Quote:
Originally Posted by alpal
Hi Lee,
2.7 to 2.5 arc seconds FWHM is fairly good -
so yes it should have looked slightly sharper.
Why have you had optical issues lately - what's changed?
cheers
Allan
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To be honest 2.7 - 2.5 is very soft and at 0.5"/px I wouldn't expect it to look any sharper than it does.
Not sure re the optical issues. I've thrown out tonnes of data due to elongation... it's not the mount, tracking/guiding is great.
Elongation exists in very short (i.e. 5 second) exposures, and
was flipping 90 degrees either side of focus... classic astigmatism. Most recently it's very elongated on
one side of focus, and much more round on the other side, which I don't understand.
I've found that a mere 8-16 microns of focus is the difference between eccentricity of 0.6 and 0.4 and SGP just doesn't get it that close most of the time.
From the testing I've done it looks like either issues with the secondary, or the primary cell. Didn't rotate the the primary mirror, nor with the camera+coma corrector, so the things
that stayed stationary are suspect (mostly the secondary, secondary mounting and primary cell). Could be the adhesive they used to stick the secondary on has changed over time. Could just be that thick chunk of aluminium that is the secondary stalk is changing temperature at a different rate to the secondary and deforming it.
I've ordered a new primary cell but it hasn't left Germany yet. Will try that first but to be honest I don't think the problem is in the primary cell... there's been a couple of minor things bothering me about it though so it was a good excuse to get a new one.
If the new primary cell doesn't resolve it, I'm going to try re-gluing the secondary with some carbon fibre in between the mirror and the stalk.
Since the optics were tested by TS before they sold it to me and I didn't have the issues in the past, I don't think it's anything inherent in the mirrors... but yeah, not 100% sure what it is yet.