Specifications

It is necessary to define few parameters before the first shot. First, it is
necessary to define the image type – choose “Light” from “Exposure”
combo box. Then choose the exposure time. If you experiment with
exposures in the dark room with a camera connected to some small
refractor, start with 1 second. Do not forget to review the image handling
options on the right side of the “Exposure” tab. Let the “Open new Light
image window” and “Overwrite image in selected window” check-boxes
checked, uncheck other options for now (we do no plan to save our first
images).
Then click the “Start Exposure” button. Camera will open the shutter,
perform 1 s exposure, close the shutter and download the image. Image is
then opened in new image window. If this is the first shot, it will probably
be far from sharp focused image. Alter the focuser and try again.
Notice that options determining the new image handling on the right side of
this tab changes with every change of the exposure type. SIPS remembers
these options for every exposure type separately. So it is possible e.g. to
define separate folders for dark frames and for flat fields.
Always check whether new image processing options are defined properly
before you start any exposure.
If you choose “Dark” from “Exposure” combo box (remember the image
handling options on the right side changes – make sure they are properly
defined), image will be captured without opening the shutter. The captured
image will represent the thermal noise, generated by the CCD chip itself,
combined with the CCD chip and camera electronics read noise. Such
images are subtracted from normal images during image calibration to
reduce the dark current effects.
Brightness and Contrast – Image Stretching
The G3 and G4 CCD camera dynamic range spans 65 536 levels. But only
imaging of perfectly illuminated and perfectly exposed scenes can result in
images with pixels spanning this range. Usually only a fraction of this range
is used, e.g. the black background can have values around 500 counts and
the brightest part of the image can have around 10 000 counts. If we assign
the black to white range to the full possible range (0 to 65 535), the image
with 500 to 10 000 counts will be displayed only in dark gray tones. This is
why image brightness scale should be “stretched” before they are displayed.
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