Quote:
Originally Posted by Atmos
From what Clive was mentioning in the other tread, it looks like astigmatism in your first one.
From what I’ve read, outside of simple doublet refractors, it’s not unusual to have different infra and extra focal images. The more complicated the optical design, the less likely that they’ll be similar.
|
Cheers Colin. Yeah, I'm of the understanding that these things are never perfect, but I'm not sure what falls within the boundaries of acceptable and normal.
Right now the astigmatism means that unless my focus is dead on, I get elongated stars. I actually thought this was to do with the backlash-not-backlash dec problem I'm having until I captured these (5 second, 2xbinned) images.
I'm also suspicious that the CO issue is having a very slight impact on SGP's AF routine, causing it to get it ever so slightly wrong, thus leading to subs with elongated stars.
Even if AF was perfect so I saw no elongation, my understanding is that astigmatism causes softness in the images. I've been getting *lots* of really bad FWHM subs lately that I was putting down to bad seeing. Basically 3" + has become the norm, whilst it used to be typical for me to get some around 1.8" on the Esprit 120 most nights, with the average probably more like 2.2" . I'm basically wondering how much these optical issues are impacting my in-focus images. 10% increase in FWHM? 20%? 50%?
Quote:
Originally Posted by peter_4059
Have you tried the inside/outside focus test (second set of images) without the paracorr?
|
Cheers Peter. I haven't yet, no. If I did try that and it became more symmetrical, what would that suggest? Incorrect spacing? Poorly behaved paracorr?