In a message dated 8/18/2005 3:09:51 PM Central Daylight Time,
bill@... writes:


>
> Thanks Jim and Roland. I am reading the suggested materials and
> will experiment with the dec setting and calibration. I absolutely
> have Dec corrections on and have left the Maxim dec value turned on
> so this gives a direction to works. I appreciate the quick
> responses. For the record, I'm using a STL11k and guiding with the
> on-board guide chip.
>
>

I'm not sure what you mean by "have left the Maxim Dec value turned on".
Maxim has a window in which you place your declination number. This should
always
be set to 0 Dec. Otherwise you may run into instability problems.

As others have pointed out, you should remove any backlash settings in your
keypad.

Other things to check:
1) Calibrate on a bright star somewhere near the celestial equator. Use this
calibration everywhere in the sky. If you don't go across the meridian and if
you keep your camera aligned the same way, you never really need to change the
calibration. I use the same calibration for many months and never need to
change it, no matter where I image in the sky.

2) Always calibrate at 1x. This will give you 15 arc seconds per second
motion during calibration. Check the parameter numbers* in Maxim after
calibration.
If they are wrong, this WILL lead to instability (either too high or too low
gain). Too high gain means the star will bobble back and forth. Too low gain
means the star can wander away and never be brought back to center.

3) Your agressiveness values are too low if you have them as low as .2.
Normally they would need to be above .5.

4) Make sure that your camera is lined up close to the RA/Dec direction. If
not, you could have instability problems. I always check my parameter number
for camera angle, and aim to be within 1 degree of 90 if possible.

5) check to make sure that your Min and Max move settings are not rediculous.
I would say that the min move should be on the order of your pixel resolution
(see below). The Max move should not be longer than about 1/3 second.
Remember, a 1 second button push command at 1x guide rate from your camera to
the
mount guide port means that the mount WILL go 15 arc seconds in whatever
direction it was commanded. Is there ever a time when you want this large a
motion?
Probably never. Even 1/3 second command is large, in that it will instantly
command the mount to go 5 arc seconds.

*Explanation of parameter numbers: These numbers tell the camera how many
pixels the mount will move for a 1 second button push. Let's say that you have a
1200mm focal length and 6.8 micron pixel. At 1x, the star will move 15 arc
seconds per second across the chip. How many pixels per second is that? Well, at
the celestial equator, the angle per pixel will be Arctan (6.8/1200) = 1.17
arc seconds per pixel. Therefore the parameter number will be 15/1.17 = 12.8.
Your parameter numbers should be near this value, if not, then your calibration
was not done correctly.

If your parameter numbers are significantly less than this for either axis,
then you have a gain greater than 1, which means that the star could bobble
back and forth at the slightest atmospheric disturbance. If your numbers are
much
larger in either axis, then your gain is much less than 1. This is stable,
but if the gain is too low, the guiding becomes sluggish and will not respond
quickly to errors as they accumulate. The guide star may even go off the chip
because the camera sends only very tiny corrections to the mount, which are not
enough to bring the star back to the center.

Roland Christen


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