Autoguider Calibration

In order to properly control the mount, MaxIm DL must calibrate the system. The exact orientation of the autoguider, the focal length of the guide telescope optics, and the speed of the motor drive all affect the calibration.

For best results, place a bright star in the field of view of the autoguider imager, near the center of the field. Take an exposure using the Guide tab’s Expose mode; the guide star will be picked automatically, or you can click on one using the mouse. Now start the calibration function by selecting the Calibrate mode and clicking Start. MaxIm DL will take a series of five full-frame exposures, moving the mount between each exposure. This will allow it to determine how fast the mount moves in each direction. The motion between successive exposures are in the +X, -X, +Y, and then -Y direction.

The star must move at least five pixels between each exposure for an accurate measurement, and preferably more. If the star does not move at least five pixels, an error message will be displayed at the end of the calibration cycle. The algorithm can be confused if another star appears in the frame; to minimize this risk, it always uses the star with the brightness most similar to that measured in the first image.

If the star does not move far enough, or moves too far (i.e. the star leaves the field), the duration of the calibration move commands can be adjusted by clicking the Settings command and changing the Cal. Time fields (measured in seconds). A longer calibration time will increase the motion of the star; a shorter time will decrease the motion. Typical values range from five to ten seconds, depending on the correction speed, focal length, and pixel size.

Using the Guide tab’s Settings dialog, you can also set a Backlash setting to run out any backlash caused by changing the direction of motion (also entered in seconds). See below for more information.

The X and Y Aggressiveness controls on the Guide tab allows you to adjust how vigorously star motions are tracked out in each axis. An aggressiveness setting of 10 means that the autoguider attempts to track out 100% of the motion, whereas a setting of 1 means that the autoguider only tracks out 10% of the motion. Lowering the setting will effective create a running average effect, which may help with tracking gradual drift while ignoring the random effects of seeing. If the setting is too low the guider may not be able to keep up with the drift rate; this may be a problem if your RA drive has large periodic error.  If your mount has very low/slow periodic error and low drift, an Aggressiveness of 3 or so should provide good results.  If your mount has larger or faster periodic error than a Aggressiveness of 8 may be necessary.  In general it is best to avoid using 10 because that can result in guider instability. Also note that the effective Aggressiveness can be affected by the accuracy of the autoguider calibration.  If, for example, there is a large amount of uncorrected backlash the calibration measurement may be low, resulting in more aggressive guiding than expected. You should experiment to determine the best setting for your particular telescope. Normally the controls for each axis can be set to the same value.

The calibration may also be adjusted manually using the Settings command. The X Speed and Y Speed parameters allow you to enter the apparent speed of motion of the star during guide corrections in pixels per second. For example, if the star moves three pixels in one second, the speed is 3 pixels per second. The number can be negative if a positive correction causes a negative motion. If the autoguider sensor is not aligned with the telescope axes, the rotation angle is entered in Angle (deg).