Guiding is compensating the remaining deviations, which are still left over from the alignment. Only by eliminating the residual errors long exposure times can be realized. A guiding camera connected to a PC is used to focus on a guide star in the sky. Via a program on the PC (e.g. PHD2) correction commands are given to the mount, which then adjusts the tracking movement accordingly.
There are two ways to integrate a guiding camera in the setup:
- via a guiding telescope, or
- via an Off-Axis-Guider
Guiding telescopes are placed outside the optical path parallel to the telescope and are especially suitable for refractors and telescopes with short focal lengths.
Setup with a guiding telescope [Source: https://www.astroshop.de (supplemented by labels)]
Telescopes with long focal lengths have a high magnification. Therefore, even the smallest tracking errors quickly become noticeable. To be able to give accurate guide star positions, a guiding telescope for such telescopes should also have a larger focal length. As a rule of thumb, a guiding telescope should have at least 30% to 50% of the telescope focal length.
For large reflecting telescopes with long focal lengths (e.g. large Newtonian telescopes from 1000 mm focal length, Schmidt-Cassegrain) it is recommended to use an Off-Axis-Guider.
Since an Off-Axis-Guider is located directly in the optical path, it uses the same focal length as the telescope and can compensate tracking errors well. As it were, the telescope is the guiding telescope.
Setup with an Off-Axis-Guider [Source: https://www.astroshop.de (supplemented by labels)]
Another reason to use an Off-Axis-Guider with reflector telescopes is that the mirrors are not absolutely fixed. During the slow tracking of the telescope, there are occasionally minimal changes in the position of the mirrors. Due to the optical law "angle of incidence equals angle of reflection" the error doubles when deflecting at the mirror. Thus, the image on the camera experiences a slight offset, which is especially noticeable at large focal lengths. In contrast to the guiding telescope located outside the system, this offset is noticed by the Off-Axis-Guider located in the beam path and can be compensated by countersteering the tracking unit.
Since refractors do not have mirrors but fixed lenses and usually have short focal lengths, a guiding telescope can be used here.
Guiding cameras are operated with exposure times of 1-3s. When using low light systems (slow optics) and because of the decoupling via a small prism, there are therefore often problems to find bright stars as guiding stars.
There are several things to consider when installing an Off-Axis-Guider
- The deflection prism should be positioned in such a way that it does not cover the chip surface.
- In theory, the distance from the deflection prism to the chip of the guiding camera is the same as from the deflection prism to the chip of the recording camera in order to have the same focusing.
In practice, however, differences of 1 to 2 mm occur here from time to time. The reason for this is that the deflection prism uses the edge rays, which often cause imaging errors in optical systems due to different path lengths.
Depending on the camera model, the correct distance can be set with a helical extension or corresponding distance rings.
- When using filters, make sure that they are installed behind the off-axis guider. With an additional filter in front of it, the already rather faintly illuminated and short-exposed image would be able to show even less bright stars.
- For Newtonian telescopes, note that it can be difficult to use an Off-Axis-Guider. The Off-Axis-Guider shifts the position of the camera further away from the telescope and the chip can then sometimes no longer be brought into the focal point. The Off-Axis-Guider should therefore be as slim as possible, or a guiding telescope must be used.
Overview table for the two guiding variants