I'm still struggling horribly with drift alignment.
I'm doing 30-second exposures with 15 seconds panning in one direction, 15 seconds in opposite direction, trying both axes every time... all I ever seem to get is perfect straight lines, not the vees I'm told to expect. If I do a 30-second exposure, though, the stars are still slightly elongated.
Anyway. I took about twenty 30-second stills and somehow, through trial & error with both az & alt, I somehow stumbled across a reasonably good alignment... so had a shot at M83 for the first time.
Orion ED80 with 0.85 flattener
NEQ6 Pro
Nikon D3300
ISO 1600
10 x 90 second subs
6 x darks
6 x flats
Thoughts, recommendations?
[Side note: I tried 3-min subs at ISO 800 but for some reason I got doubles of all the stars - not streaked, but actually duplicated seperately. What's that about?? I'm guessing something moved (it's a little windy tonight) but it happened with all four 3-min attempts and not once with my 90-second subs. Could just be a coincidence, I suppose. I can't think of any other way to get doubles like that.]
Chris, I'm new to DARV, but it works for me. Firstly, select a moderately
bright star; if it is too bright the line will be too thick, making accuracy
difficult.
Select motor speed rate 1.
Open the shutter for about 5 secs to create an obvious starting point, so you know whether the returning line is diverging upwards or downwards from the starting point.
After the initial 5 seconds run the motor for around a minute, and then for another minute or so in the reverse direction.
15 secs is nowhere near long enough to show divergence if you are fairly
close in the first place, especially if you are on a higher motor rate than 1. Hope this helps.
raymo
Using a short focal length scope you will notice the problem less than a longer focal length scope, as with the longer scope you're zooming in more. For example, if I DARV with my C8 onboard, error is usually pretty apparent at 15 seconds. With my Esprit, at about 1/4 of the focal length of the C8, not so much, and I have to push out the length of the tracking time some more.
If it looks great with DARV taking an image like you have and still seeing some elongation, check whether or not the elongation is the same (direction, length) all over the image.
Regarding the double star thing, that would be the wind...or the scope got knocked...
A longer exposure time and slower slew rate (I was using level 2, didn't actually realise there was a slower option!) makes a lot of sense. Thanks for that. The guy who sold me the mount told me he used 30-sec subs but he had a considerably longer focal length, so my little flattened 510mm will be a different story.
I might also try taking off the flattener and using a diagonal and 2x Barlow T-adapter instead. Probably not great for imaging but could dramatically speed up the alignment process.
A longer exposure time and slower slew rate (I was using level 2, didn't actually realise there was a slower option!) makes a lot of sense. Thanks for that. The guy who sold me the mount told me he used 30-sec subs but he had a considerably longer focal length, so my little flattened 510mm will be a different story.
I might also try taking off the flattener and using a diagonal and 2x Barlow T-adapter instead. Probably not great for imaging but could dramatically speed up the alignment process.
Thanks again folks.
Hi Chris, 30 seconds worked well for me given that I had an autoguider
Also make sure the scope is really well balanced with a slight tendency to fall east, that way you will eliminate some backlash issues . The idea being that the cogs always push in one direction
Just a FYI you will need to guide this mount to get much longer subs...it's what comes along with the price
The reason I suggested you look at the drift across the image is to determine which axis, if any, it was drifting in.
Yeah... much longer subs aren't on the cards at this stage, having just spent significantly more than the amount approved by the boss. Not only that but I'm determined to learn drift alignment properly and push the current unguided setup as far as I can.
But yes a guider will likely be the next investment.
Drift Alignment by Robert Vice. It's a nifty technique you use to polar align. Basically you take a one-minute exposure and, using the slowest speed setting, slew for 30 seconds in one direction and 30 seconds in the other direction. If you're perfectly aligned you see a perfectly straight line as a result; if you're out you'll see a V shape.
It's slightly more complicated than that; if you're keen, PM me and I'll walk you through it. Took a few false starts and a lot of questions, but I think I've got it down pat now (touch wood).
I'll get to guiding one day, it's inevitable, but I think it'll be a while before I'm allowed to spend any more on new astro gear...!
For now I'll do what I can with the current rig by honing my polar alignment skills. I won't be making any award-winning images, but they'll be enough to satisfy me - and more importantly I'll learn about the processing & editing side of things, in preparation for the eventual upgrade to a bigger, guided rig in a year or two.
This shot was unguided with a lucky polar alignment; planning on doing it again in a few weeks, but with a more accurate alignment and longer subs.
