NOISE -- plots variance versus signal and noise models over the top.
noise is used to calibrate the parameters READOUT and PHOTON used by
most of the pamela routines. It needs a series of fairly smooth
frames (e.g. biases and flat fields) with different count levels.
The frames should have been debiassed and flat fielded. You should
apply a flat field in which large scale variations have been removed
in order to preserve the numbers of counts while correcting for pixel
to pixel variations. Typically this is NOT the same as the one you might
want to apply during reduction.
noise works by taking blocks of pixels for each of which it computes the
absolute value of the pixel value minus the average of its 8 neighbours.
The average of these values for a block is then corrected to give a
sigma assuming gaussian statistics. The mean square is not used as it
is sensitive to bad pixels. A correction is made for the noise of the
mean of 8 pixels.
The models that you can plot are
Noise = SQRT(READOUT**2 + COUNTS/PHOTON + (COUNTS*GRAIN)**2))
where READOUT is the RMS readout noise in ADU and provides a flat
base to the noise at low count levels. PHOTON is the number of
electron or detected photons/ADU and is also called the gain.
GRAIN in the fractional flat field noise due to an imperfect flat-field
(and is really only sensitive to very short scale flat field errors
due to the measurement process described above). If you flat field your
data GRAIN should be very small or zero, but it is often visible even after
flat fielding as a steepening of the gradient of the noise versus counts
at high counts as flat field noise dominates over photon noise.
noise recognizes bad pixels.
Parameters:
FILES -- An ASCII list of the calibration files. Any number is possible.
Ideally they should extend from zero counts to the maximum
possible. You may want to add a small constant to the zero
level frames since the axes are logarithmic and you otherwise
miss many points.
XSTART, XEND -- The region of the frames to use. Allow a 1 pixel boundary
YSTART, YEND at its edge in which pixels are OK. i.e. make sure that
you are more than 1 pixel away from duff regions.
NXBOX, NYBOX -- Size of boxes to average over. Can make quite large because
the variance is not calculated globally over the boxes.
DEVICE -- Plot device
This command belongs to the class: statistics