Imaging and guiding near zenith

[johnrt]

I encountered problems last night whilst imaging. I starting guiding on my target in the east, phd was tracking flawlessly and I had a good couple of hours worth of subs.

All of a sudden as the target reached up to the zenith phd couldn't track anymore, and I got horrible elongated stars. I tried to recalibrate phd, but it wouldn't register enough movement in dec during the calibration phase. So I gave up and went to bed.

At a stab I'm guessing any errors in my polar alignment would be exaggerated at the zenith, making phd unable to cope?

Can anyone put me right on this?

Much appreciated.

[FraserClarke]

What sort of mount??

Guessing you're using a GEM, which probably means you're running into a balance problem. As the mount goes through the meridian (zenith), the weight shifts off the drive gears and gives you some instability.

If it is an alt-az mount; zenith is notoriously difficult as the rotation rate goes to infinity right at zenith. Most (all) alt-az telescopes have a zenith exclusion zone (couple of degrees) where you can't observe.

Shouldn't be anything to do with polar alignment.

[jgs001]

You don't mention how the mount is being controlled, but I'm gonna guess EQMod... there is an option in there for slew limits... this restricts the motion of the mount to certain angles to prevent eqpt damage etc, (like scopes being rammed into tripod legs). I'm thinking you hit against the slew limit and eqmod basically said no more. There are a couple of options of how this can be setup... what you have, park or flip... Park, slews the mount back to weights down, scopes up. Flip, performs a meridian flip for you, but you then need to stop PHD, possibly recentre your target, and recalibrate PHD.

[johnrt]

The mount is an eq5 and was just under control via the stock handset on my concrete pier.

I'm guessing it may be a balance issue then as the weights got higher it would become more unstable?

[FraserClarke]

Usually mounts are balanced slightly East-heavy, so that the gear drives against some weight and keeps it stable. As you come through the meridian, you might get to a point where the centre of gravity moves to the West of the mount, and the gears lose their loading and stability (the mount then tends to 'bounce' against the gear teeth as it drives). When tracking through zenith the top end of you telescope is furthest West of the pier for any given hour angle, so the effect would probably occur first near Zenith...

Try putting a little bit more weight on the east, and see if it improves it?

I'm not very experienced with GEMs though, so you might want to pay more heed others if they have different suggestions for this particular mount??

[johnrt]

Just checked my pier top, it wasn't quite level from east to west, would this have thrown it out?