Stacking

Comet Lovejoy in Cassiopeia

Since my last post I've learned how to increase the effective exposure times of my sky photos by stacking individual images. The individual photos used below were taken on March 15th when comet Lovejoy was still visible in the constellation Cassiopeia. By this time the comet was getting dimmer, its tail was diminishing, and it was getting lower in the sky. I missed a great opportunity for nice photos two months earlier. I didn't have equipment for sky tracking in January and February when the comet was bright with a long prominent tail high in the sky. Equipment was finally in hand on March 15th, so I finally had a chance to capture the fading comet.

I practiced stacking with three increasing magnifications. The first picture below is a stack of three individual 3-minute exposures with a 55mm lens. The stacking, or addition, of the images was done with a program called Deep Sky Stacker. Comet Lovejoy is the small, fuzzy, green object left of center very close to the white star Delta-Cassiopeiae (Ruchbah). No tail is visible. The "W" shape of Cassiopeia's bright stars is entirely visible. The tight grouping of stars near the lower left corner is the double cluster in the constellation Perseus. The small pink object to the lower right of the comet is star cluster NGC 457.

The next more magnified image is a stack of three individual 2-minute exposures with a 110mm lens. The green comet and white star are more clearly separated, and the comet's tail is on the threshold of visibility.

The last picture, magnified a bit more, is a stack of three individual 2-minute exposures with a 150mm lens.

I was able to achieve a uniform dark sky background in the pictures above because one of my image processing programs, Star Tools, removed brightness gradients and vignetting. The next image illustrates what the previous picture would be like without gradient removal.

Colors seem nicer in this last picture even though the center is too bright. The comet's green color looks particularly nice. I really don't know what I'm doing with color management during image processing. In the future I'd like to try stacking 9 or 10 images to better capture faint details like the comet's tail not seen in the pictures above.