ctxcal

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.

Association between keywords the number of dark current pixels

SpatialSumming	SampleFirstPixel	Number of Dark current pixels in the table
1	0	24	(12 for each channel, stored as ABABABABABABABABABABABAB)
1	>0	16	(8 for each channel, stored as ABABABABABABABAB)
2	0	12	(The dc pixels for the A and B channel have been summed together)
2	>0	8	(The dc pixels for the A and B channel have been summed together)

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
    

If run on a non-spiceinited cube, this program requires access to local mission-specific SPICE kernels, in order to find the distance between the sun and the target body. When run on a spiceinited cube, this can be determined using the camera model. Using a spiceinited cube as input has the advantage of not requiring that local mission-specific kernels be available. (See spiceinit web=true.)