How do I collimate my Schmidt-Cassegrain telescope (SCT)?
November 11, 2008
Collimation of a SCT is critical to getting the sharpest images possible, especially when viewing planets at high magnification. There are three stages to a really good collimation using the stars or a star-like source (such as a solar reflection off a distant ball bearing or Christmas tree ornament).
Before attempting fine collimation, especially in-focus collimation, always let the scope reach the temperature of the outside or ambient air. If there is a significant temperature difference between outside air and the scope’s storage area, you’ll need to wait at least an hour, maybe several hours for the scope to reach the ambient temperature. You may need to keep it in an unheated room over a period of a few days.
Seeing (the steadiness of the atmosphere) also affects your collimating ability. Heat waves and high-altitude winds move air around and cause differing temperatures of air to mix. This makes the air act like a weak lens that interferes with the light from a planet or a star by defocusing it. Choose only nights with superior seeing for collimation.
To collimated your telescope:
1. Start with a rough out-of-focus collimation. This initial step will get you in the ballpark for more accurate collimation later on. Using a medium-powered eyepiece, center a medium-brightness star in the field of view. Next, defocus the star until you can see a center dark spot (this is the secondary mirror shadow). This dark spot will not be in the center of the defocused star if the scope is out of collimation. To center this spot you will need to adjust the collimation screws located on the secondary mirror housing. Remove the logo cap over the secondary housing by gently prying with your fingernail. Turn the screws by only 1/6 to 1/8 turn adjustments, Each time you make an adjustment the star will move in the field of view, and you’ll need to re-center the star to check if the scope is in collimation.
Out-of-collimation SCT showing star going from inside to outside of focus.
Out-of-focus image of star
with collimated SCT.
2. Switch to a high-power eyepiece–about a 2.5 mm focal length is recommended. Again defocus the star just a little bit and make small adjustments with the collimation screws. When everything looks concentric and uniform, it’s now time to do an in-focus collimation. You will want to do this on an extremely calm night and use a star at or near the zenith. Your telescope must be the same temperature as the ambient air. At high magnification, a star will not look like a point, but like a small central disk surrounded by concentric rings (called the Airy Disk). You will use these rings to collimate your telescope just as you did in the previous steps.
Here are a few links on SCT collimation and the Airy disk.
Precise collimation is essential to good performance for any Schmidt-Cassegrain telescope. For SCTs, it’s done by small adjustments to the tilt and position of the secondary mirror in its cell. Celestron SCTs use 3 small Phillips or hex head screws for adjustment. Some optical tubes hide the screws under a cover that you can easily pry off with a fingernail. For Fastar-equipped SCTs, turn the triangular cover plate either clockwise or counterclockwise to reveal the collimation screws.
What may not be so easy is the inconvenience and anxiety of using a small, metal Allen wrench or Phillips screwdriver in the dark just above the vulnerable surface of your telescope's corrector plate to turn the three screws for collimation.
Several aftermarket solutions are available for this problem. The most popular is a set of knurled knobs that replaces the hex head screws. These thumbscrews let you collimate without tools, so you can easily make adjustments to the secondary without any concerns of damaging your SCT’s optics.