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.
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.
Acquiring Calibration Data
For this command a "calibration" is a set of coefficients that will be applied to each individual pixel in the image. Calibrations are conveniently stored as FITS files on disk, but they are not images. The coefficients are derived from a set of flat-fields taken at different exposure durations spanning the linear range of the sensor. For best results, multiple flat-field images should be taken at each exposure point; these will be automatically stacked to produce low-noise calibration data.
To create calibrations:
Open the Process menu GSENSE Calibration command.
The controls in the Create Calibration Sequence box at top right are used to acquire the necessary data. Set up your camera to acquire flat-field images. The detector should be uniformly illuminated with a constant light source. See Flat-Field Frame Calibration for more information on flat-field frames.
Ensure that your camera is connected to MaxIm DL using the Camera Control Window, and set the parameters in the Create Calibration Sequence box. The exposure sequence consists of a bias image and several flat-field images covering the entire dynamic range of the camera. Set Min Exposure to the shortest light image your camera can achieve, and set Max Exposure to a time (in milliseconds) after which your detector is close to saturation with your light source You may have to test several different exposure lengths to find this value. Set Number of Samples to a large enough value to sample most of the space in between these two exposure times. At least three points are required, and at least eight are recommended.
Tip: In High Gain mode, GSENSE4040 sensors start to saturate somewhere between 3600 and 3800 ADU.
Clicking the Advanced button allows you to set additional settings for the exposure sequence. Number of images to stack determines how many images are combined at each exposure time. Temperature setpoint determines the detector temperature (in °C) at which the images will be taken. Filter determines the filter used during the exposures, if available. Readout mode determines which of the available readout modes is used. StackPro modes are not available for this exposure sequence. For the GSENSE4040 sensor we recommend using the High Gain mode. Image save directory determines where the calibration images are stored. Click OK when finished.
To begin the exposure sequence, click Begin. The dialog will set the filter position, wait for the detector to reach the temperature setpoint, and take and save the images. Progress will be shown at the bottom of the dialog. If necessary you can use the Abort button to stop the process.
The generated files will be shown in the Generate Calibrations list box, along with each image's Type of exposure (usually FLAT), Duration, and Temperature.
You may also manually add existing files using the Add Input... button, or remove files using the Remove button. If manually adding files, ensure that the images were taken with the same camera, temperature, and readout mode.
Generating Calibrations
Now that your data has been acquired and is included in the list box, the next step is to generate the calibration file. To do this, click the Generate button, and then select a location to save the resulting calibration file.
The newly generated file will automatically appear in the Available Calibrations list box, along with the average temperature of its input files under Temp C, the serial number of the Camera used to take the images, the EGain electron conversion gain used to take the images, and the Image Size in pixels. Pre-existing calibration files may also be manually added to the list using Add Calibration File. Calibration files can be removed using the Remove button or Clear All.
Applying Calibrations
To perform the correction on a single image, open the image in MaxIm DL. Select the desired calibration file from the list of Available Calibrations by clicking on it with the mouse, and then click Apply to Image.
When processing your images, this calibration should be applied to Light, Dark, and Flat images as the first step, before any other processing including image calibration.
A new calibration should be used if you change any of the following:
The list of Available Calibrations is saved when you either click Apply to Image or click OK. If you click Cancel before doing that, then the Available Calibrations list will revert to the previous setting.
Automatic Calibration
To make this process easier, the Process menu 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 menu FITS Header Window. If you need to make this change to multiple images, the FITS Header Window can be used in conjunction with View menu Batch Process Window. 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: