Roger: The others are correct, the best method is the DSI method. Forget the first pages of the DSI instructions as it is just complaining the other methods. Good stuff at end. For your list:
So here's the full collimation steps.
1. Use the newtonian laser collimator to adjust the secondary mirror, the laser beam should be reflected back to the center of the collimator.
Roger: Partially correct...
-- You cannot assume the focuser is pointing down (up?) the optical axis. So you must adjust the focuser alignment to make the beam hit the center of the donut on the secondary. But FIRST:
-- Check your laser pointer is not by itself misaligned by rotating it while it is (very slightly loosened) in the focuser and observing the beam hitting the a piece of paper temporarily stuck over the secondary mirror. Put a black dot on the paper, or print concentric circles on it. Don't worry the paper mark is aligned with the secondary donut at this stage.
-- Once your collimator is proven good, then rotate your focuser to assure the beam also does not draw a circle as you rotate it.
-- Push side ways and up/down on the focuser tube (use the marked paper) with everything tight to simulate the weight of your camera, FW, etc. A lot of motion is going to affect your optics. Now you are ready to collimate!...
-- Yes, Now adjust the focuser to point at the center of the donut. It kind of "disappears" when it in the center of the donut and not reflecting off the black donut. The laser beam should be reflected back to the center of the collimator. Yes, adjust secondary to get it to reflect back to the collimator.
2. Now change the laser with a sensor or camera.
Roger: Yes, and include the reducer you plan to use.
3. Set the artificial star in the center of FOV, defocus the image in order to see the donut.
4. Adjust the primary mirror until there is no on axis coma.
Roger: 5. Then either move your artificial star to each of the 4 corners, star defocused, and take images. Fine tune primary mirror to balance any coma. All 4 corners equally good! This is following the DSI method as I recall. Write down every adjustment and the resulting effect on coma. You only need to adjust any 2 of the 3 locking positions. If you adjust all 3 you will eventually be changing the distance between primary and secondary. Just put a piece of tape over the 3rd position so you never want to use it. And don't overtighten the screws!
Remember your reducer may make coma show up all by itself. You cannot collimate to overcome the reducer effects. Different reducers are matched to different types of optics.
For step 5 I prefer to image an open cluster with lots of stars. Then with one image you can instantly see coma balance in the 4 corners, and in the center of FOV. Also, your scope will be pointing up (same as imaging) and not horizontal like the artificial star.
Threaded connections, not compression connections are better for your optical components, especially if your camera stuff is a bit heavy.
You should budget at least 4 hours doing steps 1 to 4, then 1 hour for step 5 if pointed up at the stars.
Every now an then, every few months, (or if you take out your reducer or other major adjustment), you can just do a quick check by imaging a cluster with out of focus and see all star images are balanced.
Welcome to the world of (relatively) long focal length RC imaging! Forget Andromeda unless you want to make a several images mosaic!
Edited by rockenrock, 27 September 2021 - 12:51 PM.