Setting circles question and a bit of DIY (motorising the mount)

[blusky]

Hi all,

What accuracy is typically required from an acceptable set of setting circles? Degrees? Fraction of a second?

There are compass chips around that typically give accurate readings within one or two degrees. Am thinking - would that be too coarse for digitally calibrating the mount?

Apart from calibrating the scope to two or three celestial objects - what other methods of mount calibration (preferably automatic) exist? E.g. Polaris alone would be sufficient for a Dob, no? Say, I give my best guess of the true north corrected for magnetic declination. Given latitude I rotate the scope towards Polaris and ask the user to verify the position. Once I have that and given the base is perfectly level I know where to point the scope for a given object, no?

Thanks!

[JimStan]

Consider that the moon is only 1/2 degree across, so if you wanted to use setting circles to pinpoint something that large, you would probably need accuracy at least as good as that!

Prior to the days of "goto" I had a scope with electronic setting circles. You were supposed to be able to point the scope, guided by the setting circle readout, but I hardly ever was able to get an object in the FOV of even my widest eyepiece.

I suspect that mechanical setting circles that aren't at least 6 inches in diameter are going to not have sufficient resolution to allow you to point directly at some DSO without hunting around a bit for it.

Just my experience speaking.

Jim S.

[blusky]

Consider that the moon is only 1/2 degree across, so if you wanted to use setting circles to pinpoint something that large, you would probably need accuracy at least as good as that!

Prior to the days of "goto" I had a scope with electronic setting circles. You were supposed to be able to point the scope, guided by the setting circle readout, but I hardly ever was able to get an object in the FOV of even my widest eyepiece.

I suspect that mechanical setting circles that aren't at least 6 inches in diameter are going to not have sufficient resolution to allow you to point directly at some DSO without hunting around a bit for it.

Just my experience speaking.

Jim S.

Jim, got it, thanks for the explanation. This, however, casts a shade of doubt on the ability of any GoTo mount to locate objects, no?

Once the object is acquired tracking is relatively easy, however locating the object, as you point out, for it at least to appear in the eyepiece, must be a real challenge, no?

[JimStan]

Goto mounts have gotten a lot better, but I have watched some members of our club, and others trying out their Goto scopes, and being extremely frustrated, because they cannot seem to get the desired object to show in the eyepiece even after many tries.

The secrets to getting good goto performance are getting that tripod nice and level, and having quite accurate polar alignment. If you are using alti-azmuth equipment, the clock has to be extremely accurate, and it is very important that you get the best 2 or 3 star alignment possible. If those guide stars are not centered in the eyepiece , it is quite likely that you will not find your targets without doing at least a spiral search for them.

Oh, I can't forget, use the lowest power eyepiece when attempting to goto a target. It should show up somewhere in the FOV, but remember if your eyepiece can give you at least a degree of sky at one time, you are going to have much better luck getting zeroed in on your target the first time !

I do not have a goto myself, and can be considered a bit of an old timer, but that seems to be what I have observed about the scopes I have seen others using.

I would love to hear some other's experiences.

Jim S.