Instrument Science Report NICMOS 2002-005
Implementation of the
Temperature-Dependent Dark
Correction in CalnicA
R. Jedrzejewski
December 1, 2002
ABSTRACT
The pipeline implementation of the temperature-dependent dark correction, previously
only available to users as a WWW tool, is described. The algorithm used to generate the
darks is described, along with a description of how the IDL code and data that are used in
the WWW tool were incorporated into the usual HST pipeline model of code+reference
files. This implementation necessitated the creation of a new reference file type: the Temperature-Dependent Dark (TDD) file. The design of this file type allowed many of the
hard constants to be removed from the code and put into the reference files, allowing more
convenient and flexible updating of the calibration parameters, if necessary.
Introduction
As described in NICMOS ISR 99-010 “NICMOS Temperature-specific darks” (B. Monroe), the dark current in the NICMOS detectors is a function of temperature. That ISR
describes the implementation of an algorithm to determine the dark correction in real time,
given the instrumental setup and a temperature measure. The method was implemented as
a WWW tool, allowing users to enter their exposure characteristics (camera, sample
sequence and temperature), and the tool would return a dark reference file appropriate for
those parameters. The user could apply this dark reference file, either by editing the
header parameter DARKFILE to point to this newly-generated file and re-running the
CalnicA pipeline, or by doing the same operations by hand in, for example, IRAF or IDL.
While the results of using the darks generated by this tool were a big improvement over
Copyright© 2002 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Instrument Science Report NICMOS 2002-005
using static dark reference files, the process necessary to generate and use the required reference files was rather cumbersome. There was a clear need to make the process more
transparent to the user, especially with On-the-Fly Reprocessing being introduced for
NICMOS.
Components of the Dark Current Signal
The dark current in NICMOS has three components:
•
the linear dark signal, proportion to cumulative exposure time
•
the amplifier glow, proportional to the number of reads
•
the shading component, which depends on the time since the last readout and the temperature in a complex fashion.
The first two components are straightforward to understand and implement. The linear
dark signal is basically the true dark signal, proportional to the time since the last detector
reset. It depends on pixel location, so can be modelled as an image that would be obtained
in a 1 second exposure. Typical values are a few hundredths of a count per second per
pixel, with some increase near the corners where the readout amplifiers are located. The
linear dark level varies from quadrant to quadrant, and there are several ‘hot’ pixels with
elevated linear dark current that can be as large as several counts/second/pixel. The linear
dark component is believed to vary with temperature (since it is basically due to thermally-generated electrons), but this effect is quite small and difficult to measure, so the
temperature dependence is not currently included in the characterization of this
component.
The amplifier glow component arises because the amplifiers emit a low level of infrared
radiation when they are reading out the detector pixels. This radiation is detected by the
pixels, and appears as a signal. Again, the signal depends on pixel location, as well as on
the number of reads that have been performed since the last detector reset. Typically, the
amp glow is 1-2 counts/read over most of the image, with a rise to approximately 10
counts/read towards the corners where the amplifiers are. The amp glow can be represented as an image of the counts for 1 read, and then the amp glow contribution for a given
image can be calculated by multiplying this unit image by the number of reads.
The shading component is more complicated. Here the shading signal depends on both
the time since the last read (DELTATIM in NICMOS parlance) and the temperature. The
dependence on delta time is approximately logarithmic. The temperature dependence
depends strongly on pixel position, in a way that is different for different regions of the
detector. It is the implementation of this correction that is the main subject of this report.
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Instrument Science Report NICMOS 2002-005
The final dark signal is just the sum of these components:
dark = lineardark + ampglow + shading
The IDL algorithm
The WWW tool, available at http://www.stsci.edu/hst/nicmos/tools/syndark.html, is
implemented as an html form that calls a perl script. The script is basically a wrapper
around a set of IDL calls.
At the time the CalnicA code was being designed, the IDL code lived in /data/bendyjack3/
httpd/cgi-bin/darks/ and was served by the http server run on the machine ‘bendyjack’.
Now it has been moved to the STScI Zope server, but the content should be identical. The
perl script generates IDL code that looks like the following:
@startup1
aa=getset(’/data/cdbs5/nref/hat1336fn_drk.fits’)
syndark=syndark2_2(tstep8,1,67.67)
newdark=float(aa)
newdark(*,*,*,0)=syndark
write_multi,’tmp/step8_1_6767_drk.fits’,newdark,header_from=’/data/
cdbs5/nref/hat1336fn_drk.fits’,history=[’file created by STScI web
interface’,’on date listed above in DATE keyword’,’B. Monroe and E. Bergeron’],pedigree=’MODEL 10/08/1999’,useafter=’22/08/1997’,descrip=’MODEL
NIC1 SAMP_SEQ step8 dark’
exit
This code was created by the perl script from a request that specified NICMOS Camera 1,
SAMP-SEQ=STEP8, and a temperature of 67.67 degrees K. The first line is a call to a
routine to do Camera 1-specific startup operations, such as defining common data blocks
and restoring an IDL SAV file. This file holds, among other things, the A234_TIME data
structure and the amplifier glow image. The former is just an array of FLOATs that holds
the list of NICMOS DELTATIMEs for which a temperature-dependent calibration has
been performed. The list of calibrated DELTATIMEs is given below in Table 1.
Table 1. List of DELTATIMEs for which a temperature-dependent dark calibration has
been performed.
Calibrated
DELTATIMEs (s)
255.99922
127.99308
63.997176
31.999224
15.993080
7.9935920
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Instrument Science Report NICMOS 2002-005
Calibrated
DELTATIMEs (s)
3.9938480
1.9939760
0.997624
0.388344
0.302328
0.000000
This list covers almost all of the exposure times specified in the allowed sample
sequences; the only delta times that are not covered are the tenth readouts in the MIF
sequences, much of the MIF3072 sequence, the fourth reads in the SPARS sequences
(although the difference between the actual DELTATIME and the calibrated DELTATIME
is probably negligible in this case), and the SCAMRR sequence. The only sequence
where the lack of calibration would be significant is the SCAMRR sequence, where delta
times are all 0.203000s and the shading correction is a strong function of delta time for
these short exposure times.
The amplifier glow image is stored in the variable C[123]AMPARR as a 256×256×26
array of FLOATS, with each image plane equal to the glow produced by a single read.
This means that the amplifier glow component for a given extension can be extracted as
the plane for that extension (modulo the usual zero-indexing of IDL arrays).
The next line,
aa=getset(’/data/cdbs5/nref/hat1336fn_drk.fits’)
just sets up a structure that looks like the CDBS reference file for that Camera and sample
sequence. All of the real work is done in the next line:
syndark=syndark2_2(tstep8,1,67.67)
where the synthetic dark file for the STEP8 sequence for Camera 1 is calculated for a temperature of 67.67.
The syndark2_2 routine grabs the amplifier glow array from the common block initialized previously, then locates the appropriate linear dark array image by restoring the
lineardark[123].sav file for the relevant camera. This array, C[123]_B_DARK, is then
scaled by the exposure times of the sample sequence to make a multi-plane array where
each plane corresponds to a readout.
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With the easy work out of the way, the shading component is calculated in the routine
makeshade[123](delta, temp), where delta is the array of delta times and temp is the
temperature. The makeshade routine starts by restoring the polynomial coefficients that
describe the variation of shading signal with both pixel position and temperature from the
file ‘brianshade[123].sav’. These coefficients are stored in arrays, with the names of the
arrays dependent on the implementation for that camera.
The temperature-dependent shading signal is calculated in the following way: First, the
delta times for each read in the sample sequence are calculated. The index of the calibrated delta time in the A234_TIME array that most closely matches the delta time of the
group under consideration is calculated. Then, the detector area is divided up into a patchwork of rectangular regions. Each region has its own set of polynomial coefficients. For a
given region, the shading signal is a polynomial function of position in one of the dimensions (X or Y), and constant in the other dimension. The polynomial coefficients of
position are themselves polynomial coefficients of temperature. Thus, for example, for a
hypothetical region A:
X range = X1 to X2
Y range = Y1 to Y2
Function varies along the X direction, constant in the Y direction
Shading signal is a polynomial function of X pixel, shifted to start at P1 and end at P2
(where P2−P1 = X2−X1).
The shading signal at pixel with coordinates (i,j) is given by
m
S ( ( i + X 1 – 1 ), j ) =
∑ a ( T ) ( i + P1 – 1 )
n
(1)
n=0
where
p
a(T ) =
∑ bn T
n
(2)
n=0
This is coded in the makeshade routine in blocks like the following:
pp = [poly(temp,shadefit0(*,0,deltaindex)),
poly(temp,shadefit1(*,0,deltaindex)),
poly(temp,shadefit2(*,0,deltaindex)),
poly(temp,shadefit3(*,0,deltaindex)),
poly(temp,shadefit4(*,0,deltaindex))]
where the array of polynomial coefficients a(T) is calculated as polynomials in the temperature temp, using the coefficients stored in the shadefit[01234] array. The
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Instrument Science Report NICMOS 2002-005
deltaindex parameter is the index of the delta time calibration used. The actual shading signal is calculated in the next line:
kk_l = poly(findgen1(21),pp)
where the signal is a polynomial function of pixel coordinate in the patch under
consideration.
This creates a 1-dimensional vector of the shading signal. This is converted into a two
dimensional signal by replicating in the perpendicular direction for the extent of the patch.
The total signal is calculated as the sum of all the patches; in this way both X and Y variation are possible. Finally, some patches are left blank because there is no temperature
dependence in that region. In that case, a static component, which depends on pixel location and delta time only, is added to the overall shading signal.
The last operation is of course to add the linear dark, amplifier glow and shading components into the final dark image for each read appropriate for that sample sequence.
CalnicA Reference File Design
The main problem with implementing this algorithm in the CalnicA pipeline is that many
of the parameters describing the fit are hardwired into the code. This does not map well to
the standard model of pipeline processing; make the code as general as possible and put
the things that are likely to change (due to improved calibration) in the reference files.
That way, when a new calibration is performed, only the reference files need to be updated
(with USEAFTER date processing to take care of time-dependent calibrations), not the
code itself. This seemed particularly important for NICMOS, since its cryogenicallycooled life was over, and its mechanically-cooled life is in its infancy. The significantly
different temperature régime of the CryoCooled era could well make the earlier calibrations invalid. Similarly, the IDL code looks quite different for each of the three cameras,
and yet a method is needed to process each camera with the same code, moving the camera-specific parameters to the reference files.
After much thought, a solution was developed in which all of the calibration parameters
could be moved to the reference files. A reference file format was chosen that is a combination of images and binary tables, which fits in well within the FITS format used for
reference files. The format looks like this:
1. The IDL function findgen(n) creates a 1-d array of integers with values from 0 to (n-1). So in the
example above, findgen(21) is the same as [0,1,2,3,......,18,19,20]
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Instrument Science Report NICMOS 2002-005
Primary Header Unit
LIN extension
AMPGLOW extension
SHAD binary table
STATIC extension
SHAD binary table
STATIC extension
SHAD binary table
STATIC extension
Figure 1: Structure of the Temperature-Dependent Dark reference file
The Primary Header Unit (PHU) contains basic information about the reference file. The
PHU for the NICMOS Camera 1 reference file is shown below:
SIMPLE
BITPIX
=
=
T / Fits standard
16 / Bits per pixel
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Instrument Science Report NICMOS 2002-005
NAXIS
=
0 / Number of axes
ORIGIN = ’NOAO-IRAF FITS Image Kernel July 1999’ / FITS file originator
IRAF-TLM= ’11:01:26 (15/02/2002)’ / Time of last modification
DATE
= ’2002-03-11T18:31:33’/ Creation date (CCYY-MM-DD) of FITS
header
EXTEND =
T / File May Contain Extensions
TELESCOP= ’HST
’
/telescope used to acquire data
INSTRUME= ’NICMOS ’
/instrument in use
DARKMETH= ’TEMPERATURE-DEPENDENT’ /Dark calculation method
TEMPKEY = ’NDWTMP11’
/Temperature key
CAMERA =
1 /NICMOS camera (1|2|3)
NDELTA =
12 /#different delta-times
DELTA1 =
0.0000000 /
DELTA2 =
0.30232800 /
DELTA3 =
0.38834400 /
DELTA4 =
0.99762400 /
DELTA5 =
1.9939760 /
DELTA6 =
3.9938480 /
DELTA7 =
7.9935920 /
DELTA8 =
15.993080 /
DELTA9 =
31.999224 /
DELTA10 =
63.997176 /
DELTA11 =
127.99308 /
DELTA12 =
255.99922 /
PEDIGREE= ’MODEL 18/02/2002’
/
USEAFTER= ’Aug 21 1997’
/
DESCRIP = ’Camera 1 temperature-dependent dark reference file’ /
FILETYPE= ’TDD
’
HISTORY cam1_drk.fits renamed to m3b1331gn_tdd.fits on Mar 11 2002
The DARKMETH keyword was originally included as a way of distinguishing temperature-dependent dark reference files from regular static dark files, since the intention was to
make the temperature-dependent dark reference file a _drk.fits file, just like the static dark
reference file. However, the fact that the dark reference file would then need to have a different format depending on type (not supported by CDBS), and that the dark correction
reference file would then not be backward compatible with earlier versions of CalnicA, led
to the decision to abandon the plan of using _drk.fits files to store the temperature-dependent calibration data. Instead, a new type of NICMOS reference file was created: the
_tdd.fits file.
The rest of the reference file format is split into several parts. Firstly, there is the linear
dark signal, stored in the LIN extension. This is just an image of the linear dark signal for
a 1s exposure. To determine the linear dark component for a given group, this component
is multiplied by the exposure time (keyword SAMPTIME from the extension header).
The next component is the amplifier glow signal, stored in the AMPGLOW extension.
This is an image of the amplifier glow for a single read. To determine the amplifier glow
component for a given group, this component is multiplied by the number of reads
(nreads = nsamp − ngroup, where nsamp is the value of the NSAMP keyword from
the primary header and ngroup is the number of the group in question; remember NICMOS _raw files have the order of groups reversed so that the first read is the last group).
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Instrument Science Report NICMOS 2002-005
The rest of the extensions are in pairs, with one pair for each calibrated delta time. The
STATIC components give the static shading signal for the delta time in the list of delta
times in the PHU of the dark reference file; for example, the [STATIC,3] extension is the
static component with delta time equal to 0.388344s. The SHAD component is where the
coefficients for the temperature-dependent shading signal are stored. This component is a
binary table with seven columns:
Column Name
Format
Description
xstart
int
Initial X value
for region
xend
int
Final X value
for region
ystart
int
Initial Y value
for region
yend
int
Final Y value
for region
axisvary
int
Direction of
polynomial
variation (1=x,
2=y)
pstart
int
Initial value for
polynomial
evaluation
pend
int
Final value for
polynomial
evaluation
coeff
Float array
Coefficients
Table 2. Column names in SHAD binary table
There is one row for each rectangular patch that the detector area is divided into. The columns follow the structure outlined in the previous section, where the parameters
describing a rectangular patch are described. The patches are described in a coordinate
system where the X axis is the same as the FITS AXIS1, and the Y axis is the same as the
FITS AXIS2. Both coordinates are 1-indexed, unlike IDL arrays. The PSTART and
PEND parameters are generally equal to 0 and (XEND−XSTART−1); however, there are
cases where PSTART is not zero. This arises from patches where the shading polynomial
is evaluated with an expression like the following (from makeshade1.pro):
kk_r=poly(findgen(22)-2,pp)
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Instrument Science Report NICMOS 2002-005
This particular construct was used in the process whereby the polynomial coefficients
were determined; it allowed additional pixels to the left of the region to be fitted with the
polynomial even though they weren’t part of the original fit.
The polynomial coefficients themselves are stored in the COEFF array. This is a simple 2d array of floating point numbers. Each row has the set of coefficients bn that are used to
determine the coefficients of the polynomial a(T) that are used in equation (1) to calculate
the shading signal S.
The final shading signal is determined by adding the signals from each patch, along with
the static signal from the STATIC extension. In order to ensure that the static signal is not
contaminated by a temperature-dependent component, a rectangular patch that covers the
static region is specified in the SHAD table with zeros for the coefficients.
The overall dark signal is calculated by adding together the linear dark, amplifier glow,
static and temperature-dependent components.
Creating the Reference Files
The reference files are created using three IDL procedures, one for each camera. They are
stored in the directory /data/zork4/rij/ssg/nicmos/TempDependDark/cgi/ and have the
names mkref1.pro, mkref2.pro and mkref3.pro to make reference files for NICMOS cameras 1, 2 and 3. The procedures were created specifically to work with the IDL save sets
and procedures that the WWW tool uses, so if the details of the procedures change, then it
will be necessary to edit the reference file generators accordingly.
The following listing shows the procedure used to create the Camera 1 reference file:
; This program will return a reference file for Camera 1
; Get the linear dark current array
restore,’lineardark1.sav’
; Get the amp glow array and the reference deltatime vector
restore,’dark1.sav’
; Get the temperature-dependent shading coefficients
; and the static dark component
restore,’brianshade1.sav’
; make the primary FITS header
mkhdr, header, ‘’, /EXTEND
orderedelements = [11, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 12]
ndelta = 12
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fxaddpar,
fxaddpar,
fxaddpar,
method’
fxaddpar,
fxaddpar,
fxaddpar,
header, ‘TELESCOP’, ‘HST’, ‘telescope used to acquire data’
header, ‘INSTRUME’, ‘NICMOS’, ‘instrument in use’
header, ‘DARKMETH’, ‘TEMPERATURE-DEPENDENT’, ‘Dark calculation
header, ‘TEMPKEY’, ‘NDWTMP11’, ‘Temperature key’
header, ‘CAMERA’, 1, ‘NICMOS camera (1|2|3)’
header, ‘NDELTA’, 12, ‘#different delta-times’
for j=0, ndelta-1 do begin
index = orderedelements[j]
deltatime = a234_time[index]
keyword = ‘DELTA’+strcompress(string(j+1),/rem)
fxaddpar, header, keyword, deltatime
endfor
fxaddpar, header, ‘PEDIGREE’, ‘MODEL 2/18/2002’
fxaddpar, header, ‘USEAFTER’, ‘21/08/1997’
fxaddpar, header, ‘DESCRIP’, ‘Camera 1 temperature-dependent dark reference file’
; open the output FITS file
writefits, ‘cam1_drk.fits’, ‘’, header
lineardark = fltarr(256, 256)
lineardark = float (c1_b_dark)
mkhdr, header, lineardark, /image
fxaddpar, header, ‘EXTNAME’, ‘LIN’
fxaddpar, header, ‘EXTVER’, 1
; write the linear dark array
writefits,’cam1_drk.fits’, lineardark, header,/append
mkhdr, header, c1amparr(*,*,24), /image
fxaddpar, header, ‘EXTNAME’, ‘AMPGLOW’
fxaddpar, header, ‘EXTVER’, 1
; write the amp glow array
writefits,’cam1_drk.fits’,c1amparr(*,*,24),header,/append
; Define the columns we will use
ntables = 12
xstart=1
xend=256
ystart=1
yend=21
axisvary=2
polstart=0
polend=20
coefficients = fltarr(3,5,12)
staticshading = fltarr (256, 256)
; loop over delta times
for j = 0, ndelta-1 do begin
index = orderedelements[j]
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deltatime = a234_time[index]
; Write the static component first
staticshading[0:255, 0:20] = bottoms[0:255, 0:20, index]
staticshading[0:255, 128:148] = bottoms[0:255, 128:148, index]
mkhdr, header, staticshading, /image
fxaddpar, header, ‘EXTNAME’, ‘STATIC’
fxaddpar, header, ‘EXTVER’, j+1
fxaddpar, header, ‘DELTIME’, deltatime
writefits, ‘cam1_drk.fits’, staticshading, header, /append
coefficients[*,*,0]=[[shadefit0[*,0,index]],[shadefit1[*,0,index]],[sha
defit2[*,0,index]],[shadefit3[*,0,index]],[shadefit4[*,0,index]]]
coefficients[*,*,1]=[[shadefit0[*,1,index]],[shadefit1[*,1,index]],[sha
defit2[*,1,index]],[shadefit3[*,1,index]],[shadefit4[*,1,index]]]
coefficients[*,*,2]=[[shadefit0[*,2,index]],[shadefit1[*,2,index]],[sha
defit2[*,2,index]],[shadefit3[*,2,index]],[shadefit4[*,2,index]]]
coefficients[*,*,3]=[[shadefit0[*,3,index]],[shadefit1[*,3,index]],[sha
defit2[*,3,index]],[shadefit3[*,3,index]],[shadefit4[*,3,index]]]
coefficients[*,*,4]=[[shadefit0[*,4,index]],[shadefit1[*,4,index]],[sha
defit2[*,4,index]],[shadefit3[*,4,index]],[shadefit4[*,4,index]]]
coefficients[*,*,5]=[[shadefit0[*,5,index]],[shadefit1[*,5,index]],[sha
defit2[*,5,index]],[shadefit3[*,5,index]],[shadefit4[*,5,index]]]
coefficients[*,0:1,6] =
[[yshadefit0[*,2,index]],[yshadefit1[*,2,index]]]
coefficients[*,0:1,7] =
[[yshadefit0[*,6,index]],[yshadefit1[*,6,index]]]
coefficients[*,0:1,8] =
[[yshadefit0[*,3,index]],[yshadefit1[*,3,index]]]
coefficients[*,0:1,9] =
[[yshadefit0[*,7,index]],[yshadefit1[*,7,index]]]
headerstring = ‘Shading correction for deltatime=’+strcompress
(string(deltatime))+’s’
fxbhmake,header,12, ‘SHAD’, headerstring
fxaddpar, header, ‘EXTVER’, j+1
fxaddpar, header,
fxbaddcol, xscol,
fxbaddcol, xecol,
fxbaddcol, yscol,
fxbaddcol, yecol,
fxbaddcol, avcol,
fxbaddcol, pscol,
fxbaddcol, pecol,
fxbaddcol, cocol,
‘DELTIME’, deltatime
header, xstart[0], ‘XSTART’
header, xend[0], ‘XEND’
header, ystart[0], ‘ YSTART’
header, yend[0], ‘YEND’
header, axisvary[0], ‘AXISVARY’
header, polstart[0], ‘PSTART’
header, polend[0], ‘PEND’
header, coefficients[*,*,0], ‘COEFF’
fxbcreate, unit, ‘cam1_drk.fits’, header
xstart = [1, 1, 1, 1, 1, 1, 1, 1, 129, 129, 1, 1]
xend = [256, 256, 256, 256, 256, 256, 128, 128, 256, 256, 256, 256]
ystart = [1, 22, 33, 128, 150, 161, 22, 150, 22, 150, 1, 129]
yend = [21, 32, 127, 149, 160, 256, 128, 256, 128, 256, 21, 149]
axisvary = [2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1]
polstart = [0, 0, 0, -2, 0, 0, 0, 0, 0, 0, 0, 0]
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polend = [20,
for i = 1, 12
for i = 1, 12
for i = 1, 12
for i = 1, 12
for i = 1, 12
for i = 1, 12
for i = 1, 12
for i = 1, 12
endfor
10, 94, 19, 10, 95, 127, 127, 127, 127, 255, 255]
do fxbwrite, unit, xstart[i-1], xscol, i
do fxbwrite, unit, xend[i-1], xecol, i
do fxbwrite, unit, ystart[i-1], yscol, i
do fxbwrite, unit, yend[i-1], yecol, i
do fxbwrite, unit, axisvary[i-1], avcol, i
do fxbwrite, unit, polstart[i-1], pscol, i
do fxbwrite, unit, polend[i-1], pecol, i
do fxbwrite, unit, coefficients[*,*,i-1], cocol, i
fxbfinish, unit
end
CalnicA Implementation
The CalnicA code was inherited from Howard Bushouse. He had been working on a prototype Version 4, which involved changing the order of calibration so that each calibration
step is applied to all of the groups before going on to the next step, in contrast to the thencurrent version which applied all the calibration steps to each group before going on to the
next group. The new version that incorporates the temperature-dependent correction does
the following steps, assuming the DARKCORR primary header keyword is set to
PERFORM:
•
Look for a temperature-dependent reference file specified using the primary header
keyword TEMPFILE
•
If the TEMPFILE keyword does not exist, or if it is set to ‘N/A’, use the static dark reference file pointed to by the primary header keyword DARKFILE
•
If the TEMPFILE keyword points to a valid temperature-dependent reference file, use
it to calculate the dark component for each group of the data file.
•
The temperature sensor reading to be used by CalnicA is encoded in the reference file
keyword TEMPKEY. For Cameras 1 and 2, this key is NDWTMP11, while for Camera 3 the key is NDWTMP13. The temperatures are read from the _spt file that
accompany the data files. Each group has its own temperature measurement; the
actual temperature used in the temperature-dependent correction is obtained by averaging the temperature readings from all the groups.
•
If the writedark parameter is set to TRUE (or if the calnica program is run using the
command-line parameter -write), a static dark reference file is created by CalnicA,
with the name ‘rootname_drk.fits’. This file can be compared with that created using
the WWW tool.
The ability to write out a static reference file was extremely valuable in testing the code,
since the ‘true’ dark values could be obtained using the WWW tool.
Agreement with the then-current version of CalnicA (v3.3), using static dark reference
files created using the WWW tool, was reasonably good. The differences mainly arose
from the differences in the order of calculation, not from the temperature-dependent dark
correction step.
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What’s Missing, and What’s Next
The most obvious deficiency in this code is the lack of error terms. In principle, these can
be determined by calculating the formal errors when calculating the dark components.
The next step is to add the capability to calculate the temperature from the bias signal.
This step has already been coded by Eddie Bergeron in IDL basically as a state machine
calculation. Once again, it will be difficult to incorporate this calculation step into the
code+reference file model. The most likely implementation will involve calculating the
appropriate temperature from the data before CalnicA is run, and inserting this temperature into a _spt file keyword. CalnicA will then use this keyword in calculating the
temperature-dependent dark signal. This coding has not started yet, since it seems wise to
wait until some of the SMOV issues are sorted out and then a decision can be made on the
bast way to tackle the problem.
The flatfield is also known to be temperature dependent, and a WWW tool has been written to determine the temperature-dependent flatfield correction. This can also be
incorporated into CalnicA in a similar fashion to how the dark correction was
implemented.
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