by Peter Jenkins FRAS
My guide scope is an Altair Maxi Guider which is an 80mm diameter x 328mm focal length scope. This guide scope is attached to a top mounted plate above my imaging scope and it's fitted with a QHY5II guide camera. It is attached using 2 x tube rings with nylon tipped screws.
This guide scope and my imaging scope are alligned by centering both on the same object. The camera sensors (guide and imaging) are orientated at the same angle.
The QHY5II guide camera is focused using the helical focuser as this only needs to be done once.
Guiding is used to precisely track the aparent movement of stars. Some (very expensive) mounts are capable of the accuracy required to accurately follow this movement during 5 to 10 minute photographic exposures unaided. Most inexpensive mounts (lets say sub £5000) are not so capable. These require help.
Two types of guiding are popular
1) Off axis guiding which uses a prism to pick off a small
amount of light from the imaging scope and does not require
a separate guide scope. I have never been able to make this
work (for me) satisfactorily so I use the other method
2) A separate guide scope.
Most Equatorial mounts can be controlled by computer. They can be made acurately track a star by using guiding software - most users I know, like me, use PHD Guiding (Now in version 2 - PHD2)
The guide camera takes a 2 sec image continuously) and the software is set to send corrections to the mount to ensure that the crosshairs shown stay centered on the guide star.
Its's important that the mount/scope/camera are carefully balanced (both directions) and that the Mount is acurately Polar Aligned for guiding to be at it's best.
If you look carefully you'll see that my guiding TOT is 0.55 arc secs. My image scale is 1.25 arces per pixel - so the guiding being achieved here is to within less than 0.5 pixles. Note that guiding is effected by seeing (how clear your sky is - and to be honest mine is never very good - see the section "AstroShed" and look at my site and location!!!