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The software calculates the centre of the guide star to a fraction of a pixel. I think the normal guideline in the manuals is a bright single star but not fully saturating the sensor. Not a double star as that could confuse the calculation of the centre of the star.
I found with autoguiding that not all guide stars give the same results.
The first thing I would do if I were getting poorer than expected guide errors was to try another guide star. Often a medium bright and sharp star worked better for me. Depending on your setup sometimes edge stars are not as round. They still work but I would pick a round one if I could. Sometimes you don't have much of a choice.
Check your cable works by hitting the NSEW buttons on the autoguiding tab and make sure you can slew your mount from those buttons. If not your autoguider is not connected to the mount.
With my STL I would chose the main imaging chip, have it as cold as possible, do 2x2 binning, make sure it was focused, download say a 4 second image with luminance filter, check the subframe box in the focus tab, drag a reasonably sized box around a medium bright sharp star (with no hot pixels and also no brighter star just outside the box). Then do the callibration with 15 seconds exposure time. Most of the time it worked. You should also use autodarks as the 237 chip is pretty noisy.
There may be other settings with a G11 that may need to be sorted like guide speed.
I tend to callibrate again if the object is on the other side of the meridian or far away from where I callibrated initially. Not every object.
Often times I only callibrate once. If guide errors become higher on another object then I may recallibrate in an attempt to reduce them. I am not sure it helps that much. I have read others who think it does. It can only help.
Callibration is much faster and easier on a Paramount mount and the whole thing only takes about 15 seconds and it rarely fail as above.
If its really buggy then you can copy down the callibration results from a previous time it worked and you can enter them manually if you have the same setup. I did that once when I was stumped and it worked.
Greg.
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