Using a Stellavue 80mm APO (with 0.8x reducer) and a Canon T2i, i took some images of comet Lovejoy. The main imaging run was 30 x 2 min exposures at ISO1600. The comet is bright enough that the core is blown at this exposure. So i also took a series of shorter exposure sets at 60s, 30s and 15s. Unfortunately, i could not create a high dynamic range image (HDR) with the short exposures – either automatically with the tools i have nor manually.
I was guiding on a star and not accounting for the relative motion of the comet. At 2 min expsoures, it’s manageable as the comet is not moving so fast that it would show any noticeable signs of being stretched in each frame. However, after an hour it had moved considerably in the FoV, so each frame had to be realigned to the comet’s new location. The 30 aligned frames were then stacked with a rejection filter. This provided the best image of the comet at the expense of dimming the background stars. (A simple averaging would keep each star and the result would be a line of stars giving the impression the comet was racing past them – which it is.)
The part of the tail visible in the image above is 2.2deg wide. This works out to 3.3Mkm in actual length and that’s not all the tail. The distance from earth to the comet was about 85 Mkm when the image was taken.
To get an image with the background stars, i reprocessed just 5 of the images and aligned on the stars. Even with a 10min lapsed time, the comet is still not stretched too much in the final image.