Thanks, Dick
If I check the calibrate checkbox, as I now plan to do based on your comments, will ACP also calibrate the guider images?

Since I have an off-axis guider (my guiding sensor is included in my SBIG filter wheel--not the camera), I have heard from another wiser-then-me ACP user that I might get better guide star images and thus better tracking by having ACP calibrate the guiding images--and maybe select a better guide star.

Of course, the pixel size of the sensor for the guide camera is not the same as that for the main imaging camera.
And the array for the guide sensor is only 648 px x 486 px instead of 2750x2250. So it is hard to see how to use the main camera calibration frames for the guide sensor.

I can see that I have quite a bit to learn!
Many thanks for the help.
Warmly, Roger