lronaccal performs radiometric corrections to images acquired by the Narrow Angle
Camera aboard the Lunar Reconnaissance Orbiter spacecraft. The LRO NAC camera
will make observations simulteously with the HiRise camera.
The LRO NAC detector has a total of 5064 pixels, divided among an A
channel and a B channel. The pixels alternate between the two channels:
ABABABAB, etc. Images from LROC NAC 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 LROC NAC 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 from the masked pixels
that lie along each edge of the image.
If SpatialSumming is 1 the dark current pixels are averaged together then
this average is subtracted from all image pixels. If SpatialSumming is 2,
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.
