If I was happy doing up to 4min subs with nice round stars, good results etc, then when I went to eight min exposures and I am seeing elongation in my stars, would that be polar alignment, needing more drifting?
Yes, most likely polar alignment and periodic error in your mount. 4 minutes unguided with round stars is pretty good though - you must've been pretty good in your drift alignment already.
You need to start guiding - either manual or auto guiding.
It is a 102mm Celestron refractor on an EQ6. I don't really know how to do PEC, otherwise I would have done it. I do use EQMOD, so I suppose I should give it a whirl. I always guide, no matter how short the exposure and although it seems to be ok, in hindsight, maybe the guide scope is a bit "rickety", I need to shore that up, too.
interesting your using a Guide scope and getting drifting tails on stars after 8 min. :S ide be trying to like you said make sure the guide scope is 100% solid.
Also look into how you have mounted the main scope as if you are getting flexure thoughout the mount as it starts moving to track? dunno if this is a problem.
PEC is quite easy to do, basically as the worm drive goes around there are inaccuracys in the milling process, the computer onbord drives the worms and accounts for the PE within them, essentially speeding the drive up and slowing it down to get a "near" perfect drive system. Normally to do a PEC Callibration you have to guide on a star for a compleate cycle of the worm (one end to the other and normally between 5- 30 min depending on your mount) once this is done it stores the information. You do this for both drives for maximum effect.
Im unsure about the eq6 drives on how they are done as yet, though ill be picking my mount up either today or tomorrow and will definately be playing with it this week/weekend. So bare with me till then and ill give you instructions on how to do it all.
