Cosmic ray detection was performed in stacks with the same dither position.
A new version of the CRREJ task in STSDAS was used, which is capable of
handling different exposure times and time variable background levels. The
task uses a noise model and calculates the expected noise for each frame
from the measured signal level and the known detector read noise. The sky
level is measured by finding the mode in the histogram with bin sizes of 1
ADU.
At each pixel CRREJ looks at the stack of frames
and selects some initial pixel value, which can be the minimum or the
median across the stack. Because even with 7 frames or so the median can be
corrupted by cosmic rays, we have used the minimum which
should be very rarely corrupted by cosmic rays. One disadvantage of the
minimum is that occasionally a data drop out may be picked as the minimum.
Another disadvantage is that the exposure times in a stack vary a lot. So
when images are scaled to the same exposure time the
amplitude of the noise is much larger those with short exposures
times than in those with long exposure times, which means they are more
likely to populate the extremes of the distribution, and are therefore more
likely to be chosen as the minimum. Then when measuring the difference
between the initial guess and the individual pixel values, the fact that
the minimum is lower than normal has to be taken into consideration when defining
the CR detection threshold. This has not been done perfectly,
but seems to work reasonably well. This all matters only in the first
iteration. On subsequent iterations the average is used. Pixels are
rejected which deviate from that mean and the remaining pixels are averaged
together forming the base value which enters the following iteration.
The output of CRREJ is a cosmic ray mask for each input image.
These masks will be made available in a highly compressed form.
The parameters used for CR detection are stored in the headers of the
combined stack files.
The rejection was done using three iterations, the
kappa-sigma clipping thresholds were 6, 5, and 4 in these iterations.
When a pixel is flagged as cosmic ray
all pixels in some neighbourhood around it are considered suspect. The
relevant parameter is called radius in CRREJ, and a radius of 1.5 was used,
i.e. the full 3x3 neighbourhood of a given pixel. For these suspect pixels,
the thresholds are multiplied by a parameter PFAC which can be anything
from zero, meaning that all suspects are automatically rejected, to one.
We have chosen PFAC=0.5. The suspect pixels are therefore
subject to clipping at the 3 sigma, 2.5 sigma and 2 sigma level.
All these input parameter values have been experimented with extensively.
The pixel values for pixels thrown out by a PFAC=0 choice, but not by
PFAC=0.5 choice were compared. It was found that the average value of these
pixels is 1/2 ADU higher than the sky. So there is some CR contamination in
these suspect pixels. However with PFAC=0 the number of
additionally rejected pixels would large. With PFAC=0.5 only 2.5% of the
pixels are typical rejected as CR-hits, compared to 7.5% with PFAC=0. This
means if a stricter contamination control would be exerted, one would
reject a large number of pixels, which would entail a higher noise level.
The choice is between accepting a little higher corruption in the data
versus cleaning those pixels out at the expense of higher noise. The 1/2
ADU is a fairly low level and puts a some extra flux and structural noise
into the combined frame, but this is expected to average out to a small
contribution over many frames.
With the chosen PFAC=0.5 parameter setting roughly 2.5 % of all the pixel
values are rejected due to suspected CR hits, entailing an overall
signal-to-noise decrease by about 1.25 %. There is some additional noise
contribution due to undetected CR hits, but its amplitude can presumably
assessed only by doing some realistic simulations.
Copyright © 1997 The Association of Universities for
Research in Astronomy, Inc. All Rights Reserved.