Comet Holmes
Night of November 2-3, 2007    Rutherford College, NC

 

This photo was produced by stretching and combining the next two images to display increased dynamic range. Details are below.
Click this link for a 1280-pixel version. (See below for a better try.)

 

30 seconds

 

10 seconds.

Notes:

So there I was all packed up for a stargazing trip to New Mexico when I got a clear night with a bright comet in it. Like I'm going to complain? So I unpacked the minimum kit required to take a quick shot: the Astro-Physics 5-inch F6 refractor, A-P telecompressor, Canon 20D, Losmandy G11, a Stiletto focuser, and a small battery pack. No guide scope, no guide chip, no computer, no remote cabling, no cooled CCD tonight.

I focused on Alpha Persei and then swung over to the comet and made a few frames ranging from 1/4 second to 30 seconds in RAW format (I used RAW mode for its 12-bit depth, to defeat most in-camera sharpening, and to avoid jpeg artifacts; I used the self-timer to release the shutter in the absence of my electric cable release). The 10- and 30-second images were OK while the rest were pretty dim. The 30s frame showed a little trailing in dec (no surprise: I didn't dig out a flashlight or other illuminator for the pole-finding reticle). I cleaned up the 30s frame and saved it. Then I saved a second version of the same frame with the stars removed and the left side of the histogram (that's "the shadow detail" for old-school photographers) stretched like crazy to reveal the outermost green ionization globe.

Then I put the 10 second frame, the unstretched 30 second frame, and the stretched 30 second frame on separate layers in Photoshop and rebalanced their opacity and blend modes to try to get lots of dynamic range while still preserving a modicum of the visual impression. The resullt is the image above, at the top of this page. Not bad, and the data will still be here to play with more when I get home. Now I have to repack.

 

Later that same night:

I was inspired to go back outside and take a cut at capturing the dim ion tail first shown in images by Ray Gralak and Jerry Lodriguss and posted to the A-P and SBIG users groups. In my photo, features extending rightward at 2 and 3 o'clock through the cyan halo of the comet are possibly (but by no means certainly) instrumental in origin. (The camera's amplifier noise begins to appear as a magenta glow in the extreme lower right, so you know I worked over the histogram pretty aggressively.) The dim, narrow streamer extending down and to the right through and just beyond the aquamarine halo (look near the 4 o'clock position) is the comet's faint ion tail. Squint, or click here to view a ludicrously stretched, histogram-equalized image as a guide.

 

270 seconds: 9, 30-second sub-frames aligned and summed in MaximDL, presented in Photoshop.
The brightest star in the frame is about magnitude 6.2, and the image reaches to around 14.3.
There's a little "orange peel" noise in the sky because I'm really stretching it to get that faint, filament of a tail.
Click for a 1280-pixel version of this image.

Same data, reprocessed in MaximDL. 1280 here.

 

© 2007 David Cortner
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