Zenith balance?

[ollypenrice]

Following a question from Fay and thinking about tracking accuracy on the zenith, here's an idea I thought I'd try on you.

Away from the zenith you are likely to have a slight imbalance. Some people deliberately align away from the true NCP so the guider always pushes one way. An imbalance will have the same effect, with camera end or objective end heavy. But on the zenith this imbalance stalls becuase the heavy end is directly above or below the light end, perhaps encouraging the OTA to oscillate across the backlash.

In this case, what about a weight off to one side as shown in the (abysmal) sketch? You could use a magnetic weight placed on a short and normally empty arm. It ought to restore a little imbalance and kill the oscillation, no?

Olly

[martin_h]

I suppose that if you had a side by side setup you could add a weight to the guide scope side? Or just slide the whole lot over a bit?

[Coco]

Presumably this image would be incorrect if the scope had been travelling from the east heading west, it would fall on the gears, but I fully understand your meaning...

Image looks fine to me ... Some of the worlds greatest engineering problems have been solved on the back of a fag packet..

[skybadger]

Is this something you have experienced or a hypothetical question ?

I can't image the time window for this to occur last longer than a few minutes every time you image something on the zenith.

If you are using an equatorial, zenith imaging can only happen at the equator for LHA=0, otherwise the minimum observing LHA that allows access to the zenith is increasingly +/- from 0 as latitude increases. Thinking about it I think the minimum LHA to achieve zenith viewing must be equal to the latitude. Proving that could be interesting.

What this gets us too is that youre RA imbalance for driving will remain since you are operating across the equatorial but you may lose dec due to oscillation. IF you are perfectly polar aligned, that shouldn't worry you.

I tend to operate in fully balanced mode because if I don't there are sticky spots in the driver where I can lose fine guiding due to imbalance load on the dec wheel.

Did I miss something becuase I don't think the situation in your drawing can happen.

regards

Mike

[ollypenrice]

Thanks Mike. The question is hypothetical but based on reported difficulties. I know that literally being on the zenith won't be for long if at all but when in the general area it can get tricky. I was wondering why that might be and trying to think of an experimental solution.

Olly

[Fay]

I had at least 3x600 subs, one after the other, where all stars were egg shaped, on two occasions when imaging around the zenith. I will save one of the subs next time to show you

Fay

[fwm891]

Olly, Fay et al - In a lot of the old ATM books it seemed common to apply a weak spring to pull back or forward against or with the drive respectively...

From the egg shaped stars I'm getting I think I need one

[ollypenrice]

I often wonder about a weak spring as you describe but haven't though of a way of contriving one. Perhaps a clock sprin type woud be ideal.

Olly