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Taking Images (part 3)

Initial Thoughts Second Thoughts More Thoughts

More Thoughts

Setting Up

Here's a trick: When polar aligning the scope, the first thing that must be done is to point the tube at declination 90 degrees North. I find that the easiest way to do this accurately is to set the tube to nearly 90 North, and then move the scope base until the scope is pointed towards an area relatively rich in stars. At this point rotating the scope about its RA axis will cause the stars to appear to move in arcs. The trick is to adjust the Dec axis until the arc get smaller and smaller and you can see the point where, if there were a star there, it wouldn't be moving at all. Now your Dec axis is really set to 90 degrees North. OK, so maybe you don't think it's so easy, but it really is accurate. In fact, try it and compare it to the results that you get by using a level to make the base horizontal and then to make the tube exactly vertical. For me there was a significant difference.

Capture Software

As I mentioned previously, I wrote some image capture software for use with the QuickCam. If you're interested, here it is. Please note, however, that it is unsupported, and comes with minimal documentation. Have fun.

Taking images

For bright planets and the Moon, I polar align the scope and use the electronic paddle that came with the ETX (not the AutoStar.) As I've said before, I think the simple paddle drives the scope more smoothly than the AutoStar, and the whole thing works more smoothly in equatorial mode than alt/az. Anyway, it works pretty much as I've described previously.

For dim objects, here's how I do it now: I polar align the scope and then do a 2-star AutoStar alignment. After verifying that the scope points to where I tell it to, I point the scope at a bright start, remove the eyepiece, and attach the QuickCam. Then I hook it all up to my laptop and start AstroCap (my image capture program.) I have a special black Polartec hat that I put over the end of the scope and take a dark frame. After removing the hat, I focus on the star. With the dark frame available, I can set the exposure all the way up to 254 and the out-of-focus star image becomes apparent. As I focus, I reduce the exposure time to both speed up the capture process and to prevent the image from being overexposed.

You'll like this next bit: After focusing (a long process) I unhook the laptop and plug the QuickCam into a long parallel extension cable which goes into my house and attaches to a computer there. Then I take my AutoStar, which is also on an extension cable, and toss it into the window. I go inside, put my laptop next to the other computer, and fire up SkyTools, which I use to generate finder charts. With AstroCap running on the main computer, and SkyTools on the laptop, I then tell the AutoStar to point my telescope at a target. Usually, it's pretty close, and using a 1 degree wide finder chart I can generally figure out where the scope is pointed and slew it until the target is centered.

To capture dark frames, I run outside and put the hat over the scope tube, take about 10 or 20 frames, and then run back out and take the hat off again.


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