New NICMOS Readout Sequences
Chun Xu
Space Telescope Science Institute
Introduction
In order to make better use of the pre-defined readout sample
sequences, NICMOS group added four new readout “sparse
sampling” (or SPARS) sequences, called SPARS4, SPARS16,
SPARS32 and SPARS128, in addition to the existing sequences
SPARS64 and SPARS256. The new sequences replace the
rarely used MIF sequences. The SPARS readout sequences are
recommended for observations of fields where only faint
objects are present. Another advantage of using SPARS
readout sequences is that these sequences primarily sample
the readout time equally so the data can be better calibrated
(off the pipeline if necessary) as far as the shading is
concerned. The SPARS readout sequences consist of 2 rapid
initial readouts at times 0.303s and 0.606s, followed by multiple
readouts at equally sampled time series. Table 1 lists all the
time sequences for these new SPARS readouts.
Abstract
On 12 June 2005, four new NICMOS multi-accum sequences (SPARS4, SPARS16,
SPARS32, SPARS128) were implemented in the NICMOS flight-software. All of the SPARS
sequences have two initial readouts at 0.303 and 0.606 seconds followed by equally
spaced readouts. Observations with the new sequences were carried out in August 2005
(Proposal 10721). The observed dark currents are well matched by the dark current model
used in the present pipeline.
The advantages of having these new sequences is that observers may choose timing
patterns that optimize the handling of cosmic rays with equally spaced reads without
having the extra amplifier glow caused by the logarithmic initial readouts of the current
STEP sequences. This is especially desirable for fields with moderate to low dynamic
range. Another advantage of these new sequences is a reduction in thermal variations
that give rise to fluctuations in bias levels, called "shading".
Table 1. New Sequences and Readout Time (seconds)
SPARS4
0.303 0.606 3.996 7.997 11.998
15.999 20.000 24.001 28.002 32.004
36.005 40.006 44.007 48.008 52.009
56.010 60.011 64.012 68.013 72.014
76.015 80.016 84.017 88.018 92.019
SPARS16
0.303 0.606 15.996 31.999 47.999
63.996 79.997 95.997 111.997 127.997
143.998 159.998 175.998 191.998 207.999
223.999 239.999 255.999 272.000 288.000
304.000 320.000 336.001 352.001 368.001
SPARS32
0.303 0.606 32.002 64.001 96.000
127.999 159.999 191.998 223.997 255.996
287.996 319.995 351.994 383.993 415.992
447.992 479.991 511.990 543.989 575.989
607.988 639.987 671.986 703.985 735.985
SPARS128
0.303
0.606 127.996 255.996 383.996
511.996 639.997 767.997 895.997 1023.997
1151.998 1279.998 1407.998 1535.998 1663.999
1791.999 1919.999 2047.999 2176.000 2304.000
2432.000 2560.000 2688.001 2816.001 2944.001
Figure 1. The comparison between the observed darks, the model darks and their
differences. The 3 panel images are for 3 cameras, 4 columns in each panel are for 4
sequences. In each column, there are 3 sub-columns, which are observed darks, model
darks and their differences. The smoothed difference images infer that the model and
observed darks are well matched. Note that the there are some quadrant by quadrant
offsets in the difference images, these offsets are caused by incomplete removal of the
pedestal.
Data Reduction and Analysis
The observations of dark current took place in August 2005. All
the observation were scheduled in SAA free orbits to avoid
excessive cosmic ray persistency, with the three cameras
operating in parallel.
The data are calibrated through standard pipeline. The resulted
ima files off the pipeline contain practically all the dark images
at the corresponding readouts. These darks should be
matched exactly by the model dark images used in the pipeline
if the models are correct. To perform such comparison, we
generate corresponding model dark images with calnica in iraf
by setting keyword darkcorr to “PERFORM” in the raw files
and setting writedark option to “yes” in calnica. We then
compare the generated model dark images with the observed
darks (i.e., ima files off the pipeline) extension by extension.
Figure 1 shows the observed darks, model darks and their
differences side by side, extension by extension, for all 3
cameras and all 4 new readout sequences. We only list first 9
out of 25 readouts to save the space. This figure shows
visually how well the model matches the observed darks.
Figure 2, The mean value (in DN) of the difference between observed and model
darks are shown for each readout in a full 25 read sequence. Different SPARS
sequences are color coded as indicated. In general, the model darks are
consistent to better than +/- 10 DNs compared to observed darks. Differences
are due to incomplete removal of pedestal and time-dependent effects related
to the bus voltage and aft-shroud temperature.
Figure 2 plots the detailed differences between the observed
darks and model darks.
Acknowledgements:
The author would like to thank Louis E. Bergeron, Ilana Dashevsky and Keith Noll
for their valuable comments.