At night: primary for the center, secondary for the corners

The frame

Point at a rich star field about 70° up (the Milky Way is ideal). Focus, then rack the focuser out (moving the camera away) until you see donuts about 30 or 40 pixels across: note how far, and always do it the same way. Take a 5 to 30 second frame (10 or 15 work well), with good tracking.

Step 1: the primary removes the coma in the center

Look at the donut in the center. If it’s brighter on one side, there’s coma. The Deep Sky Instruments rule: tighten the primary’s pull screw on the bright side (or loosen the opposite one). Small turns: 1/4 to 1/16 of a turn, smaller each time. One frame after every turn.

Step 2: the secondary balances the corners

Now look at the stars near the edge, all at the same distance from the center. In a collimated RC they look the same as seen from the center: all round, all “flat” (short axis toward the center) or all “pointed” (long axis toward the center). What matters is that they match, not that they’re round.

If they come out pointed on one side and flat right across from it, the mirrors are misaligned. The rule: tighten the secondary’s pull screw on the side of the flat stars.

And back to the primary

Touching the secondary puts a little coma back in the center: remove it with an equal and opposite touch of the primary. Repeat both steps until the center is clean and the corners match. Two or three rounds are usually enough; with practice, 10 to 15 minutes.

Heads-upDon’t use the secondary to clean up the center: it’s the most common mistake. It leaves the center free of coma and the corners badly unbalanced.

How to turn a pull screw

Each adjustment point has a screw that pulls the mirror and one (or two) that push. To tighten the pull screw: back the push screw off a little, turn the pull screw as needed, then snug the push screw back with similar force. To loosen, the other way round. That way the mirror goes exactly where you send it and isn’t left loose or pinched.

Knowing which screw is which

The rules say “the one on the bright side”, but in the image you can’t tell which screw is on that side: it depends on how the camera is rotated. Deep Sky Instruments covers one quadrant of the aperture and sees which part of the donut disappears. The Astrolarius assistant does it differently: it asks you to tighten one screw 1/8 of a turn, measures how much and in which direction the image changes, and works out how far to turn each one.

TipTightening the pull screw on one side is the same as loosening the one on the opposite side. If a screw is already very tight, use the opposite one.