Re: tracking issue

On 6 Apr 2005 13:16:41 -0700, "spiral_72" <spiral_72@xxxxxxxxx> wrote:

>First, I am still learning, but something I have noticed bothers me. I
>have a permanent equatorial mount setup for my 8" Orion scope, polar
>aligned using the drift method. I consider the performance of my
>mount/drive to be excellent considering the equipment I use. However,
>there seems to be a repeatable difference in the tracking error between
>planets and deep sky objects.
>
>I expect the movement of the planets is different than that of
>background objects due to the plants' orbiting motion. For
>observations made at 150X, should I be able to notice this difference
>during a 15 minute period or is something else going on? I run a single
>axis drive on the R.A. It seems that planets require a faster drive
>speed with no noticed error in declination. I am certain the
>scope/mount is balanced. The mirror/tube assembly is ambient
>temperature so I wouldn't expect this to be caused by temp changes. I
>don't have any way to test for drive RPM irregularities.

Right now, Jupiter is drifting west with respect to the stars at about 3
arcsec per 15 minutes. Saturn is drifting east with respect to the stars
at about 1 arcsec per 15 minutes. So you would need a faster tracking
speed for Jupiter, and a slower one for Saturn, but only by ~0.01%. And
in both cases, there is a declination drift component, too, although
somewhat smaller. I don't think you could easily detect this drift.

Mars is drifting east at about 34" per 15 minutes (about 5.5 Martian
diameters). That's probably fast enough to be noticeable- but you should
expect to slow down your drive to compensate, not speed it up (and still
only 0.2%).

It seems to me you are probably seeing some other problem unrelated to
the drift of the planets. Possibly an alignment problem- the actual
drive rate changes with position in the sky because of refraction. Mars
is very low now, and depending on when you observe there could be quite
a difference in the altitudes of Jupiter and Saturn.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

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