Polar and 3 Star alignment

[RobF (Rob)]

Just how clever are GOTO mounts?

If you don't have azimuth and latitude exactly aligned for SCP, can the GOTO ever successfully locate AND track without drift?


(been mucking around with drift alignment for first time tonight, thus the cogitating....)

[g__day (Matthew)]

Depends on the mount and hand controller. For instance in polar unaligned mode the SkySensor2000-PC can point and track accurately up to about 20 degrees off the pole given a 3 star align (and adjust for refraction of the air using King rates) - whether its tracking DSO, planets, the moon or satellites, but I'd expect you'd get field rotation even if the central spot tracks perfectly.

But the thing is if you're photographing something you don't want any source of error, pointing, tracking, flexure, mirror shift or flop, focus change through temperature variance, seeing error - nothing. So over time you go to significant effort to eliminate each of these sources of error. Most gear I expect won't track well if you're mis-aligned, and even then track well is subjective. At long focal length on long duration "track well" has to be pretty precise - down to arc seconds per hours maybe.

So yes learn drift alignment - and realise mirror shift / slop on an SCT or Newt - even tiny amounts - could just slightly alter your polar alignment precision. Barely noticable on 3-6 mintue shots, but significant when you go beyond 10 minutes at longer focal length.

[peter_4059 (Peter)]

Rob,

If you are setting up in the same location each time, one way to achieve quick setup is to make some marks on the ground where the tripod legs are located and an index mark on the mount to show the azimuth position after you have drift aligned.

Then next time you set up you just put the tripod in the correct location, level it and put the mount on with the correct azimuth. As Matthew says you will still need to drift align for long exposures but this method works fine for me up to a few minutes. I have three bricks sunk into the ground, each with a hole drilled with a masonary bit where the tripod leg needs to sit.

Peter