Home Up

 

Collimating a Schmidt Cassegrain Telescope

In many ways a schmidt cassegrain is easier to collimate.  In the first place, these types of telescopes hold their collimation better than a newtonian.  In the second, in my opinion, I have been able to get a much better collimation too.

Unlike my recommendation to perform the collimation of a newtonian during daylight hours, collimating a schmidt cassegrain is most easily done at night.  The only thing you should have already done while it was still light outside is to become familiar with the location of the collimating screws on the front of the telescope.  If you can't see three small allen screws, they may be under a cap in the center of telescope schmidt corrector plate.

I suggest that you do this collimation procedure with the scope in a straight through configuration - that is without a 90 degree prism.  Any misalignment of the prism with the body of the telescope will result in a bad collimation.  After your scope has been polar aligned and has reached ambient temperature, take a look at Jupiter or Saturn.  Remember this image, because we'll come back to it after completing  collimation and you'll see what a difference it will make to your views!

First, point your scope at a fairly bright star.  The star should be as high in the sky as possible while still allowing yourself enough room to get your head up to the eyepiece.  This will minimize atmospheric turbulence effects.  Put in a moderate powered eyepiece, about 100X.  Focus on the star.

The next steps will involve making the image of the star blurry - both outside of focus and then inside of focus, and closely comparing the images.  We will adjust the collimation screws until we have centered the airy disk (diffraction rings) exactly around the star and the rings look the same both inside and outside of focus.  If you see any spikes in the diffraction rings, these are caused by currents of air in your closed schmidt cassegrain.  You should allow your telescope cool down some more before starting the sensitive collimation procedure.

For no reason in particular, let's start by turning the focus outside of focus.  Keep turning the focus knob a couple of turns until you see faint concentric diffraction circles around the star.  Look closely at these circles.  Are they exactly centered on the star or are they off center?  With this image  in your mind, take your allen wrench and turn one of the collimating screws about 1/4 of a turn clockwise.  Remember which one you turned.  Now look at the image in the eyepiece.  Did the diffraction circles become more concentric? (This is what we're trying for.)  If not, turn the same screw back counter-clockwise a quarter of a turn and then continue counter-clockwise for another 1/4 turn.  Look at the image in the eyepiece.  If this made the rings more concentric, you may try another of the screws - making SMALL turns and then comparing the image before moving on.  In this way, by working with all three screws, the diffraction rings can be centered exactly upon the star.

Once you're pleased with the results, rack the focus to inside of focus.  Are the rings still concentric?  If not repeat the previous procedure until they are.

That's stage one and you probably have a pretty decent collimation, but you can do even better.  Move your scope over to a star that's about half as bright as your first star.  Repeat the procedure, but what you'll notice is that you will be able to see fainter diffraction circles that are closer to the star.  This will allow you to be even more precise.  Go slow, sometimes turning the allen screws only a 1/16 of a turn until the circles are as evenly spaced and concentric as you can get them.  You are done with the collimation!

(You may continue with a star even less bright if you wish, but you typically will need excellent steady skies.)

Now go back and look at Jupiter or Saturn to see the difference your work and patience has yielded.

    

Before (Out of Collimation) After (In Collimation)

Further Reference:  Advanced SCT Optical Adjustments

 

11/2011