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Grab Command
Let's take a dark frame with your new camera. If you haven't done so
already, screw in the nosepiece and put the rubber cap on it. While the
shutter was designed to block light from the telescope for dark frames, it
can't block flooding room light from leaking around the edges of the shutter
blade. That's why you should cover it under bright light illumination.
Now, use the Grab command in the Camera menu and set the dialog as
shown below:
Here we're asking for a 1 second dark frame of the full CCD. Click the OK
button and in a few seconds you'll see an image that looks a lot like the
Image 1 from the tutorial. Hot pixels, readout glow and salt-and-pepper
noise, they're all there. Congratulations: You've just taken your first dark
frame! That wasn't so hard was it?
Let's experiment with the settings in the Grab dialog. Try changing the
Exposure Time to 10 seconds and grabbing another image but this time
watch the LED on the back of the camera. It blinks while the camera is
exposing the image. That's a handy thing to remember when you're in the
observatory. Don't bump the telescope when the LED is blinking.
Anyway, the new image should look just like Image 2. Try setting the
Exposure Time back to 1.0 and the Dark frame to Also. What happens
here is the camera takes a dark frame first, and then takes a light frame,
subtracting the dark frame from it. The result should be uniform noise
without hot pixels and without readout glow.
Had we had the camera on the telescope we would have seen the object
but because we had the nosepiece covered we took a dark subtracted image
of the dark. If you take enough of these, and stare at them long enough
you'll see all kinds of patterns in the random noise, including even pictures
of Elvis!
You can read about the other items in the Grab dialog by clicking the
Help button, but in general the Grab command is used to take a single
image and optionally do an automatic dark subtraction.
SBIG ST-i Manual
18
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