Video Astronomy Mounts: Alt/Az
There are two main types of tracking mounts that are used in video astronomy, an Altitude/Azimuth or Alt/Az and Equatorial Mounts. I’ll be talking about Alt/Az mounts in this post. An alt/az mount basically swivels left/right and up/down, similar to how a camera tripod works. Dobsonian mounts are a special type of an Alt/Az mount for larger telescope tubes. When tracking an object you generally have to move it in both left/right and up/down to keep an object in the field of view.
Alt/Az mounts have the following advantages:
The biggest disadvantage of Alt/Az mounts is field rotation. Because the earth is rotating, objects will appear to rotate as they move through the night sky. For example, when the moon transits across the sky, features that appear up when looking to the east as it rises will appear down when it reaches the west when it sets. Because a camera on an alt/az telescope, doesn’t rotate, the object will appear to rotate in the image. Software can correct for this by derotating the image but when those images are stacked, it will leave dark areas around the border of the image.
Field rotation also limits how long of an exposure you can take, but it depends on where in the sky the object is located. Objects located to the East/West are typically well suited to longer exposures. I’ve done exposures as long as 30 seconds with my 130SLT. Objects near Zenith or the poles require shorter exposures.
If you’re not doing long exposures or many stacks of images, field rotation shouldn’t be a problem. For cameras like the Revolution Imager which can send images directly to a monitor, you’d likely never notice it.
Below is my Celestron 130SLT on the Nexstar SLT mount (alt/az).