Daylight Collimation of a SCT or Maksutov-Cassegrain

by Robin Casady

Checking the optical alignment on Schmidt-Cassegrain or Maksutov-Cassegrain telescope can be easily done in broad daylight. It just requires looking into the front of the scope from a very specific location.

Finding this location is not difficult. Set up the telescope so the OTA is level and at a height where you can easily look straight into it. Position your eye in front of the OTA about as far away from the front as the focal length of scope. For a scope with a focal length of 2700mm you would stand almost nine feet away.

Look into the scope and find the shadow of the central obstruction (CenOb).

Position your eye so the shadow is just slightly larger than the CenOb.

Move your head until the CenOb and the shadow are concentric. Your eye is now in the correct location to judge the alignment of the optics.

You will notice several light and dark rings. If they are perfectly concentric, the scope is properly collimated.

If the rings are not concentric, the scope is out of collimation.

If you wish to adjust the collimation, it is advisable to set up a white card with a pinhole located at the correct eye position. This makes it easier to keep your eye in the right place, and lets you go back and forth to the scope to make adjustments.

I use a photo light stand and clamp to hold the card in place. The light stand has a vertical adjustment the makes it easier to set it to the right location.

I've found that I can collimate a scope with a fair amount of accuracy by using the white card. Only on a very steady night can I improve upon it with a star. The other night, in average seeing, I checked a Maksutov at 300x on a star and it seemed fine. When I checked it with the daylight method, it was obviously out of collimation. I was quite surprised.

Blue is telescope, black and white are reflections
Align your eye so the shadow is concentric with the central obstruction (CenOb).
This telescope is out of alignment. The CenOb shadow is concentric with the CenOb, but the other rings are not concentric
This telescope is properly collimated. All rings are concentric.