Last updated on
4/11/
2006

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Collimating Your SCT

As I described on the Cassegrain Optics (OTAs) page, cassegrain scopes combine a spherical primary light-gathering mirror at the rear of the scope with a glass correcting plate at the front. On the back of the correcting plate is a secondary (hyperbolic-shaped) mirror that reflects light from the primary back through a hole in the center of the primary, through which you observe the image .  In the less-common Maksutof-Cassegrain scopes, the secondary mirror is a reflective spot applied directly to the back of the corrector plate because the curvature of the rear of the corrector plate is exactly the hyperbolic curve needed for the secondary mirror.  The relative alignment of the primary and secondary mirrors is fixed and cannot be, nor ever needs to be, adjusted.

In Schmidt- Cassegrain (SCT) scopes, the rear of the glass corrector plate is flat - the required, complex curvature of the corrector plate is ground into the front of the plate only. Thus for SCTs the hyperbolic secondary mirror must be a separate mirror - it is held in a mechanical assembly that is mounted in a hole drilled in the center of the corrector plate. Because of the requirement for this mechanical arrangement, a precise optical alignment between the primary and secondary mirrors cannot be designed to be automatic like it is in a Mak - the alignment of the secondary mirror can be, and must be, adjustable because it can shift slightly due to vibration when the scope is transported. (This situation is also true of the secondary mirror in Newtonian scopes, including Dobsonians.) The process of adjusting the secondary mirror in an SCT (or Newtonian) is called collimation.

You can determine whether your SCT requires collimation by defocusing the image of a star and observing it while you move back through proper focus, to being defocused in the other direction. If the central black spot is off-center from the set of rings in the images, as shown in this diagram (shamelessly copied from one of the Celestron SCT manuals), then you need to collimate your scope:

Experienced SCT users will tell you that the first step in collimating your scope is to understand why it is important by reading Thierry Legault's Collimation page (look at the astrophotography on his web site - he very much knows whereof he speaks).  Basically, precise collimation is somewhat less critical for observing DSOs but extremely critical for planetary observing.

How to Collimate Your SCT

An SCT’s secondary mirror pivots about a central post, like a three-way see-saw, and the angle of pivot is adjusted by three screws that are visible at the front of your scope. (Some scopes have a dust cover over the secondary mirror holder - if this is true for your scope remove the dust cover according to the directions in your scope’s instruction manual to expose these three screws.  Once you gain experience in collimation you probably won’t replace this cover.) These three screws will be either Phillips-head [more logically called “plus” (+) screws, as opposed to “minus” screws, by the Japanese <grin>] or Allen-head screws. In either case you would need to use a tool to adjust them.  This is totally idiotic - you must find a way to adjust these screws at the front of your scope while observing through an eyepiece at the rear of your scope.  Just for fun, try a dry run on this in daylight, but don’t actually use a tool because you need to do this while observing a star at night.  Even if you figure out a way to do this in daylight, you collimate your scope at night so you risk scratching the front of your corrector plate with the adjustment screwdriver or Allen driver in the dark, which is a Bad Thing to do. 

So the first step most folks take in collimating, is to replace the factory-installed screws with screws that have a knurled head you can turn with your fingers without a tool, which is what the manufacturer should have provided in the first place (I’m not making this up).  You can purchase a set of these three knurled-head “collimation knobs” for $14 from Jim Henson’s ScopeStuff web site, or similar ones for $16 from the Bob’s Knobs web site (which also has a selection of these knobs for less-common SCTs).  If you have a Meade LX90 or one of the 8” LX200’s you should order the “shorty” knobs, which will avoid a clearance problem that prevents the scope’s dust cap from fitting on.  The regular knobs are OK for the 10” and 12” LX200’s, as well as Celestron SCTs.

You need to be careful in how you replace the factory screws with the new collimation knobs, so that you don’t throw your scope’s collimation way off in the process. Even if you purchase a set of Bob’s Knobs instead of Jim Henson’s, you should still use the installation instructions Jim Henson provides on his ScopeStuff site - they are better than those provided with Bob’s Knobs. (Jim’s instructions are given for a Meade LX90 scope but that doesn’t matter - they are applicable to any SCT.)  Basically, Jim’s process involved resetting the scope’s secondary after each of the three screws is replaced, which is an excellent idea that really works to give you the same collimation in your scope when you’re finished, that you had before replacing the screws.  (If you think about it, that will make sense to you.) Note that if you live in a house which is long enough that your SCT, at one end, can achieve focus on something at the other end of the house, you can cut a hole in a cardboard box, put a light behind the hole, and cover the hole with a piece of aluminum foil with a small pinhole in it, aligned with the light source, and use this as the target for Jim Henson’s instructions.  (And with this target and a high-power eyepiece you can actually achieve pretty good collimation itself, when you proceed to collimate your scope, if the pinhole is small enough and far enough away from the scope.)

Many SCT users are afraid of the process of replacing the collimation screws (and of collimating their scopes in general) but if you follow the ScopeStuff instructions above, and the collimation instructions identified below, everything will work out fine.

Once you have the knurled collimation knobs in place, collimation involves aiming your scope at a reasonably bright star and tweaking the knobs according to the instructions given at any of the following:

Note that most folks in the northern hemisphere use Polaris as the star to collimate their SCTs because it doesn’t move much during the time it takes to collimate a scope.
 

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