GSENSE4040 sensors, used in cameras such as the SBIG Aluma AC4040, are known to exhibit a residual fixed-pattern structure that is not completely corrected by dark/flat frame calibration. This can be corrected using the GSENSE Calibration dialog.
The dialog allows creation and application of special calibration files to images. This additional calibration step should be applied to all images taken with affected cameras, prior to any other processing. As this function should be applied to calibration frames as well as light frames, functionality has been added to Set Calibration to automatically perform this extra step.
Acquiring Calibration Data
For best results, the calibration images should be taken at approximately the same temperature setpoint as the light frames; some degradation in correction performance will be noticed with a sensor temperature change of 10C.
The first step is to acquire calibration data for your camera. The camera sensor needs to be evenly illuminated with a constant light source - same as when taking flat-field images. See Flat-Field Frame Calibration for more information on flat-field frames.
You will want to acquire 5-10 images for each of:
The different illumination levels can be achieved by selecting appropriate exposure times. Although the absolute minimum allowed is 3 steps of illumination, we strongly recommend at least 8 steps. Additional steps at low illumination levels can be helpful.
Since reading out a CMOS camera generates extra heat, we recommend adding a Delay Between exposures in the Autosave Sequence. This helps ensure that the sensor temperature is stable for each exposure.
NOTE: The GSENSE4040 sensors start to saturate somewhere between 3600 and 3800 ADU. You will want to avoid saturation.
Generate Calibrations
Add your images to the Generate Calibrations section using the Add Input... button.
The files will be shown in the Generate Calibrations list box, along with each image's Type of exposure (usually FLAT), Duration, and Temperature.
Click Generate... to start processing. You will be prompted to select a filename for your calibration data. You may wish to build multiple calibrations; for example, for different sensor operating temperatures, readout modes, gain adjustments, etc.
Calibration Images
Once the processing is complete, a file will be generated and added to the list of Available Calibrations. The calibration data is stored on disk in FITS format - though it is not an image, it is a matrix of calibration parameters.
To use a calibration, select one from the list and click the Apply to Image button. This will apply the calibration to the currently selected image.
Automatic Calibration
To make this process easier, the Process ribbon Set Calibration dialog can apply GSENSE calibrations automatically, during regular calibration. For best results, all dark and flat-field frames should also have the GSENSE Calibration applied; Set Calibration will also do that automatically. Please note that you must do this prior to the creation of master frames; you should not apply GSENSE calibration to already-stacked master frames as it will produce inaccurate results.
Troubleshooting
While it is recommended to use the same temperature, camera, and electron conversion gain between calibrations and exposures, the tool will continue even if they differ. If the calibration does not seem to correct the image properly, check that these all match.
StackPro readout mode can cause undesirable behavior unless handled correctly. When processing images taken with an older version of MaxIm DL, or taken with another software package, the FITS header will not have the NSUBF keyword. This keyword tells MaxIm DL how many subexposures were captured to generate the image. To ensure NSUBF is included, be sure to use an SBIG Aluma AC4040 with the latest release of DL Imaging drivers, along with the current release of MaxIm DL.
If you are processing images that do not have NSUBF, e.g. taken with older releases of MaxIm DL or a different software package, you can manually add the NSUBF keyword using View Ribbon FITS Header Panel. If you need to make this change to multiple images, the FITS Header Panel can be used in conjunction with View Ribbon Batch Process Panel. You can calculate NSUBF knowing the exposure duration of the image, and the Target Subexposure configured in the camera's Setup dialog. The number of subexposures is determined as follows: