Quote:
Originally Posted by Placidus
Coming along marvellously! A great result and fantastic for the circumstances.
There's no sign of field rotation there. Stars look great. If the polar alignment is off along the same hour angle as where you are photographing, then you not only get nasty field rotation, your guiding can get lost in the dec backlash. Conversely, if by good luck your polar axis is off along an hour angle that is at 90 degrees to where you are photographing, then at least for an hour or so you get no field rotation and consistent guiding. Perhaps the pier moved in a direction that was harmless or even helpful given where you were photographing. (With our fork mount, we intentionally put the polar axis 0.04 deg west of the SCP. That gives excellent behaviour when photographing say 60 deg either side of the meridian. And 2.5 tonnes of steel reinforced concrete in solid rock makes it stay put!)
Best,
Mike
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Thanks Mike! :-)
There's definitely some movement in the subs from last night, which I believe to be rotation. PHDLab estimates a misalignment of over 29' and there's visible elongation in the subs, either NE or SW. PI reported quite high eccentricity; two which didn't even make it into this were 0.7+. All's I know is that when stacking the 10 "bad" frames with 20 "good" frames, the elongation disappeared and I was left with pretty round stars. Happy days!
I think I'm going to have to do some more work on the pier. In trying to save myself some money I've got a clearly inferior solution. I think it's the mounting to the pier that's the problem; I'll be looking into buying a proper pier top plate to hopefully sort this out. I've seen alignment change as much as 70' in a week; whereas parking the tripod on the same footing I'd end up about 15' after several weeks.
My footing is okayish, it's mostly just the dodgy DIY pier. Pier footing is 110x110x60cm (only 60 deep). It's in clay mixed with granite, but to get below that point in our soil would require the footing to go down over 3m. It moves a little but it's not too bad, certainly not 70' in a week bad.
I also intentionally misalign to improve guiding; I've got 2884ms (thanks PHD2!) of backlash in DEC so I try and keep corrections to one side. I'm going to be trying PHD's new backlash compensation feature to see if I can use that to correct in both directions whilst dialling in better alignment.
Quote:
Originally Posted by gregbradley
Awesome Lee. You must be happy with that. Lovely round stars too.
Greg.
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Thanks Greg! :-) Pretty happy with the stars! You wouldn't think 1/3 were bad throw-aways eh? Could be a bit tighter like yours and and Mike/Trish's, but I'm still pretty happy.