Maksutov telescopes have to be collimated


Fig.2: View of a non-collimated Newton without a collimation eyepiece


Fig.3: Three steps of collimation for a Newton
 A.The easiest way to collimate your telescope during the day is when it is bright and all screws, nuts and other parts on the telescope are clearly visible. The well-known warning in advance: Do not point the telescope or the side opening of the collimation eyepiece directly into the sun!
Place the collimation eyepiece directly into the focuser and point the side opening towards the sky or a light surface. The collimation eyepiece should be pushed in so far that you leave a little space between the secondary mirror and the lower edge of the collimation eyepiece when you look through the viewing hole.
Newton telescope (without secondary mirror / eyepiece rotation system)
  1. Look into the tube from the front and check whether the secondary mirror is nicely positioned in the center of the tube. It is best to measure this with a ruler or a slide gauge. For Newtons with a very short aperture ratio (e.g. f / 4), the secondary mirror must be positioned a small offset further away from the focuser. Also check whether the main mirror is nicely centered in the tube. A quick look is usually enough. All components must also be at least roughly adjusted so that you can at least see the main mirror in the secondary mirror.
  2. If there is an eyepiece in the extract, remove it. Look through the focuser. You will see the secondary mirror and the secondary mirror holder, as well as reflections from the secondary mirror and its holder, the primary mirror and your eye (see Fig. 2). This figure shows the sight with both mirrors decollimated and the secondary mirror not optically centered under the focuser. It is best to place or stick a white sheet of paper opposite the focuser on the inside of the tube. This helps center the secondary mirror, especially in slightly dim lighting conditions.
  3. Now place the collimation eyepiece in the focuser and look through the viewing hole. You will notice that the field of view has narrowed a little. Instead of your eyes, you will now see the reflection of the 45 ° light reflector. Now check whether the secondary mirror appears nicely centered under the focuser. The crosshair serves as a reference for this. The center of the secondary mirror should coincide exactly with the crosshairs. At this point, forget about the reflections in the secondary mirror, they will only come into play in the next few steps. If the secondary mirror does not appear centric, correct the position of the secondary mirror. If the two cannot be brought into agreement, check whether the focuser is mounted at right angles and correct this if necessary. At the end of this step it should look something like Figure 3a.
  4. Now correct the inclination and rotation of the secondary mirror until the reflection of the entire main mirror appears nicely centered. A point (e.g. with a felt pen) in the middle of the main mirror is very useful as a reference. At this point, forget about the reflection of the secondary mirror in the main mirror. You only need this in the next step. At the end of this step it should look something like Figure 3b.
  5. Now is the time to correct the tilt of the main mirror. The point in the middle of the main mirror must be exactly in the center of the bright reflection of the 45 ° light reflector in the collimation eyepiece. Now it should look like Figure 3c. The reflection of the main mirror is centric to the secondary mirror and the reflection of the secondary mirror is centric in the reflection of the main mirror. If everything is now centered with the crosshairs of the collimation eyepiece, then the telescope is collimated.
  6. The final fine-tuning of the collimation is best done with a strongly magnifying eyepiece on a star. To do this, you usually only need to correct the main mirror a little until the diffraction image of the star appears nicely centered. Harold Richard Suiter's "Star Testing Astronomical Telescopes" is perfect for this.

Schmidt-Cassegrain
With the commercially available Schmidt-Cassegrain telescopes, collimation is best done using a star with a strongly magnifying eyepiece. However, the collimation eyepiece is very practical for very strong decollimation. Usually, with commercially available SCTs, only the inclination of the secondary mirror can be adjusted a little. But be careful - with many the three adjusting screws are very short and there is usually no safety device! Sometimes a third turn of the small screw is already too much. It is better to put the telescope in a somewhat inclined position so that the secondary mirror cannot fall on the main mirror.