ST-4 Autoguider

I use an ST-4 autoguider from SBIG for guiding prime focus astrophotos. Although it takes a few minutes to set up, it sure beats manual guiding with a reticle eyepiece. I have not experimented much with using the ST4 as an imager and have only acquired a couple of CCD images with it.

I have experimented with the various ST4 guiding parameters and have determined the values that appear to give the best results.

GM-8 with Traveler

I am quite happy with the way the ST-4 works with the GM-8 mount. The refractor gives nicely focused off-axis stars, and I have no difficulty finding suitable guide stars. Here are the ST-4 parameters I like to use:

EA=1, b=2-4, ba=A, SA=2-5, FL=S, C1=10, C2=10, H1=0, H2=0

I aim for a reading from the ST-4 of 15 to 40. I try to keep the exposure (EA) constant at 1 second and vary b. Sometimes I change ba to faint mode (F) if the guide star is very dim. If the star is too bright, I decrease EA and then set averaging (AA) to give corrections at 1 second intervals.

The GM-8 is set to 2x guide rate, and the dec backlash compensation is set to about 5. The PEC function of the GM-8 is not used. The mount is aligned using the polar axis scope, which seems more than adequate for astrophotography. I have not had to use the drift alignment procedure.

The CCD head needs to aligned correctly with respect to the mount before performing the ST-4 calibration procedure. This is done by first orienting the label on the CCD parallel to the camera body. I then peer down the optical axis and note the angle between the CCD head and the top of the OTA (or the declination axis). The CCD is then rotated by this same angle and in the same direction (i.e., 20° clockwise). The ST-4 is calibrated every time the scope is moved.

LX200 SCT

I no longer have this scope, but this is what I found using the ST-4 with the LX200.

EA=1-3, b=3-4, ba=A/F, SA=7-10, FL=S, C1=5, C2=10, H1=0, H2=0, AA=1-4

Initially, I had trouble tracking on objects with DEC less than -15. I attributed this to atmospheric effects since the objects are within 40 degrees of the horizon. An alternative explanation may be problems with balancing the optical tube. I hang a weight on the front of the tube to offset the weight of the ST-4 head, and the dangling weight might introduce some periodic flexure.

The symptoms I experienced were large oscillations in RA and/or DEC. They sometimes lead to loss of tracking star and inevitably resulted in streaked or elongated stars. To eliminate these oscillations, I used a short exposure time (EA=1-3) and a rather high value of the scintillation adjust (SA=7). Sometimes I averaged over several exposures (AA=1-4) if the atmosphere was very unsteady.

My drive had a hysteresis in DEC of about 3 sec when using the guiding drive rate. However, were no benefits were noted from setting the H2 parameter. Also, although the scope had a rather long focal length (2000 mm at f/10), I did not noticed any difference between the short and long focal length settings (FL parameter).


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