Additional optics and back focus
Telescopes usually have their focus point near the telescope exit in order to be in the focus point during visual observation with the eyepiece. The distance from the telescope exit to the focal point is called the back focus.
With refractors, there is usually an eyepiece focuser at the telescope exit into which the eyepieces are inserted. If the focus point is too close to the completely inserted focuser, it is often difficult to bring the camera into the focus point with accessories (such as off-axis-guider or filter wheel). Therefore, refractors especially designed for astrophotography have a larger back focus.
In Newtonian telescopes, the back focus is defined by the small secondary mirror and is therefore often very close to the rear end of the focuser. Here, it is sometimes very difficult to reach the focus point when many attachments are present.
Schmidt-Cassegrain telescopes can move the focus point very far back by adjusting the primary mirror, so the latitude is much greater with these. By turning the focusing knob counterclockwise, the focus point can sometimes be moved up to 900 mm behind the telescope exit.
For astrophotography there are additional optics that fulfill special requirements:
- Reducer (reduce the focal length)
- Flatteners (flatten the image so that stars are displayed up to the corners without aberrations)
- Coma corrector (corrects coma error in Newtonian telescopes)
- Barlow lense (increases the focal length)
These are introduced into the beam path in front of the focus point. With reducers and barlow lenses, the focal length and thus the position of the focus point now changes. In order to set the correct working distance between the chip and the accessory optics, the manufacturer always gives a distance measurement, which is the new back focus. The distance is usually defined from the locating surface of the accessory to the chip.
If another filter is placed in this area of the working distance, the working distance increases by approx. 1/3 of the filter thickness.
The additional optics are always designed for a specific telescope and calculated as if they were located close to the telescope exit. However, there are sometimes differences and tolerances here, whereby the specified working distance varies somewhat. If and how the working distance has to be changed can be determined from the first frame. The images of the stars at the edge of the image provide information on how to proceed.
Source: https://www.teleskop-express.de (translated)
With the Schmidt-Cassegrain telescope - as described above - the focus point can be set very far from the telescope exit. The specifications for the focal length are always related to a focus point close to the telescope exit. The further away the focus point is, the larger the focal length. As a rule of thumb for Schmidt-Cassegrain telescopes, each additional millimeter brings a magnification of the focal length of 3.1 mm. (Source: https://agenaastro.com/articles/guides/upgrading-the-focuser-on-your-sct.html#increase paragraph 4.4)
This means, for example, that an eyepiece focuser with 100 mm length at the end of the telescope requires a shift of the mirror, which increases the focal length by 310 mm. This in turn has an influence on the reducer effect. A reducer with a reduction factor of 0.63x now does not reduce the focal length of a Schmidt-Cassegrain telescope with e.g. 2032 mm to 1280 mm, but due to the changed position the longer focal length must be used, so that with reducer a focal length of 1475 mm results.
Therefore, it should be taken attention to place the reducer as close as possible to the telescope, and to shift the focus point by turning the focus knob back towards the telescope exit (turning the focus knob clockwise). This keeps the focal length as small as possible, and thus makes the optics even "faster" (see slow and fast optics in the menu point 'Basics' - 'Physical quantities' - 'Aperture ratio / f-number').