When setting up a guide scope, the main consideration is the comparison of the pixel scale of your imaging rig compared to your guiding rig.
The pixel scale is just 206 * (pixel size) / (focal length).
Assuming your 30 mm guide scope is f/4 that would be a focal length of 120mm. Using a common guide camera such as the ASI120MM which has a pixel size of 3.8u, the pixel scale is 206 * 3.8 / 130 = 6.0 arcseconds/pixel.
Now say you are using a 800 triplet at f/4.8. This gives a focal length of 384 mm. With an ASI1600 with a pixel scale of 3.8u, for example, the pixel scale is 2.0 arcsecond/pixel.
These days with advanced guide algorithms in software like PHD2 the centroid of a star can be found to a fraction of a pixel. Most people suggest the guide pixel scale no more than 4 or 5 times that of your imaging scope.
So with the 30mm guide scope you should be good up to say 600mm focal length (assuming your imaging camera has a pixel size around 3.8u).
Check Your Mount’s Tracking First
The most important thing to do before ever trying to guide for real is just setup the scope and calibrate PHD2 then go in the setup menu in PHD2 and disable guide corrections then turn on guiding and just watch your mount track.
PHD2 will track the guide star but send no corrections. This will get you a feel for the native tracking of the mount. If it is nice and smooth looking you will probably be able to guide. If it is jumping around occasionally, especially in RA, then you may have mechanical issues that are going to make guiding difficult. Also balance or cable drag, etc.
This is a good guide for PHD2 best practices.
Originally Written by Michael Fulbright
Adapted for the website by Matt Lochansky