How does a mount track?

[Kirkus (Kirk)]

I'm still asking beginners questions, and I'm sure this one will seem extremely basic, but I just want to make sure I have this right.

On an equatorial mount, my LXD75 for instance, if I have perfect polar alignment and my scope is tracking an object in the sky, the only axis that should move is the R.A., right?

I'm having a heck of a time getting my mount aligned and auto-tracking well. My tracking software isn't working as well as it once did, and I'm sure it's due to a problem with my mount/polar alignment.

I think if I understand what's happening, I'll be better equipped to troubleshoot the problem(s).

[Merlin66 (Ken)]

Yes is the answer.
Dec corrections should only be necessary when observing close to the horizon where you may get some atmospheric abberations.

[montewilson (Monte)]

When I started doing astro imaging in the late eighties, there was rarely a dec control. If there was, it was a tangent arm. The RA control was all that we used. If you are well aligned you will need to make minor adjustments in Dec only and corrections in RA to account for inaccuracies in the worm gear.

HTH

[OneOfOne (Trevor)]

The only time there would be "automatic" changes in both axes would be if you were using an autoguider. If the drive was not moving at the correct speed though, I would expect the object would slowly drift out of alignment, but moving the drive in RA should move the object back to its original position, confirming it was caused by incorrect speed.

If the optical axis of the tube is not parallel to the polar axis of the mount it I think it would appear to move out of alignment, however, I don't know how you would check this, but I am sure someone else on the forum would be able to help you out with this if they felt this could be the problem.

[gary]

Originally Posted by Kirkus

I'm still asking beginners questions, and I'm sure this one will seem extremely basic, but I just want to make sure I have this right.

On an equatorial mount, my LXD75 for instance, if I have perfect polar alignment and my scope is tracking an object in the sky, the only axis that should move is the R.A., right?

Hi,

Keep in mind that there is no such thing as a perfect polar alignment, not
even from a theoretical viewpoint. At best there is an optimal polar alignment for
the part of the sky you happen to be imaging in at any one instant in time. The
effects of atmospheric refraction mean that, from a practical viewpoint, it
is better to align the polar axis of the scope with the refracted pole rather than the
true pole.

For extended imaging, the effects of refraction combined with the geometric,
eccentric bearing and gravitational flexure errors within the mount mean that
you will also need to be making tiny corrections in Dec as well, which your
autoguider should provide.

You might like to consider posting a sample image and a brief description.
Some of the more experienced astrophotographers on the forum, such as
Monte, might be able to offer a diagnosis and suitable prescription.

Good luck!

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Terry B]

I agree with the other comments.
One point though is that if your polar alignement is off some mounts will correct both directions to compensate for this. An example is the vixen SS2000.

[Kirkus (Kirk)]

I'm actually using tracking software that has performed very well in the past. But not so much now.

In the past, when the tracking software has worked flawlessly, it's been when my Autostar has told me that my polar alignment was very very good.

The times I've tried recently however, my Autostar is telling me that my polar alignment is off... and the software hasn't been able to track worth beans.

So, I've put 2 and 2 together in my head, and am assuming that the software isn't tracking because I can't get my mount correctly aligned.

The software is constantly trying to correct on the DEC. axis. That's why I asked my original question. According the guy who wrote the tracking software, corrections to the DEC. should be minimal.

I'm just trying to wrap my head around it. LOL.