Autoguiding - Exposure Time vs Minimum Motion

Exposure_Time-vs-Min.Motion

Originally published 12-May-2010

Regarding, "Typically you will want to autoguide with exposures of 4-5 seconds to overcome effects of seeing ...", (from a Yahoo Group post), I believe there is a different school of thought developing on that point. From my experience, the key to avoid chasing the seeing is to increase your minimum motion rather than increasing exposure time. This enables using a so-called "mid-range" mount and making quick, responsive corrections. To my way of thinking, you want to choose an exposure time that provides a solid SNR but does not saturate. Then, separately, adjust your minimum motion/movement to accomodate seeing conditions. Using PHD Guide as one example, I typically examine the star profile and shoot for a nice sharp spike rather than a plateau. But, the error most commonly made is to leave the minimum motion set too low and then increase the exposure time, which bloats the guide-star. For example, I myself use an Orion ST80, 400mm f/5, with a Q-Guider, which has 5.2um pixels. The resulting image scale is of course 2.68 arc-seconds/pixel. But at the default minimum motion setting of .15 (pixels), that turns out to be .40 arc-seconds. That means PHD would issue a correction if it detected a move of .40 arc-seconds. That would be fantastic seeing, no? Even if you double that to allow for movement in either direction, that's still .80arc-seconds. For most of us, seeing is around maybe 2 arc-seconds. In practice, I typically set exposure to 1.5 seconds. But, if the star I want is bright enough, I'll go down to 1 second, again, to get a nice sharp guide star, and let the min. motion take care of chasing the seeing. I've used anywhere from .25 pixels to .40 pixels min. motion. I've also seen it advocated to reduce aggressiveness. That is the last factor to adjust - again, it has nothing to do with seeing. Once you have the above factors determined, if you're consistently overshooting, i.e., bouncing back and forth (overcorrecting), reduce it. If your corrections generally take 2 or 3 moves in the same diection to get the star back where it belongs, increase it. So, in summary, use exposure time to provide good data for the centroid calculation and deal with chasing with the parameter designed for that purpose - the Minimum Motion Setting.

Thanks and regards,
Bill Shaheen Gold Canyon, AZ\ USA

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