Rick Veregin:
We have all had nights like this. I don't see how this could be a mount issue--unless the alt-az is not locking properly. This is purely mechanical, if you have everything locked down on your altitude and azimuth adjustments, and your tripod is properly locked, then there is nothing in the mount that would cause your polar alignment to drift. If you want, you could put a small piece of tape across where your alt and az rotate on the mount, if the tape gets pulled you know there was a shift there, so something wasn't properly tightened, or there is some issue preventing it to tightened properly. The dec/ra lock have nothing to do with polar alignment, unless they are not locked when you are actually doing alignment, but then your initial alignment would be poor.
One more point, make sure your tripod is really stable. The tripod needs to be properly tightened open so it can't move. And make sure the feet are well settled in place. A leg may get caught in a stressed position on setup and as the telescope slews that leg pops a few mm. I always take hold of the legs and give each a good shake after putting the feet down to make sure they are really in a stable spot. If your legs are 1 meter apart, then 10 arc-min is only 3 mm shift in the legs--if your legs move that little relative to each other it could explain the shift in alignment. Make sure your scope is balanced properly, if there is a lot of weight shifting around as the scope moves it can tilt you setup. I am not sure you can assume your deck is that stable, if you are standing on the deck next to your rig doing the polar alignment, then walk away, the deck can shift a few mm for sure. You can test this by doing polar alignment, then move to a different location on the deck and recheck, make sure as you walk around your alignment is not shifting. Also, with night cooling, contraction of wood can move things as well.
Great advice, thanks Rick. I will make sure I pay attention to all the things you said. Your last sentence in the first paragraph--In a strain-wave mount like the AM5, there are no RA-DEC locks, it's not even possible to move the telescope manually, it's all done electronically. Manual slewing isn't possible. But it was suggested to me that I do a PA, then partially lock the ALT_AZ locks and do it again, and then lock them down snugly. If the deck is the culprit I'll really have to re-think this because it's the only spot in my yard where I can plate solve and polar align.
Andy Wray:
Jerry Gerber:
Andy Wray:
I've had nights like that where something moved in my setup and I never really quite worked out what what it was. I wouldn't jump to the conclusion that it is a problem with your mount. Try again another night, be extra careful with how you set it up and polar align and you'll probably find it all works fine. We all have nights like you have described.
Well, I will try again when the seeing is better. And you're probably right, maybe I did do something inadvertently that moved the polar alignment. At least I got all my darks, bias frames, flat darks and flats complete!
I note that you mentioned taking darks, bias frames, flat darks and flats. I would like to suggest that you drop the bias frames and just stick with darks, flats and flat darks.
Yeah, my OCD kicked in so I did both bias and dark flats. I'll find out through experience when I finally get some lights which work best with my camera. I've read so much controversy about which works best, and that it doesn't matter whether dark flats or bias frames are used that I do what seems most practical--experiment and go with personal experience!
