How to Collimate a Schmidt Cassegrain Telescope (SCT)

In this article I am going to discuss the collimation of SCT's this is a worry for a lot of people, but in fact it is quite simple and even more important that an aligned finderscope or clean eyepieces. 

The first thing I would suggest before getting started a useful tip when collimating an SCT is to use Bob's Knobs, these are knobs that can be turned by hand and replace the screws found in the secondary mirror.

Once you have done that it allows you to adjust the collimation a lot more easily because they are larger and easier to use and you do not have to use a screwdriver or Allen key right next to the delicate objective glass.

I would like to point out that it is best to not calibrate your telescope outside at night focusing on a star instead to use an artificial star. This allows you to collimate your telescope inside your house and not have to try and do it in the dark outside which makes everything a lot easier.

So, the first thing you want to do is to put your artificial star as far away as you can from your telescope inside your house and then point the telescope at the artificial star and try and line it up in your eyepiece.

Then you want to try and make an airy disc by slowly moving the focus so that it is out of focus and so you can see a blurred star like a circular blur around it then you want to look for where the largest bulge is and you want to sharpen up this side or let it in or let it out 

To do that first you need to put an object across the front of the telescope such as your finger and you will see this come through on the image now if it is closest to one you find that screw that is closest to the bulge. You need to let it in or let it out and I usually turn the screw about an eighth of a turn and then I will do the opposite direction for the other two screws.

You keep on doing this until you have managed to flatten it out the bulge and have a symmetrical airy disk.

Once you have finished your calibration by defocusing one way. Then defocus in the other direction by the same amount. If it is still calibrated then everything is fine otherwise restart the calibration process.

If you would like to see what all these steps look like please check out the episode below.

I get commissions for purchases made through affiliate links in this article.