What would cause Polar Alignment to move in an obs?

[swag72]

About a month ago I managed to get my PA pretty sorted with the help of PEMPro. I am observatory based and so figured that this would stay OK for quite a while. Last night I noticed that my 30 minute subs really had some drift on them, and so tonight I am back out with PEMPro to check alignment.

Imagine my surprise when it showed itself to be rather out indeed and in both azimuth and altitude. So I am wonering what may cause this? Nothing has moved or been moved since the last time.

Would welcome some thoughts on this. I don't mind having to check it monthly but do wonder why it has changed so much and what causes the change.

[nmoushon]

If your obsy has pretty extreme temp changes from night to day that might do it as the metal expands and contracts will loosen bolts that are super tight. i.e. like the ones that are standard for RA/DA movement.

Also if your obsy has recently been built then it could be your peer settling. Not sure if yours is new or not but just one thing that could do it.

[astrofox]

Maybe its movement due to ground water. An Obsy is a pretty sensitive instrument to measure ground heave either through water logging or soil drying out. Its not unusual to see cracks in buildings caused by ground subsidence to move a couple of milimeters.

[swag72]

Thanks both - I am inclined to go with heat changes on account of me living in Spain and the temp in the obs ranging from over 40 degrees to the low 20's. I did wonder if it could perhaps be heat. The pier has been in since September last year and the obs is built on a flat roof, so no water to worry about. Perhaps general subsidence? I will keep an eye out for cracks.

[nmoushon]

Your obsy on your roof?! Cool ! But that would definately heat up you obsy by at least 5-10 degrees higher than the temp given by the weatherman. So I would go with the change in temp. I don't think you'll have to worry about cracks as it really isn't that heavy. Not much you can do about the change in temp unless you put an AC in but doubt thats worth it. Don't know if its as hot over in Spain this year as it is here in the US put if its anything close it sucks and wish the best.

[ollypenrice]

In astronomical terms a roof is far from inherently stable. We aligned Yves' mount in early December, here, on a mainly mature ground based concrete slab. One foot stands on a new extension to the old mature concrete base. Using a full format chip in a scope with 2.4 metres of focal length we are still doing half hour subs without any issues whatever. Nothing has been touched, so I think that sometihing is moving quite a bit for you, Sara. No idea what, mind!

Olly

[swag72]

I don't *think* I have much movement, I can move about frrely in the obs without affecting the guiding graph at all. But as I have learnt to never under estimate anything, lets assume it's possible. Is the temperature fluctuation equally as likely? That I do know happens!

[JamesF]

Might be worth marking the positions of the alt and az adjustment bolts for the future so you can check to see if they've moved at all?

James

[swag72]

Good point James, will mark them later.

If they do move, I wonder if the upgraded bolts will halp?

[JamesF]

I can't immediately see why they would. A locknut on each of the adjustment bolts might prevent them moving at all, but could make it a pig to get set up.

James

[steppenwolf]

The altitude bolts on the HEQ5 are not exactly precision and the interface with the adjustment tongue less than optimal so it could be some movement here, perhaps even a slight bending of the bolts over time.

[swag72]

The altitude bolts on the HEQ5 are not exactly precision and the interface with the adjustment tongue less than optimal so it could be some movement here, perhaps even a slight bending of the bolts over time.

If that's the case then a bolt upgrade would be beneficial, yes?

[JamesF]

I'm not convinced, Sara. If the current bolts are a poor fit then it may well improve things if they were replaced, but if the problem is caused by, say, the mount body and the bolts expanding and contracting by different amounts in the heat then it may not. Or it might just move the problem elsewhere.

The other side of the coin is, I guess, that if you're absolutely stuck for ideas and that's the only thing you can think of to change that might stand any chance of making a difference, then you have little to lose (bar the cost of the bolts) by changing them.

Should you decide to replace them (or even if you don't I'd get the mount nicely aligned once more and mark reference points on as many moving parts as possible. If this happens again then perhaps you'll have some evidence as to the cause.

James

[swag72]

Thanks for that James. I have marked the bolts, it was a little tricky!

[JamesF]

Yes, I can imagine

I'd also mark the position of the mount base against the top of the pier and the position of the two halves of the mount at the alt-pivot. Depending on how mad you want to go there are lots of other things you could do as well, but I think the tricky bit is going to be noticing that something has changed. We're only talking about tiny changes, after all.

James

[steppenwolf]

I agree with James, if this is a temperature thing then new bolts won't help - I was thinking more in terms of fatigue in the rising bolt but upgrading the bolts to the Astro Development ones is a worthwhile improvement anyway and nothing to lose apart from the £20.00 cost!

[alan potts]

Sounds a bit silly but has someone knocked one of the legs, I don't see how if it was set correctly it would have moved, mine don't. Is the mount old, has a bearing broken, it could be many things. I would give the mount head a good going over to see if the is any play anywhere.

Alan.

[swag72]

Don't have any legs Alan (well I do but the mount doesn't!!) as it's all pier based. Nothing has been knocked as far as I'm aware and it's only me who goes in there. The mount is 2.5 years old and it does have some play in the DEC axis that I've been unable to get rid of, but it managed 11 / 11 30 minute guided subs last night, so the movement isn't affecting it too much!!

[nmoushon]

Just remember when you mark you bolts that the movement is very very small and will take some time before you can clearly see some movement. If any. I would suggest maybe checking on it a couple times a day to see if you can even see any movement.

[pete_l]

About a month ago I managed to get my PA pretty sorted ... Last night I noticed that my 30 minute subs really had some drift on them, ... Imagine my surprise when it showed itself to be rather out indeed and in both azimuth and altitude.

You're not suffering that the affliction of a saggy drawtube, are you.

ISTM if the mount/pier are pretty solid, which by all accounts they are, there's not much opportunity for them to move. Any movement is more likely to be in the smaller components than in the large ones.