The difference between star guides.

In order to find the target quickly, the seeker needs a large field of view. In order to get a large field of view, the magnification should not be high. The magnification of a general viewfinder is several times. And the viewfinder is required to be coaxial with the main mirror. In other words, the center seen by the viewfinder should be what the main mirror sees. The center of the viewfinder is aimed at a star. If you switch to the main mirror, you should also see the stars, and the more in the center of the main mirror, the better. The discoverer is different.

The star guide mirror is used to track the target. In order to have high accuracy, the magnification of the star guide mirror should be greater than that of the main mirror. The greater the derivative, the higher the accuracy.

Sometimes it is even required that the focal length of the star guide mirror is more than twice that of the main mirror. For the convenience of guiding stars, we usually need to find a bright star to follow, but the target we shoot is not necessarily this bright star, and sometimes it may even be a nebula invisible to the naked eye. That is to say, the star guide mirror is not necessarily coaxial with the main mirror, and the target that the star guide mirror looks at is not necessarily the target of the main mirror. If you shoot a target near Polaris is, you may even adjust the star guide mirror near the celestial equator. The difference is tens of degrees.