Questions about guiding, coma and collimation

[thegableguy (Chris)]

Sorry to lump two different questions in together - seemed fairer than starting two new threads.

What is considered a good benchmark for successful guiding / alignment? I'm halfway through tonight's capture and all adjustments seem to be under 1", with maybe one little spike per minute on average going just beyond 1". Is that good, average or bad? It's the best I've managed so far but maybe I can do better...? I haven't touched any of the settings in PHD2, it's all just default settings (mainly because I don't know what any of the settings actually do).

Secondly, can someone help tell me what the results look like when a reflector needs collimating, and how they're different to having an improperly spaced coma corrector? Does one produce stretched stars all over, and the other only at the edges? Or does one create yellow haloes around stars and the other not? Or does improper spacing basically mean you can't focus properly? Any help welcome.

[glend (Glen)]

I can not comment on PHD because i don't use it.
Re collimation, have you done a star test to check if it is out? Simply pick a star, centre it, and wind the focuser outward until you see the ring structure. There are plenty of examples online of good and bad collimation. With a star test you want to see well spaced concentric rings. Any mis-shapen circle or offset centre is likely bad collimation. I suggest you do the star test without a coma corrector, it won't help a centred star anyway.
In you images if all the stars are showing egg shapes, including the middle ground, it is likely a mount tracking issue and not coma. Keep at it.

[Atmos (Colin)]

With my EQ6, under really good seeing it would sometimes get down as low as 0.4" RMS in PHD but it wouldn't remain that way. On average it would sit around the 0.6", that I guess would be my long term average.

[thegableguy (Chris)]

Originally Posted by Atmos

With my EQ6, under really good seeing it would sometimes get down as low as 0.4" RMS in PHD but it wouldn't remain that way. On average it would sit around the 0.6", that I guess would be my long term average.

Ah okay. So 1" isn't too bad for a night of good seeing, but it could still be better. Thanks for that, I'll try for better alignment next time.