ISIS 3 Application Documentation
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Radiometrically calibrates a CTX image
ctxcal performs radiometric corrections to images acquired by the Context Imager aboard the Mars Reconnaissance Orbiter spacecraft. The CTX camera will make observations simulteously with the HiRise camera.
The MRO CTX detector has a total of 5000 pixels, divided among an A channel and a B channel. The pixels alternate between the two channels: ABABABAB, etc. Images from CTX may or may not include all pixels in the acquired image. There are special summing modes that are utilized on-board the spacecraft to average detector pixels to combine them into a single output pixel value. The value of the ISIS label keyord, SpatialSumming, indicates the number of samples that were summed and averaged to result in the pixel values stored in the file. Note that this will reduce the number of samples in the output image by a factor of at most the SpatialSumming mode value.
The CTX camera has the ability to acquire images of differing sizes in both line and sample. The starting hardware detector pixel for the acquired image is specified by the ISIS label keyword, SampleFirstPixel. The first pixel in the detector is indicated by a value of 0.
Dark current pixels are taken for each line and are stored in the Isis cube as a table, named "Ctx Prefix Dark Pixels". The number of dark current pixels in this table is dependent on the SpatialSumming and the SampleFirstPixel keywords from the labels. The following table shows the association between these keywords and dark current pixels.
If SpatialSumming is 1, the dark current pixels for the A and B channel are averaged separately, then the A channel dark average is subtracted from the A channel image pixels and the B channel dark average is subtracted from the B channel image pixels. If SpatialSumming is 2 the dark current pixels are averaged together then this average is subtracted from all image pixels.
The equation ctxcal implements is as follows:
For any pixel i, on line L, compute r in counts/msec r(i,L) = ( inputPixel(i,L) - darkCurrent(L) ) / (flatPixel(i,L) * exposure)where r is the average signal being generated at the focal plane (in DN/msec), darkCurrent is the dark current term (in DN/msec), flatPixel is the pixel dependent gain, and exposure is the exposure time in milliseconds.
If the IOF parameter is chosen, the output pixels are converted to relative reflectance (I/F) using the solar radiance, with an albedo 1 target at normal incidence at Mars perihelion distance (2.07e+08 km) and a CTX response of 3660.5 DN/msec.
For any pixel i, on line L r(i,L) = ( inputPixel(i,L) - darkCurrent(L) ) / flatPixel(i,L) ) / iof iof = 1.0 / (exposure * w1) w1 = w0 * (2.07E8)^2 / dist^2 w0 = 3660.5 dn/msec dist = distance between Sun and Mars in kilometers exposure = line exposure duration in milliseconds
An uncalibrated CTX image.
The resultant radiometrically calibrated cube
Flat Field file (pixel dependent gain). If no file is entered, the default will be used.
|Internal Default||Internal Default|
The user has the option to request the output of ctxcal to be in either units of percent reflectance (IOF="YES") or in average signal level at the focal plane (IOF="NO") in DN/msec. Ctxcal has a default I/F (percent reflectance) factor of 3660.5 DN/msecond response with an albedo 1 target a normal incidence at Mars perihelion distance (2.07 e+08). Conversion to I/F takes place after calibration and pixel-to-pixel variation corrections.