Focus Command - Santa Barbara Instrument Group ST-i Operating Manual

Note: Dark subtracted images have an offset of 100 ADU added to them to
ensure values near zero are not clipped.

Focus Command

Using the Grab command to focus your telescope would drive you crazy,
taking a single image at a time, tweaking the telescope, etc. Instead we use
the Focus command, which is like Grab but it takes image after image
automatically. Use the Focus command in the Camera menu and set it up
in the Focus Setup dialog as shown below:
Click OK and the camera will go into Focus mode. You'll see image after
image appear in a single window at a rate of several frames each second.
The images will vary slightly from image to image, mostly in the random
background noise, but in general they'll all look a lot like Image 1. You'll
also notice that a new Focus Mode dialog appeared.
Here's a brief description of the key items in the Focus Setup and Focus
Mode dialogs.
Exposure Time – You'll be able to see most objects in a 1 second
exposure. Use shorter exposures (down to 0.001 seconds) for
faster rep rates when focusing on bright stars or longer exposures
when trying to center dim objects. The first time you focus it helps
to use an exposure of about 3 seconds so that you can see even out
of focus stars. As the stars are brought to sharper focus you can
reduce the exposure time.
SBIG ST-i Manual
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