The Set Calibration command allows you to initialize the calibration function. Calibration is used to overcome limitations in cameras, as well as other instrumental effects (e.g. vignetting). Calibration greatly improves the signal-to-noise ratio and photometric accuracy of images. For more information on calibration and these tools, please see Image Calibration under Guide to Astronomical Imaging.
Calibration is performed using a group of calibration images taken with the same camera under the same conditions as the actual image. Calibration images must be full-frame images, although they can be binned. The Bias Frame is used to remove offsets, the Dark Frame is used to remove variations in dark current, and the Flat-Field Frame is used to remove pixel-specific differences in sensitivity, as well as variations in illumination across the camera sensor. For all three types of calibration frames, it is best to acquire multiple frames. This reduces the effect of readout and thermal noise in the calibration frames themselves. For more information on calibration, please refer to the appropriate sections of the manual.
MaxDSLR allows you to specify multiple groups of calibration images using the Set Calibration command. During calibration, MaxDSLR will select the three groups of calibration frames - bias, dark and flat - that best match the currently active image. MaxDSLR will select the flat-field whose filter name matches that of the image to be calibrated. If no flat-field with a matching filter name exists, then MaxDSLR will attempt to find a flat field with no specified filter name and apply that group to the images. If the image to be calibrated is a subframe, then MaxDSLR will perform calibration using the matching area from the calibration images. This allows you to set up calibration frames for both the guider and main camera, under a variety of conditions, without needing to switch manually between different groups of calibration images.
Groups of color images are handled slightly differently from monochrome images. For color images, the filter name is always TRICOLOR and cannot be altered. When selecting the calibration group to use when calibrating a given image, MaxDSLR will only select groups which match the color state of the image that you're calibrating, meaning that only color groups will be considered for a color image, and only monochrome groups will be considered when calibrating a monochrome image.
Note that RAW images from DSLR cameras will be treated as either color images or monochrome images, depending on the Convert to Color option in the File menu Open command.
Since calibration can be complicated, you may wish to use the Calibration Wizard to help you select the correct settings.
MaxDSLR allows you to create an arbitrary number of groups of calibration images. Each image group is identified by the properties of the images that it contains, including the image type (bias, dark or flat), filter used for flat field, duration, image size, binning and camera temperature. All images in a group must be the same size. These input images are combined to produce a single master frame for each group, which is then used for calibration. Each image group has different properties specifying the algorithm used to combine the images. Groups of dark frames have additional settings that control whether and how they are scaled to match the exposure duration of the image to be calibrated.
The Calibrate Bias, Calibrate Dark and Calibrate Flat check boxes are used to enable or disable the use of the associated type of group for calibration. If bias and dark subtraction are both enabled, then the dark frame will always have the bias frame subtracted from it. Flat fields are themselves calibrated before being used to calibrate a light frame, but it is possible to disable the bias subtraction and dark subtraction steps using the Dark-Subtract Flats or Bias-Subtract Flats check boxes.
The Add Group button allows you to create a new, empty group. The group is assigned the filter name that is present in the combo box next to the Add Group button. If <AUTO> is selected, then the group will be assigned the filter name of the first file that is added to it. Remove Group is used to delete the selected group, and the Clear All Groups button deletes all of the existing groups.
The Group Properties for the selected group are displayed below the list of groups. Each group contains a set of images that are combined to produce a master image, that latter being used for image calibration. Clicking the Add button displays a dialog that allows you to add new files to the list. Remove deletes the currently selected files from the group. Normally, the list of files shows the full path to each file. If you click Show File Names Only, then only the file name will be displayed. You will be prevented from adding images with incompatible properties to the group (for instance, you can't add a 256x256 image to a group that already contains a 512x512 image). Also, if a group contains a master calibration frame, then that is the only file that may be present in the group. You may not add a master frame to a group that already contains other images, and you may not add any other images to a group that already contains a master.
The Apply Boxcar Filter setting is available only for groups of flat fields. When enabled, this setting causes MaxDSLR to apply a boxcar kernel filter to the master flat generated from the group. It is useful for raw images from DSLRs and one-shot color cameras, in which adjacent pixels vary in intensity due to the different filters covering them. If a flat field is applied normally MaxDSLR will calibrate out all of the color information. The solution is to blur the flat field slightly.
The Combine Type combo-box allows you to specify the algorithm to be used for combining the images in the group into a single, master image. The available algorithms are Average and Median.
Median can be useful for suppressing “hot” pixels, cosmic ray hits, and other types of impulsive noise. Median combining strongly suppresses outlying pixel values, but it is slightly less effective than averaging in reducing the overall noise level. A median calculation produces an RMS noise level about 25% higher than that when averaging, but combining more calibration frames can compensate for this. Flat-field frames are normalized prior to the median calculation so as to compensate for any change in overall illumination levels..
The Dark Frame Scaling mode allows you to select one of several dark frame scaling modes. The available modes are Auto-Scale (applicable to FITS and SBIG format files only), Auto-Optimize, Manual, and None. The Scale Factor is entered as a multiplication factor to be applied to the dark frame. Auto-Optimize is particularly useful for non-temperature regulated cameras.
If Apply To All is checked, then any change to the group settings (excluding the addition or removal of files) will be propagated to all of the groups. Normally these settings affect only the currently selected group.
Instead of creating the groups and adding files to them manually, the Set Calibration command provides the ability to automatically divide a set of images into appropriate groups, by clicking the Auto-Generate button. The Source Folder will be scanned, and any FITS images present will be divided into groups containing images with similar properties. If Include Subfolders is checked, then the Auto-Generate command will also find all of the files in the subfolders of the Source Folder, and add them to groups as well.
The speed of image calibration can be greatly improved by the use of master frames. A master frame is the combination of all images in a group, that has been pre-calibrated using the appropriate bias (and, in the case of a group of flats, dark) groups. Once generated, calibration will no longer need to load and combine the images in a group whenever that group is needed.
You can access the master calibration images created by Set Calibration using the Create Master Frames command. You must create the master frames immediately after using Set Calibration and before calibrating any images.
The Replace w/ Masters button can be used to automatically generate calibration groups that use master frames. When clicked, the Set Calibration dialog will automatically generate the master frame for each group, and save it to disk. The files are saved in the folder specified by the Source Folder control, and each file's name is generated based on the description of the file. Once all of the master frames are successfully saved, the file list for each group is changed. The existing files are removed from the group, and the single master frame that was constructed from them is added. No other group properties will be changed.
Settings are saved between sessions. If you calibrate an image without first running the Set Calibrate command, the previous calibration files will be loaded.
The Advanced Calibration settings should only be changed by advanced users.
The Pedestal setting is used to compensate for noise in the calibration frames and in the image being calibrated. This value is added to the final calibration frame so that all of the pixel values are positive, and is recorded in the PEDESTAL keyword of the FITS header. It should be set to 3x the standard deviation of the background noise for the image, as displayed in MaxIm DL's information window.
If enabled, Remove Bad Pixels setting causes MaxIm DL to apply the settings from the most recently selected bad pixel map in the Remove Bad Pixels command to the calibrated image. The Remove Bad Pixels command runs immediately after the image has been calibrated and the pedestal has been added to the image.