Keeping a star precisely positioned throughout a long exposure session used to be a job reserved for people with an iron will and patience second-to-none as they continuously made minute adjustments to their scope to keep everything aligned.
Nowadays we AstroScientists can use technology to do the job for us with an Auto Guider.
An Autoguider is a digital camera, combined with some basic image processing gear that allows it to monitor a star and send position adjustment signals to the telescope mount when it detects movement of the star. This allows the mount to stay precisely positioned on its target throughout a long exposure.
The Auoguider can be connected to a second scope mounted on top, or on the side of the main scope, or it can be connected to the main scope using a ‘pick off mirror’ to deflect a portion of the incoming light towards the autoguider. Each arrangement has its pros and cons:
When the Autoguider is attached to a secondary scope, it can be directed toward a different target than the main scope. This allows a bright star to be used for guiding while a dim object is being imaged.
When an autoguider secondary scope is attached on top of the main scope (Piggy back) the additional weight of the second scope needs to be balanced by moving the counter weights on the mount. This may require additional weights.
When an autoguider and secondary scope is attached on the side top of the main scope (Side by Side) additional mounting adapter plates, or a specific ‘Side by Side’ used to mount both scopes. While this incurs additional weight, the weight is applied closer to the mount and so has less impact on balance.
When the autoguider is attached to the main scope its field of view is fixed to that of the main scope target. This can mean that suitable guidestars are hard to find.
Once you autoguider is attached you will typically use a PC to view the stars in its field of view and select a star in the field for the autoguider to track.