Why an RC is different

Two mirrors, two jobs

On a Newtonian it’s enough for the primary to point the right way. On an RC both mirrors are hyperbolic and both are adjustable: their axes have to be parallel and also coincide. And there’s a trap: there’s a position where the secondary leaves the center free of coma even though the axes don’t coincide. What’s left is unbalanced astigmatism, and it shows up in one corner. That’s the infamous “perfect center, bad corner”.

That gives you the division of labor, the same in every technical source: the primary mostly moves the coma in the center; the secondary, the balance of the corners (and a little of the center). Doing it the other way round, using the secondary to clean up the center, is the most common mistake, and it never converges.

The center marks lie a little

The ring painted on the secondary marks its mechanical center, not the optical one, and the two can be anywhere from a few thousandths of an inch to a tenth of an inch (about 2.5 mm) apart. The baffles aren’t necessarily perfectly centered either. That’s why an RC can “look” collimated in a collimation scope and still perform badly, and vice versa. Daytime tools get you close; the stars have the final say.

Closed tube or truss

On the closed-tube GSO 6" and 8", the focuser bolts to the primary mirror cell: every time you move the primary, the focuser tilts with it, and the weight of the camera adds leverage. That’s the source of the forums’ “endless loop” and of the “never touch the primary” advice. It’s not that you can’t: it’s that you have to check the focuser every time. On 10" and larger, and on truss or carbon scopes, the focuser and the primary are separate.

The bigger the scope, the finer the work

Collimating a 6" or 8" is a lot more forgiving than a 12" or 14". At the same focal ratio, the same angular error does more damage the bigger the mirror. Big mirrors also weigh more, shift more as the tube tilts, and take much longer to reach ambient temperature: until they do, the donut “boils” and any measurement will fool you.

If your RC is 12" or larger: set it out an hour or two early, collimate at the altitude you’ll be imaging at, and expect more rounds than with an 8". That’s normal.

Give the screws names

Looking at the scope from the back, starting at the top and going clockwise: A, B and C on the primary; X, Y and Z on the secondary. Label them with tape. Never touch B and Z: two screws per mirror are enough to move it in any direction, and this way it doesn’t creep over time.

Heads-upOn the GSOs, the secondary’s center screw sets the mirror spacing at the factory: don’t touch it to collimate.