On Monday, March 30, 1998 6:32 PM, Evan Garber

> I have posted some CCD shots taken for less than 1 minute in alt/az on
> an LX-200 showing poor tracking. Please be so kind as to take a look at
> these and post your opinion as to what is actually going on. Given the
> short length of the shots as well as there bubble like appearance, I do
> not believe field rotation is the culprit. All were taken around 9 P.M.
> - 10 P.M. south east. from Northern California on a rare decent night.)
>
> (I particularily would like to get Doc G.'s, Mike Hart's and Don
> Dillinger's opinions.)
>
> GOTO: http://www.dnai.com/~evang/badpic/problem.htm
>
> My guess is that Don Dillinger's assessment Stiction in DEC axis), in
> response to the prior posts, is correct.
>
> I would also like to know whether anyone else is obtaining the same type
> of results.
>
> THANKS!!!!
>
> Evan Garber
>
>

To attempt to duplicate Evan's results, I shot a few CCD images.
A precision leveling (0.005" /ft) was completed on a balanced 12"
LX-200 in the Alt-Az mode last night using a Pictor 416 CCD camera
rather than an eyepiece. A one star alignment was used to set
tracking rates to enable precise centering of the alignment star in a
subsequent 2 star alignment. The x,y coordinates of the imaging
chip were used to precisely center the star on the chip.

Images were shot at 1,2 and 4 minute exposures at 3000 mm fl,
close to twice the focal length of the 10" f /6.3 used above. Optimal
imaging targets were chosen to isolate altitude and azimuth drives.
Four minute exposures were needed to produce about 1/2 the drift
shown on the linked images including some images where the drive
speeds were greatest, such as at 80 degrees altitude.

I was able to reproduce somewhat similar results (though not so well
defined) from time to time in both axis of my modified 12" LX-200. It
appears that Evan is getting this in both axis as well. Whether the
altitude axis is worse is difficult to tell.

The equatorial mode outperforms the Alt-Az mode on my 12" LX-200,
however, it is easier for it to do so since only one axis is tracked and
at reasonable speeds at that. The drive is further fine tuned with PEC,
precision drift alignment. Further, I may make slight changes to the
RA drive frequency. The Alt-Az mode does not offer this degree of
control.

In the equatorial mode, Dec stiction can be seen with a reticle
eyepiece aligned with the RA axis. Dec stiction will cause the Dec
axis to NOT move perpendicular to the RA axis for several arc minutes,
then straighten out when the left fork bearing stiction is fully released.
When you reverse Dec direction, the above will repeat itself. You could
probably duplicate this test in the Alt-Az mode with a leveled scope
focused on a known vertical structure such as a radio tower. For
manually guiding, small to moderate Dec stiction is not critical, but for
calibrating an autoguider, any RA movement when the autoguider
expects only a Dec movement will cause an "unexpected movement
in y axis" and calibration failure. This can be helped by changing
calibration times for the Dec axis short of driving the star off the chip
to allow the RA coordinate to return to the correct expected x coordinate.

Since you are moving to a wedge and you have a mid-sized Meade
SCT lightly loaded, I recommend the Decectomy (Dec bearing
replacement surgery) be postponed until more compelling evidence
for the need is demonstrated.



--
Michael Hart
Husen Observatory

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