by Benjamin Gomes-Casseres
Here's the weblink to related photos that shows the processing steps described below:
http://www.alliancestrategy.com/Astro/McNaught/Mcnaught.htm
- Manchester, VT (43deg 10min N; 73deg 4min W)
- Very dark and clear; comet was about 7-10deg above horizon; 55F with slight humidity
- June 19, 2010 at 2:35-3:06 AM EDT; in the image above, the comet is located at its 3:03 AM position (see explanation below)
- Takahashi E160 astrograph (6" F/3.3) with field flattener
- Image scale is 1.5deg x 2.3deg (so comet tail is over 1.5deg in the image, trailing off the edge)
- Takahashi EM200 mount; unguided; tracking stars
- Canon 450D (modified by Hap Griffin)
- 10 subs of 3min each (total exposure 30min); 1600 ISO
- 5 darks, 8 flats, 14 bias files; processed as usual
- Captured with Mike Unsold's ImagesPlus 3.82; processed with ImagesPlus 3.82, Photoshop C4, and Noise Ninja
- Procedure: (1) comet aligned and stacked; (2) stars aligned and stacked; (3) images blurred and recombined as explained below.
- Explanation of procedure: The following procedure is ingenious; it was invented by Bernhard Hubl, who provides excellent instructions here. Because the comet moved visibly between the exposures, it is located in a different spot for each sub exposure. As a result, aligning on the comet yields star trails (first image below); aligning on the stars yields a trailing comet (second image). In this procedure, the subs aligned on the comet are first combined with a technique that eliminates most of the stars, leaving the comet on a dark background (using IP 3.82). The same subs are then aligned on the stars and combined as usual (using IP 3.82); the trailing comet is then blurred out of the image, leaving the stars (using CS4). Finally, the two images are added to each other (CS4), reuniting the (now stationary) comet with the (similarly stationary) stars. Some manual stamping out of stray star smudges in CS4 was done to clean each image prior to recombining and Noise Ninja was used to reduce noise in the comet image. Color and exposure was balanced in IP 3.82 after each stacking, with minor adjustments in CS4. In this case, the International Space Station can be seen in the top right corner of the second image below (it crossed the field in the sub taken at 3:00-3:03 AM; see here). This trail was erased from the final image above to focus attention on the comet in its star field.