Understanding Calibration Groups - SBIG STF Series User Manual

3.4 Understanding Calibration Groups

MaxIm LT allows you to set up the calibration frames into multiple groups of
files to provide a high level of calibration automation and flexibility, including:
Automatic creation of master calibration frame libraries
•
• Fully-automatic calibration of images sequences involving different
exposures, filters, and binning
• Autoguider per-filter exposure scaling without needing new dark frames
Eliminating the need to shoot autoguider dark frames during a session
•
Automatic selection of flat-darks
•
As previously discussed, calibration requires bias, dark, and flat files. In
practice, users can often take exposures with different settings in a single
imaging session. Individual exposures may vary by exposure time, sensor
temperature, binning, filter band, subframing, and different cameras may even
be in use (e.g. an autoguider versus the main camera).
MaxIm LT can perform exposure scaling to account for exposure duration (Auto
Scale) and to some extent temperature (Auto Optimize). The software can also
automatically extract a subframe from the calibration frames. Differences in
binning, exposure bands, filters, and of course cameras cannot be
accommodated except by using multiple calibration frames. Switching back
and forth manually can be time consuming, particularly if complex exposure
sequences are taken with different filters and binning, as is typically done for
LRGB imaging and photometry.
A Calibration Group is simply a set of one or more calibration frames that are
taken under the same conditions. For instance, a set of 10 dark frames taken
at -20C with a specific camera would be one group. A set of flat frames taken
with a green filter would be another.
These groups can be automatically generated by scanning a folder or folder
tree. All image files with matching characteristics are grouped together, based
on the information in their FITS headers. Alternatively, the user can choose to
manually create calibration groups.
All the files in a group are combined using an average, median, sigma combine,
or SD Mask algorithm to make an internal "master frame" that is used to
calibrate images. This master frame is not visible to the user.
Some users prefer to generate and save the master calibration frames so they
can be quickly reloaded and used at a later date. The Set Calibration command
allows you to do this. This step is completely optional and is not required for
the calibration process to work properly.
When master calibration frames are generated and saved to disk, the list of
groups in the Set Calibration window is automatically replaced by these master
frame files. If bias subtraction is enabled and suitable groups are available, the
darks and flat masters will automatically be bias-subtracted. Similarly, the flat
masters will be dark-subtracted if dark subtraction is enabled and suitable
groups are available.
SBIG STF SERIES - CAMERA USER'S MANUAL
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