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In general, guiding is most accurate when using the shortest exposure time possible. In order to get the shortest exposure time in a given situation, try the following techniques.

First (in Preview mode), set the zoom level at least to level 2 so the star will appear larger.  The highest zoom level is 4 and it will make the star appear largest.  Focus at a high level of zoom, adjusting focus until the star appears smallest.  Also monitor the BRI reading.  It should increase as you close in on best focus.  While focusing, keep the BRI reading below 100 by using the – (reduce exposure time) button. This avoids saturating the CCD’s pixels and overexposing the image and will also help to achieve a better focus.

If the exposure time is too short to show the faint star you need, lower the noise value in steps until the star reappears. You’ll also see more electronic noise at low noise level settings.

if there is very little background noise, then continue to decrease both the exposure value and the noise value until you have the fastest exposure time possible without adding too much noise to the overall image.