Off axis guiders
You probably know that to get a good photo of a nebula through a telescope you need to use an exposure of a minute or more. This means your telescope’s mount needs to track the star – not just approximately, but precisely.
There are some mounts that, once they’re set up correctly, will track beautifully. They do this by having high-tech sensors and magnetic drives which are near to perfect. The problem is that these mounts cost vast amounts of money.
More reasonably priced mounts, like my NEQ6, are driven by gears. But even high-quality gears like this can’t be made perfect. Even if they're close, they get dirty, the lubricating grease moves around unevenly, or something else happens to the gears. As well, random wobbles occur.
When this happens, the telescope shifts a tiny bit left or right, and it shows in your photo. For very wide-angle images, like those through a camera lens rather than a telescope, this is fine. But for telescopic images using high magnification scopes, or for astronomers like me with an irrational desire to get a better photo, the image might be ruined.
So how can you get a precisely-tracked image?
Autoguiding is a feedback method that keeps the mount pointed precisely at the target. It uses a second “guide” camera that simply watches a star, and detects when your mount moves off target - even a tiny bit.
Your guide camera can be mounted on a smaller scope bolted to the larger one, or you can use what is called an off-axis guider.
This is a tiny periscope that peers through the same
telescope as the main camera. It reaches into the scope without obscuring the
main sensor. You can see the end of the periscope in the photo through the
front of my scope. As you can see in this "down the barrel" photo, the periscope doesn’t get in front of the main sensor.
When the guide camera detects a slight movement, it sends a signal to a computer, which then sends a signal to the mount to straighten up.
This way, reasonably priced mounts can get 20-minute exposures or more.