Schmidt-Cassegrain Collimation: Your Guide to Pinpoint Stars
Unlock Your Schmidt-Cassegrain’s Full Potential: An Easy Collimation Guide
Schmidt-Cassegrain telescopes often get a bad rap, and many amateur astronomers prefer to invest in other optical designs. It’s true that, in the field, you often find small-diameter refractors delivering better planetary results than our larger telescopes, even those boasting excellent optics and significantly larger apertures! So, what’s the deal with this observation?
Thierry Legault, a name that needs no introduction in the world of astrophotography both in France and abroad, explains that Schmidt-Cassegrains are indeed excellent telescopes, but they are incredibly sensitive to collimation – the alignment of the optical elements within the tube! In fact, even a very slight collimation error (a mere quarter turn of a screw on the secondary mirror adjustment) is enough to lose two-thirds of your optical resolution. Under these conditions, a good 250mm SC ends up performing like an 85mm refractor!
| Before Collimation | After Collimation |
|---|---|
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The best way to collimate a Schmidt-Cassegrain is to observe a defocused star at a certain magnification and, through successive adjustments of the secondary mirror screws, achieve the best possible image. However, while this technique is perfectly explained on Thierry Legault’s website, it can be quite challenging for beginners.
The method I’m about to propose is, in my opinion, simpler for newcomers to implement and should help you achieve excellent collimation.
A quick heads-up: Before you start this procedure, make sure to set your telescope outside well in advance so it has enough time to reach thermal equilibrium.
This method uses a Hartman mask and a webcam.
Place your mask so that the holes align with the A, B, and C screws of your secondary mirror:
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Now, close the hole facing screw C with a paper or cardboard stopper. You should now have this setup (the blocked hole appears in white):
Point your telescope at a particularly bright star. Adjust your webcam’s contrast so it only displays the brightest pixels. Now, focus your Schmidt-Cassegrain until the two stars (remember, the third one is masked) merge into a single point of light.
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Next, move the stopper to the next hole (clockwise), which is hole A:
Do not touch screws A and B. Instead, adjust only screw C until the two stars merge into a single, perfectly round point.
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Continue this method with another rotation. Move your stopper to the next hole (clockwise), which is hole B:
Refocus precisely… And adjust only screw A until the two stars merge into a single, perfectly round point.
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Move your stopper back to its initial position (hole C):
Refocus precisely… And adjust only screw B until the two stars merge into a single, perfectly round point.
Keep repeating this process with successive rotations. You’ll find that this method converges very quickly!