Collimating the C5+

last updated 24 June 2006

Collimation is important for any telescope, but much more so for SCTs. The reason is the magnifying effect of the curved secondary in SCTs. For ordinary reflectors, a small change in the alignment of the mirrors has a correspondingly small change in the final image. But in SCTs, the secondary typically acts as a 5x magnifier, so even small changes in alignment translate to large changes in the final image.

In the C5+, as with most SCTs, the only collimation adjustment possible is to the secondary mirror. The primary mirror is aligned at the factory and the telescope must be returned in order to realign the primary.

Adjustments to the secondary are made using the three Phillips head screws in the secondary fixture located at the center of the corrector plate in the front cell. Here are the steps involved:

Warning: These instructions--especially regarding the screws and the directions in which they should be turned--are specific to the C5+. The basic principles are, in my opinion, valid for most SCTs, but SCTs may vary in the kind of screws used to adjust the secondary, and the direction in which they should be turned.

First, I'll assume that you typically observe with a star diagonal. Leave it in. Do not confuse collimation with the star test, which is best run without the star diagonal. Have the diagonal point up and away from the focusing knob. (This is for the C5+, whose focusing knob is in this position. For many other SCTs, the focusing knob should be directly right of the diagonal, so align the diagonal accordingly.) Throughout this procedure, look through the eyepiece with the telescope directly in front of you, rather than to the side. Use an eyepiece with about a 10 mm focal length. (In general, use an eyepiece with a focal length in mm about equal to the focal ratio of your scope.)
As in the star test, find a star of about second magnitude. Polaris is a good star to use which is visible year-round in the northern hemisphere. Southern hemisphere users can use alpha Pavonis, which is visible most of the year and can be found at (J2000) RA 306.412, Dec –56.735 (RA 20h 25m 39s, Dec –56d 44m 6s).
For coarse collimation, center the star and rack it out of focus in either direction until the blur fills up about 1/10 of the field of view (by diameter). You should be able to see the shadow of the secondary as a dark spot somewhere toward the center of the blur.

If it is not in the center, you will need to center it by using a Phillips head screwdriver to adjust the secondary alignment. Suppose that the shadow of the secondary is leaning toward the 3 o'clock position, as shown above. Now come around to the front of the telescope. Using the up direction of the diagonal as 12 o'clock, find the secondary screw that comes closest to either 3 o'clock or 9 o'clock. If the screw is in the 3 o'clock direction, turn the screw clockwise; if the screw is in the 9 o'clock direction, turn it counter-clockwise. In short, turning a screw clockwise pushes the disc away from the screw's clock position; turning it counter-clockwise pulls the disc toward the screw's clock position. If your screws are placed as shown below, you should turn the rightmost screw clockwise.

Use small turns--at most about 1/10 of a turn. Repeat this step until the shadow is centered in the blur, as shown below. Remember to center the star after each adjustment.
For fine collimation, focus the star as tightly as you can. You should see the star as a very small disc, surrounded by a thin halo. This is the Airy disc, surrounded by the first diffraction ring. Now, rack the star out enough to see about three diffraction rings. The disc will shrink until it is barely visible somewhere near the center of this "bullseye" pattern (see below).

Using the same procedure that you used to center the shadow of the secondary, center the disc, so that it looks something like the image below. Again, remember to center the star after each adjustment.

Then rack the star toward focus so that only two diffraction rings are visible. Somewhat surprisingly, this pulls the disc out and emphasizes its misalignment. Again, turn the screws until the disc is centered. At this point, your screw turns should be extremely small, probably only a couple of degrees, just the slightest give on your screwdriver.

That should do it! If you had to do a lot of realigning, you'll be pleasantly surprised at how improved images are, especially with the planets. As I've mentioned numerous times to other observers, fine collimation can make the seeing much better! :)

Caveats

If you can't see an Airy disc or the star skips around, either the seeing is too bad or the C5+ needs to be cooled down. Experience will tell you which is the case. If the seeing is too bad, you'll only be able to do the coarse alignment. Then again, if it's that bad, you won't be able to see that much detail on targets anyway, right...?

The C5+ tends to hold its collimation better if the screws aren't too loose. If they feel loose, then anytime you would turn a screw counter-clockwise, instead turn the other two clockwise. That should have the same effect on alignment. Similarly, if a screw you would turn clockwise is too tight, then you should loosen the other two instead. It's unlikely to be too tight to actually damage the secondary (though I suppose that is possible, so take care), but long before you get that tight, the secondary will deform slightly, resulting in a "pinched optics" kind of appearance.

After the collimation procedure, you may find that your finderscope is no longer perfectly aligned with your telescope, especially if you made large changes.

Additional Information

A more involved discussion of collimation and its importance can be found on Thierry Legault's page at http://perso.club-internet.fr/legault/collim.html.