Telescope Reviews: Optical aberration in homemade refractor. Help...

I see a lot of different theories that haven't been properly defined.

1. Is the mount polar aligned? If not, it is tracking error.

2. Is the tracking engaged? If not, it is tracking error.

3. Is the mount NOT polar aligned--a whole LOT of not polar aligned? The error could be field rotation. Field rotation is limited to 1. alt-az mounts 2. VERY POORLY aligned German equatorials and 3. pretty well aligned german equatorials over long exposure periods. It is not to be confused with sidereal motion correction. You can have an alt-az mount tracking a long-length galaxy dead center and it will rotate inside the field of view, staying centered. That's field rotation. That's why fork owners get equatorial wedges and some Newt owners opt for equatorial platforms, or equatorial mounts.

4. Another possibility is periodic error, but this typically does not "arc". On a well polar aligned scope it will make a linear back and forth movement.

I think that several errors are present. I think there is some shutter shudder going on in the first picture. The long arcs on the longer exposure indicates that the mount is either grossly not polar aligned or:

5. There is a large amount of backlash in the mount. This would be unsurprising. You get everything set up and centered but don't approach from the correct angle. You snap the picture but the mount doesn't begin to track for thirty seconds or so till the worm catches up with the worm gear on the mount head. This backlash can be quite large on (a) older American mounts from the 1960s (for example) and (b) inexpensive Chinese mounts.

One test of large backlash is to use the paddle to move the mount in one direction, so that a star moves from one side to the other. After traveling abruptly reverse direction. On one side, and sometimes two, depending on what's going on, there could be a delay before the worm catches up and starts moving the mount in the opposite direction. That's your backlash and though it might seem not a problem when you reverse at paddle speeds, it can be quite a delay when you've got the mount tracking at sidereal.

Which of these things it is I can't say, but Photon Wrangler will want to confirm polar alignment, confirm how the backlash is working and oh yes:

6. confirm that the clutches are engaged so that the mount is in fact tracking!

These are basic issues. There are a few others

7. confirm mount is set for N hemisphere (often times they aren't).

8. confirm power supply

9. confirm power is turned on and all wires are reliably delivering power to motors

10. Confirm dec wire is going to Dec motor! And RA wire to Right Ascension motor. A frequent set up error. If the wires are reversed the mount isn't tracking.

11. Make sure mount is set for sidereal and not lunar. Sometimes we do moon stuff and forget to set it back. The moon is going more slowly or "backwards" relative to the rest of the stars so if you're tracking at lunar rate you will get streaks as if the drive were turned off, but not as bad.

hope that helps and good luck!

Greg N

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