Double Stars Star Shapes

[TrevorW]

Got some interesting star shapes in corners is this due to double stars or caused through focus, seeing or tilt issue or any other aberration that plagues astro images :)

[joshman]

looks like a combination between being out of focus, plus possible back spacing?

[Dave882]

Hard to say exactly but my guess would be backspacing if in all the corners. If just 1 or 2 corners affected then tilt or collimation. If it’s all over the frame then guiding or more sever issues with the above.

[TrevorW]

usually top and bottom left of image central stars seem fine, thanks :)

[Drac0]

Sounds like it may be sensor tilt if it's all on one side. I don't know what software you use but ASTAP can give you a measure of sensor tilt if present. Other software can too, but ASTAP is what I use.

Cheers,
Mark

[Startrek]

You problem may be a combination of minor issues !!
I’ve been testing my new 8”f5 Carbon fibre Newt over the past 2 weeks and have experienced the similar issues , perfect round stars in the central region and slightly ovoid stars on the left hand side and bottom corner.

A couple of things or a combination resolved the issue -

1/ Found one primary mirror rubber retaining clamp too tight ( so possible pinched optics ) Re adjusted so it can wiggle

2/ Re aligned my Spider vane assembly so each vane is within 0.25mm from centre.Still have a minor issue with the top and bottom diffraction spikes , particularly the top spike , as they don’t tighten up like sharp swords compared to the left and right. I’m sure the spider support holes are not perfectly at 90 degrees or right angles to each other. This does cause an issue with focus drift as it’s noticeable if you pixel peep. Most folk wouldn’t worry but I’m going to work on it.

3/ Rotated secondary mirror slightly to square it up with focuser using a small right angle square

4/ Re collimated scope with Cheshire and Laser

5/ Possible tilt , so I filed my home made 15mm spacer to within 0.30mm discrepancy on all 4 quadrants ( Stefan Buda of Beamtech has posted new machined black anodised 15mm spacers to me , should receive tomorrow)

Performed some Star tests last night under crappy seeing and stars were round to edge of field

Problem solved !!

Oh, also tried different backfocus spacing for my TS Optics GPU coma corrector , 55mm default , 55.5 , 56.00 , 56.5 and 57mm. Didn’t resolve the ovoid stars so the issue wasn’t coma. Left spacing at the recommended 55mm

Cheers
Martin

[TrevorW]

Thanks, I should be able to determine if its a collimation issue easy enough with my RC will do a check when clouds go away :)
see attached full frame

[TrevorW]

image resized to 3200x