Learning Curve

DSLR Sky Photography

On a clear windy March 15th evening I fled glaring neighborhood lights for a serious first attempt at wide angle DSLR sky photography. I carried my tripod-mounted sky tracker with Nikon D40 camera to a darker nearby field, about a quarter mile from home, where I could actually see Polaris in the polar alignment scope instead of reflected lights. Once the mount was polar aligned I patiently took systematic exposures to gain practical experience with the camera and mount. Eventually, after two hours, I quit when the temperature dropped near 40 degrees, wind gusted stronger, and my fingers got too numb to operate the equipment.

I learned some valuable lessons during my time in the field. First, exposures longer than two minutes captured too much sky brightness from light pollution. In the future I'll use shorter exposures. Second, the camera mount and tripod were shaky. Every time I adjusted or moved the camera the mount was nudged and polar alignment was destroyed. I need to check and adjust polar alignment after every camera adjustment. Third, I should try setting ISO to 800 instead of 1600. Maybe that will reduce noise a bit. Fourth, I need to take more than three images for stacking in order to increase effective exposure time. Maybe I'll try for 9 images in the future.

Capturing images is only the first step towards producing a good picture. The camera's raw images need to be processed to remove flaws and bring out all the details. I downloaded three new image processing programs and spent hours learning to use them, mostly by trial and error. One of these programs, Star Tools, does a great job removing brightness gradients and vignetting which plague all my sky photos.

The first decent image obtained with my new equipment is the following picture of Jupiter and nearby star cluster M44, the Beehive Cluster, in the constellation Cancer. This is a single 120-second exposure through a 55-200 mm telephoto lens set at about 150 mm. I subtracted a dark frame, but did not apply a flat field. Stars are nice and round indicating good tracking, but colors are not vivid, and bright Jupiter at lower left is very overexposed. Color management is still a mystery to me.

The following picture of the Pleiades star cluster in the constellation Taurus reveals stars down to magnitude 11.8 as well as hints of blue reflection nebulosity present in the cluster. But the bright stars are overexposed, and the blue color seems too light. This is also a single 120 second exposure through a 150m lens with dark subtraction but no flat field.

I'll try stacking some images in my next post.