Tuesday, November 26, 2013

Checking Out a New Tracker...

One of the things that make astronomical imaging more difficult is that the Earth rotates, so anything more than a couple second exposure is either trailed or must be tracked to counteract the Earth's spin.  I've got a couple tracking mounts, a 30 year-old Byers Cam-Trak, and a year-old Vixen Polarie.  While both work well with normal-to-wide-angle lenses, use of a telephoto or small telescope usually requires setting up the Big-Boy mount, currently my Astro-Physics AP1200.  It would be nice to have something between the two to handle more payload, or even a couple cameras, say if we have a spectacular comet in the sky!

My friend George e-mailed a couple weeks ago that he wanted to sell his Astrotrac system, complete with pier, wedge, alignment scope - the works!  Unlike most precision mounts that use a worm gear, this device uses a tangent screw that tracks for 2 hours before needing to reset.  With a tangent screw, the drive rate needs to vary slightly as it extends, but with microprocessor control, it is easy to do.  Reading the website claims a payload of 30 pounds, compared to the 7 pounds of the Polarie - something worth a try! 

With the recent wet weather passed, clear and cold returned, and last night I went out for a couple hours to check out the mount.  Unfortunately, the sun sets so early there was no chance to avoiding setting up in the dark, and not wanting to travel too far on a "school night", I went down the Sonoita road near the Greaterville turnoff and set up at a roadside table.  The sky was great as I figured out how it worked in the dark.  The setup is shown at left as Orion rose in the east.  I mounted my 70-200 lens on the hastily machined mounting plate and tried my first exposure - shown at right.  It was trailed!  I was attempting to use it without looking at the manual, and had I done so, would have seen I needed to push the arrow button twice - the second to start tracking...

That startup glitch behind me, I started working on a 3-frame mosaic of  Andromeda-to-Triangulum, home of two of our nearest and brightest galaxies to be seen from the northern hemisphere.  Using the Canon XSi, I only took 2.5 minute exposures, and took 5 exposures per field, including in-camera noise reduction, so total investment was 75 minutes, and I was on my way home by 10:30.  Evidently I barely finished in time - the camera lens was just starting to dew over as I ended my exposure sequence.  As the temperature dropped, the lens dropped below the dew point, higher than normal because of the recent rain.  The mosaic was assembled with the Microsoft ICE program, and north in the image is approximately to the lower left direction. The Andromeda Galaxy (M31) and its two companions are at far left, the Triangulum Galaxy (M33) at upper right center.  Beta Andromeda (Mirach) is the bright star at upper left center, and the three stars at lower right form the Triangulum constellation.  The star cluster at bottom center is NGC 752.  Not a bad image given 12.5 minute exposure total per frame with a 70mm lens (F/3.2).

So wide angle shots don't push the mounting system too hard, but given the short time I had available to test, at least I know more about how it works now!  Shown here are the pair of galaxies at the full camera resolution, M31 and companions at left, and M33 at right.  Of course, the above mosaic was reduced in size mightily to fit the blog's 1600 pixel limit.  Next time I'll spend more time at longer focal lengths - one or two short exposures at 200mm seemed to track pretty well...  I feel a little better-prepared in case ISON survives its trip around the sun!

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