Guiding with ST-4

ST-4 is a very capable guider, able to achieve pin-point stars on even the really long CCD exposures. Here is my usual session with ST-4:

  1. Turn on ST-4 and let it warm up (cool-down, actually) for about 15 minutes

  2. Find and center a suitable guide star with a reticle eyepiece inserted in the  guide scope

  3. Turn off all backlash correction on the mount (TVC setting of 0).

  4. Judging by the brightness of the star, set the exposure time for ST-4. Usually between 0.5 and 3 seconds).   I first increase the Boost parameter (b) if the star requires an exposure greater than 1 second.

  5. Leave the bA, the brightness adjust parameter, on A.

  6. Set SA parameter to 5. If the seeing is truly awful (you may ask why I'm out imaging on night like this), set it even higher. 

  7. Set FL at S, as the longest focal length I usually image with is about 1200mm.

  8. Set G11 guide speed at .6x sidereal rate

  9. Set C1 and C2 parameters, usually to 10-20 depending on focal length

  10. Set H2 parameter to a number between 20 and 30, as I have some backlash in DEC

  11. Take a dark frame

  12. Switch to the Find and Focus mode

  13. Insert the ST-4 head into the guide scope, and focus to get a high brightness value. I've guided with brightness around 5, but 10 or above is recommended.

  14. Rotate the guider so as to get the X axis parallel to RA motion and Y axis parallel to DEC:
    As I slew the mount in RA, watch the coordinate display on ST-4. If only the X number is changing then ST-4 is perfectly aligned. If a small change in Y occurs while changing in X, I may leave it and worry about a better alignment. Usually a change of 1 in Y per 15-20 units in X is desirable.

  15. If the guider was rotated in the previous step, re-focus it again to get the highest brightness.

  16. Press the UP button on the mount hand paddle to take up all backlash in the Y-UP direction. Stop when the Y number start to increase.

  17. Calibrate ST-4

  18. After calibration, set the H2 backlash setting back to 0.

  19. Decrease guiding speed of the mount to 0.5x sidereal. In effect, this reduces aggressiveness of the guiding corrections.

  20. Put the mount into Photo mode to prevent accidental slewing.

  21. Start guiding and watch the guiding performance. Let ST-4 guide for 5-10 minutes, especially if the errors seem large.

  22. If the guider overshoots when correcting (oscillating +/- errors that seem to increase) try lowering the mount guiding speed even further.

  23. To judge seeing and periodic error, I sometimes want to turn off all corrections from ST4 to the mount, and just watch the ST4 error display. This can be done by unplugging the autoguider cable, or with Gemini, simply going into a Menu mode disables all corrections.

  24. For DEC corrections, consider building a device that blocks corrections in one direction. Here is a design I use by Wesley Erickson.

  25. If the errors are large, and display shows the dreaded E for either or both axis, consider recalibrating the guider.

Copyright 2003 Paul Kanevsky