror that gave triangular star images :eek:  by using bubble wrap as the cushioning when making the mirror and not rotating the mirror. Guess what? Bubble wrap has a triangular pattern of bubbles. And it was a very large mirror nominally 40mm thick!!

Anyway Clive, follow Peter's advice and rotate the primary.  Also check that the mirror supports are free to float and not seized.

Nigel

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Smithfire

Smithfire

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Thanks for the replies guys.

I guess i unclamp then rotate the mirror and then reclamp?

The irony was that even with this pinched pattern I had a fantastic session last night. M42 was beautifully wispy and detailed, and I could split the trapezium without to much fuss.

Clive

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Ibbo!

Ibbo!

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Many many moons ago I had a newt with pinched optics and yes it was the secondary as it was glued down in 3 places.

I lifted it gently and put some felt between the mirror back and the diagonal holder-job sorted

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niallk

niallk

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I see the clips as being a there to simply prevent the mirror falling out. On one scope of mine they are about 3mm away from the mirror. I suspect yours are still too tight.

+1 - the clips on my Obsession are as Moonshane says more 'mirror retainers' and there is around a centimetre of clearance.

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Smithfire

Smithfire

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You could well be right guys. As i mentioned, 3 of the adjustment screws have free play and one is at its stop. I will get the cell out, loosen all the screws and untighten the rubber mirror holders until there is a 1.0mm gap.

Pretty sure it isnt the primary because thats stuck to the mirror holder with glue.

Clive

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Moonshane

Moonshane

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phew. just loosen the clips so there's an obvious but small gap between the rubber and the mirror and then re-insert the mirror 

make sure you leave plenty of time to cool (10 minutes per inch of aperture) and I suspect you'll see an improvement.

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