Tracking and Stacking
One of the challenges of astrophotography is taking long exposures of dim objects in the sky. Since the Earth is rotating on its axis and moving through space around the sun, objects in the sky appear to move in the sky from the perspective of the surface of the Earth. In order to hold an object steadily in view through the telescope, the telescope has to move as the Earth does. There are various mechanical means of accomplishing this, and however it is done, it is referred to as “tracking” of the telescope. It has to be very precise. Any errors in the tracking of the telescope will cause the photographic image through the telescope to be streaked. (Stars will look like little lines instead of dots.) One of the current tasks facing the technical team here at the Morgan County Observatory is to improve the tracking precision of our 16” Cassegrain telescope. Currently, our telescope can only track to a precision of about 2.5 arc seconds per minute. That is a small error in everyday terms (2.5 arc seconds is less that one thousandth of a degree) but is way too much for astronomy. Fortunately, there is a modern technique which allows us to get long exposures even with less than perfect tracking. Instead of one single long exposure, we can take a series of short exposures and with fancy software combine and align them into one composite image. This procedure is known as “stacking” and is allowing us to get pretty good pictures with what we got now. We are still going to fix the tracking error, however. More on that in a future post.
The Globular Cluster M3
This spectacular star formation is found in the constellation Canes Venetici. The photograph was taken through our 16” Cassegrain telescope using the ZWO ASI533MC Camera and is a composite of 52 frames of 6.7 seconds exposure for a total effective exposure time of 348 seconds or just under six minutes.