I had another opportunity to photograph the spectacular spiral galaxy M33, also known as the Triangulum Galaxy.

Last year I acquired 30 three minute exposures of M33 at ISO 800, and the resulting image can be seen in this gallery. This time I acquired 15 six minute exposures at ISO 400 and combined them with the previous exposures, producing this image representing 3 hours of total exposure time.

If you compare this image with my previous image of M33, you will see the benefit of increased exposure time for faint deep space objects. The lower noise and increased detail are obvious.

While M33 is one of the nearest galaxies to earth at approximately 3 million light years, the surface brightness is fairly low. Under my light polluted skies, it is difficult to see this galaxy at all through the eyepiece of my 8" reflector. Individual photographic exposures show a hint of the spiral structure, but combining multiple exposures and processing the results to increase the contrast are necessary to bring out any detail.

This image and my previous image of M33 were acquired with a Canon Digital Rebel XT at the prime focus of an 8" newtonian reflector. The telescope was mounted on an Orion Atlas equatorial mount, with an 80mm f/11 refractor riding piggy-back on the reflector. At the prime focus of the refractor was a Meade DSI-C CCD camera acquiring continuous images of a nearby guide star. The guide star images were processed with PHD Guiding software running on a laptop computer, and movement of the guide star was fed back to the equatorial mount in real time to adjust the position of the mount, eliminating tracking errors that would cause blurring and/or streaking of the images. The computer interface with the Atlas equatorial mount was achieved using the incredible public domain software EQMOD. Automated camera control of the Digital Rebel XT was accomplished with DSLR Focus software.

The individual images were calibrated, registered and combined using Iris Software, and the composite image thus produced was finished in Photoshop.