STIS Instrument Science Report 97-02
The STScI STIS Pipeline VII:
Extraction of 1-D Spectra
Steve Hulbert, Phil Hodge, and Ivo Busko
February 1997
ABSTRACT
This report discusses the extraction of 1-D spectra performed by CALSTIS-6. CALSTIS-6
processes flat-fielded 2-D ACCUM images to produce a collection of one-dimension spectra. In the first iteration of CALSTIS-6 an unweighted extraction algorithm will be used.
The resultant flux- and wavelength-calibrated spectra are stored in 3-D binary tables.
1. Introduction
This reports presents a subset of the spectroscopic reductions planned for the STIS calibration pipeline. Following the original design described in “Plans for the STScI STIS
Pipeline I: Overview” (Baum & Hodge, STIS ISR 95-006) this report deals with CALSTIS-6. This part of CALSTIS takes the flat-fielded science image and extracts onedimensional spectra. This report is organized as follows:
•
Section 2 summarizes the basic spectroscopic calibration processing and data flow as
they apply to 1-D spectral extraction.
•
Section 3 describes details of the possible extraction schemes.
•
Section 4 describes the data quality and error propagation performed by CALSTIS-6.
•
Section 5 summarizes the output data format for spectra.
•
Section 6 lists specific programming requirements.
2. Extraction of One-Dimensional Spectra in CALSTIS-6
The extraction of one-dimension spectra from the flat-fielded (or CR-rejected) twodimensional image is a multistage process. The processing of spectroscopic ACCUM
observations in CALSTIS-6 is summarized in Figure 1 on page 2. Figure 2 on page 3
depicts a representative spectrum (first-order or single echelle order) in a hypothetical
1
point-source input image. Figure 2 also provides the definition of the coordinate system
used throughout this report.
Figure 1: CALSTIS-6 Processing Flow.
(_flt or _crj)
Extract 1-D
Spectra
Subtract
Background
Assign
Wavelengths
Assign
Fluxes
Apply Heliocentric
Wavelength
Correction
(_x1d)
2
Figure 2: The STIS coordinate system for spectroscopic observations
AXIS 2 / Slit Position
Spectrum
AXIS 1 / Dispersion
Note: For the case of observations using the cross-disperser, AXIS1 becomes Slit Position and
AXIS2 becomes Dispersion.
Read the input data
The input data consist of a flat-fielded (_flt) or cosmic ray rejected (_crj) 2-D science
image. In addition to reading the input image, CALSTIS-6 reads header keywords from
the input image that describe the instrument configuration: DETECTOR, OPT_ELEM,
APERTURE, CENWAVE. These keywords are, in turn, used to select the appropriate calibration data from the calibration reference tables. Table 1 lists the set of calibration
switch keywords that controls calibration processing and the keywords containing the
names of the supporting reference table.
Table 1. CALSTIS-6 Calibration Switch and Reference File Keywords
Calibration Step
Calibration
Switch
Calibration
Reference
File(s) Needed
Read small scale geometric corrections
SGEOCORR
SDSTFILE
Extract 1-D spectra
X1DCORR
SPTRCTAB
XTRACTAB
Subtract background
BACKCORR
XTRACTAB
Apply dispersion solution
DISPCORR
DISPTAB
INANGTAB
APDESTAB
MOFFTAB
ESPTAB
Convert to absolute flux
FLUXCORR
APERTAB
PHOTTAB
Convert to heliocentric wavelengths
HELCORR
3
SGEOCORR: Read Small-Scale Geometric Distortions
If SGEOCORR is set to “PERFORM”, CALSTIS-6 corrects for small scale geometric distortions in the MAMA detectors. These distortions are not adequately removed by
the dispersion or spectrum tracings. The corresponding reference file, SDSTFILE, contains the distortion offsets for each pixel in the MAMA image. For CALSTIS-6
processing, all AXIS2 positions in the input image must be modified by the AXIS2 small
scale distortion deltas in the small scale distortion file. Since we do not interpolate pixels
in the dispersion direction, no corrections are made to the AXIS1 positions prior to reading or extracting pixel values. Instead, the AXIS1 deltas are used to correct the assigned
wavelengths.
X1DCORR: Extract 1-D Spectra
CALSTIS-6 assumes that a 1-D spectrum is present and that it can be extracted independently of the X1DCORR calibration switch. In the pipeline, CALSTIS-0 determines
if CALSTIS-6 should be called based on the value of the X1DCORR calibration switch.
Initially, X1DCORR will be set to “PERFORM” for observations using one of the echelle
gratings and to “OMIT” for all other spectroscopic observations.
Locate the spectrum
The nominal location of the spectrum is specified in the spectrum trace table,
SPTRCTAB and is given by (A1CENTER, A2CENTER) from this table. These coordinates are not constrained to be integers. The nominal position along the slit must be
modified to include the previously updated position information found in the header. The
possible position deltas are listed in Table 2.
Table 2. Possible Offsets to Nominal Spectrum Center
Keyword
Description
SHIFTA1
Offset determined from the associated WAVECAL caused by the limits of the
MSM repeatability and by thermal drifts.
SHIFTA2
MOFFSET1
MOFFSET2
Commanded offsets for MAMA observations to reduce degradation of MAMA
detectors
The nominal A2CENTER position of the spectrum (i.e., the position of the target in
the AXIS2 or A2 or slit direction) is calculated as follows (where the variables are as
described in Table 2.
A2CENTER = A2CENTER + SHIFTA2 + MOFFSET 2
(Equation 1)
4
SPTRCTAB also contains the description of the distorted shape of the spectrum. The
shape is stored as a vector consisting of pixel offsets (in the AXIS2 direction) relative to
the nominal center of the spectrum (A2CENTER). This spectrum trace is used to find (and
eventually to extract) the 1-D spectrum. See Table 7 on page 16 for a description of the
columns in the spectrum trace table, SPTRCTAB.
The location of the spectrum is improved by “searching” in the vicinity of the nominal
location of the spectrum by performing a cross-correlation between the distortion vector
and the input spectrum image. Figure 3 on page 6 schematically shows the cross correlation process. The search extends for ± n pixels around the nominal center, where n is read
from the MAXSEARCH column in the XTRACTAB table. At each AXIS2 position in the
search range (which differs from the nominal center by an integer number of pixels) a sum
of the counts along the spectrum shape is formed. This sum is created by adding the value
of one pixel’s worth of data at each of the AXIS1 pixel positions. The pixel extracted in
the AXIS2 direction is centered on the spectrum position (A2CENTER + pixel offset) and
may include fractional contributions from two pixels. Quadratic refinement (as shown in
Figure 4 on page 6) is used to locate the spectrum to a fraction of a pixel.
The final A2CENTER becomes:
A2CENTER = A2CENTER + CRSCROFF
(Equation 2)
where CRSCROFF is the offset found during the cross correlation. If the cross correlation
fails, the value of CRSCROFF is set to zero, a warning message is written to STDOUT,
and the A2CENTER calculated prior to the cross correlation attempt is used as the location of the spectrum. CRSCROFF is written to the output science header.
An alternate method for performing the cross correlation may be employed. In this
case a 2-D template is created from the spectrum trace table. The cross correlation is carried out between the 2-D template and input image. Quadratic refinement is used as above
to refine the position of the center of the spectrum to a fraction of a pixel.
Extract the 1-D spectra
The extraction of the spectrum is defined by a triplet of extraction “boxes” found in the
reference table, XTRACTAB. Figure 5 on page 7 shows a schematic representation of the
extraction boxes. For each pixel in the dispersion direction, CALSTIS-6 sums the values
in the spectrum extraction box. The extraction box is nominally one pixel wide and x pixels tall, centered on the spectrum. (Remember that we determined the center of the
spectrum in the previous step.) The height of the extraction box may include a fractional
part of one or two pixels. In the case of a fractional pixel, CALSTIS-6 will scale the
counts in the given pixel by the fraction of the pixel extracted. Thus, each pixel in the output spectrum consists of the sum of some number (or fraction) of pixels in the input image.
5
The extraction of the 1-D spectrum is shown in Figure 6 on page 8. See Table 9 on page 17
for a description of the columns in the extraction parameter table.
Cross Correlation Range
AXIS2 Pixel
Figure 3: Repeated Extraction Strategy for “Finding” Spectrum Center
Spectrum Trace
Spectrum
AXIS1 Pixel
Figure 4: Quadratic Refinement Used to “Find” Actual AXIS2 Center of Spectrum
Actual AXIS2 Position
Sum
Nominal AXIS2 Position
AXIS2 Pixel
The extraction of the spectrum allows for unweighted or optimal extraction. The
extraction algorithm is selected based on the value of the reference table parameter,
XTRACALG. This flag has possible values of UNWEIGHTED and OPTIMAL. See Sec-
6
tion 3 for a description of the extraction algorithms. The value of XTRACALG should be
written to the header of the output spectrum data file. As a first cut, CALSTIS-6 will perform unweighted extraction of the 1-D spectra. Implementation of the optimal extraction
algorithm in CALSTIS-6 will be done at a later time.
At the end of the 1-D extraction step, a spectrum of gross counts/second is produced.
The conversion to count rate is as follows:
R
G i = -----i
t
(Equation 3)
where
i is the pixel number,
G i is the gross counts/sec in the spectrum at pixel i,
R i is the extracted raw counts, and
t is the exposure time.
Figure 5: Extraction Box Geometry
Background tilt angle
Background Extraction Box
Spectrum Extraction Box
Spectrum
Background Extraction Box
1 pixel
BACKCORR: Subtract the Background
If the calibration switch BACKCORR has the value of “PERFORM”, the background
is calculated and subtracted from the extracted spectrum. The background is extracted
above and below the spectrum and a function is fit to the background. The fitting function
is restricted to a zeroth or first order polynomial fit and is a function of the AXIS2 position. The polynomial order, BACKORD, is read from the XTRACTAB table and written
to a header keyword of the same name in the output spectrum table. Average background
(c/s/pixel) values are calculated from each background bin. A complete accounting of the
7
fractional pixel contributions to the background is required. In the case of BACKORD=0,
a simple average of the two background bins is computed. For BACKORD=1, a linear fit
to the background values as a function of AXIS2 position is computed. A background
value is then interpolated at the center of each pixel that contributes to the extracted spectrum. The background in the spectrum extraction box is totaled and subtracted from the
sum of the spectrum box. The total background at each pixel in the output spectrum is
saved and eventually written out to the output data file.ry
AXIS2 Pixel
Figure 6: Extracting the 1-D Spectrum
Spectrum Extraction Box Length
AXIS1 Pixel
In general, it is not assumed that the background or sky is aligned with the detector
pixels. To accommodate this, the definition of the background extraction apertures
includes not only a length and offset (center-to-center) but also a linear tilt to assist in
properly subtracting the background. This tilt, as shown in Figure 5 on page 7, is taken
into account when calculating the average background in the background extraction boxes.
DISPCORR: Assign Wavelengths
Wavelengths are assigned using dispersion coefficients from the reference table,
DISPTAB when the calibration switch, DISPCORR is set to “PERFORM”. IF DISPCORR is equal to “OMIT”, no wavelengths are assigned. Offsets introduced by using
apertures other than a reference aperture (used to derive the coefficients) will be removed
using coefficients in INANGTAB. Offsets introduced by the monthly MAMA dither offsets will be removed using coefficients in the MOFFTAB table. In the case of echelle
8
observations, small shifts introduced by the tilt of the spectral features will be removed
using coefficients in the ESPTAB table.
The DISPTAB table of dispersion contains coefficients for fits to the following dispersion solution:
2
2
s = A 0 + A 1 mλ + A 2 ( mλ ) + A 3 m + A 4 λ + A 5 m λ + A 6 mλ
2
(Equation 4)
where,
λ is the wavelength in Angstroms,
s is the detector AXIS1 position,
m is the spectral order, and
A i are the dispersion coefficients.
For each pixel in the AXIS1 direction, a wavelength will be calculated. First, any modification to the dispersion coefficients due to spectrum offsets must be made. Table 3 on
page 9 lists the possible offsets and the appropriate corrections. For each integer value of s
in the AXIS1 direction, a wavelength is calculated. The wavelength value must be solved
for iteratively using the Newton-Raphson method (see, for example, Press, W.H. et al.
Numerical Recipes in C. 1992. p. 362).
Table 3. Modifications to the Dispersion Coefficients Caused by Offsets
Correction
Ref Table
Incidence Angle
INANGTAB
Algorithm
Definitions
Ai = Ai + c1 s
A0 = A0 + c2 s
2
Ai
dispersion coefficients
ci
incidence angle coefficients
s
aperture offsets in the axis 1
direction calculated as difference
of relative aperture centers (arcsec)
MAMA Offsets
MOFFTAB
A i dispersion coefficients
Ai = Ai + o1 x
A0 = A0 + o2 x
2
oi
MAMA offset coefficients
x MAMA offset (MOFFSET1)
(pixels)
Echelle Spectrum Tilt
ESPTAB
A 0 = A 0 + y tan θ
A i dispersion coefficients
y axis 2 offset from nominal
A2CENTER during spectrum
locate process (pixels)
θ
9
spectrum tilt angel
FLUXCORR: Convert to absolute flux
If FLUXCORR is set to “PERFORM”, the raw counts are corrected to FLAM (erg cm2 s-1
Å-1) using reference files PHOTTAB and APERTAB. Execution of the flux conversion calibration step requires that wavelengths have been assigned. Corrections for
vignetting and echelle blaze are handled within the PHOTTAB reference files–at this time,
no attempt will be made to decouple all of the various sources of response variation seen
with STIS. The conversion to absolute flux is calculated as:
hc
F λ = -------------------------------------- N λ
A HST R λ T λ λ∆λ
(Equation 5)
F λ is the calibrated flux at a particular wavelength,
h is Planck’s constant,
c is the speed of light,
A HST is the area of the unobstructed HST primary mirror (45238.93416 cm2),
R λ is the throughput of the STIS instrument configuration at a particular wavelength when a clear full aperture is in place,
λ is a particular wavelength,
∆λ is the dispersion (Å/pixel)at a particular wavelength,
N λ is the net count rate at a particular wavelength, and
T λ is the aperture throughput at a particular wavelength.
HELCORR: Apply heliocentric corrections to the wavelengths
The correction of wavelengths to a heliocentric reference frame is controlled by calibration switch HELCORR and DISPCORR—if both switches are set to “PERFORM”
then the correction is made. The functional form of the correction (shown below) requires
the calculation of the heliocentric velocity (v) of the earth in the line of sight to the target.
v
λ helio = λ  1 + --

c
(Equation 6)
λ helio is the heliocentric wavelength,
λ is a particular wavelength,
v is the component of the velocity of the earth in the direction of the target,
and
10
c is the speed of light.
The derivatives of low-precision formulae for the Sun’s coordinates described in the
Astronomical Almanac are used to calculate the velocity vector of the earth in the equatorial coordinate system of the epoch J2000. The algorithm does not include Earth-Moon
motion, Sun-barycenter motion, nor light time correction from the Earth to the Sun. This
value for the earth’s velocity should be accurate to ~0.025 km/sec during the lifetime of
STIS. (Note: the uncertainty of 0.025 km/s is much less than the ~2.6 km/s resolution
obtained with the STIS high dispersion echelle gratings.) The algorithm used in CALSTIS-6 should be identical to that used in CALSTIS-7.
The value of heliocentric velocity, v , is written to the header keyword, HELIOVEL in
the output header.
3. Extraction Algorithm
Extraction of one-dimensional spectra for point sources can be described by:
∑ W sλ ( C sλ – Bsλ )
s
N λ = -------------------------------------------W
∑ sλ
(Equation 7)
s
where,
N λ is the net extracted spectrum,
W sλ is the weighting applied to each pixel in the spectrum,
C sλ is the observed count rate at each slit position and wavelength, and
B sλ is the fitted background (and/or sky) count rate at each slit position and
wavelength.
Unweighted
In the case of unweighted extraction, the factor W sλ has the value of 1 at every pixel
in the spectrum and is 0 outside of the spectrum. As implemented in CALSTIS-6, a
rectangular extraction box centered in the slit direction on the spectrum is used to identify
that portion of the 2-D spectrum to be summed to produce the output 1-D spectrum.
Optimal
The case of optimal extraction is described by Horne (1986) with additional work specific to handling the problematic case of distorted spectra by Marsh (1989). Optimal
extraction has as its goal identifying a set of weights that maximizes the signal-to-noise
11
ratio in the extracted spectrum. The optimally-extracted spectrum is found by using Equation 8 where:
P sλ
W sλ = -------V sλ
(Equation 8)
P sλ is the spatial profile of the spectrum and
V sλ is the variance of the background/sky subtracted data points.
The two items of importance in this method are good error estimates for the original
data points and an accurate knowledge of the spatial profile. Assignment of error estimates
to the data points is handled in CALSTIS-1. The method for determining the best spatial
profile is under investigation.
4. Data Quality and Error Propagation
Data Quality Propagation
The data quality value for any pixel in an extracted 1-D spectrum is the “or-ed” value
of all data quality values that were used to produce the spectrum. At this time CALSTIS6 does not set any additional data quality values.
Error Propagation
CALSTIS-6 will propagate errors during the conversion of counts to count rate, the
extraction of the 1-D spectra, the subtraction of the background, and the conversion to the
absolute flux. In those cases where fractional pixels are coadded to form a sum, the relative error from each fractional part is combined to form the error associated with the sum.
The errors associated with the calibrated data are part of the output data products. These
errors are consistent with the highest level of flux calibration applied to the data (i.e., if the
data are only processed as far as background subtracted count rates, the error estimates are
those associated with the count rate values). In equations 9-11 below, the deltas ( ∆ ) represent the associated errors for a given measurement.
The gross count rate spectrum is calculated as:
R ∆R
C ± ∆C = --- ± ------t
t
(Equation 9)
C ± ∆C is the count rate,
R ± ∆R is the raw counts, and
t is the exposure time.
12
The net spectrum is calculated as:
2 1⁄2
2
N ± ∆N = ( C – B ) ± ( ∆C + ∆B )
(Equation 10)
N ± ∆N is the net spectrum and
B ± ∆B is the background.
The flux calibrated spectrum is calculated as:
F ± ∆F = N × S ± ( ∆N × S )
(Equation 11)
F ± ∆F is the flux calibrated spectrum and
hc
S = -------------------------------------- is the flux conversion factor from Equation 6.
A HST R λ T λ λ∆λ
5. Data Formats
A complete spectrum consists of seven arrays containing: wavelengths, gross count
rates, background count rates, net count rates, absolute fluxes, absolute flux error estimates, and data quality flags. A given spectrum will be written to one row of a FITS binary
table extension. Additionally, each row will contain columns describing the spectral order
and number of data points in the spectrum. Table 4 lists the table column labels, units, and
datatypes. To accommodate a single spectrum per row, the table cells will contain arrays
of numbers corresponding the physical quantities of the spectrum. NELEM gives the number of elements in these vectors for a given spectrum.
Table 4. Column Definitions for Output Binary Table
Column Name
Datatype
Units
SPORDER
I*2
NELEM
I*2
WAVELENGTH
R*8[n]
Angstroms
FLUX
R*4{n]
erg cm-2 s-1 Å-1
ERROR
R*4{n]
erg cm-2 s-1 Å-1
DQ
I*2
GROSS
R*4[n]
counts/sec
BACKGROUND
R*4{n]
counts/sec
NET
R*4[n]
counts/sec
Table 5 summarizes the details of the FITS files created by CALSTIS-6. The mapping is effectively one output table extension to each input group of image extensions.
13
Table 5. Output spectrum FITS definition
Output
Source
Primary header
Primary header of input science data file
Primary data
empty
1st binary table extension
header
image extension header from 1st science extension [sci,1] PLUS definition of binary table
1st table extension data
extracted spectra from 1st science extension
nth table extension data
extracted spectra from nth science extension
6. Programing Requirements
I/O
Routines that perform I/O should be isolated (in code) from routines that perform
numeric processing. HSTIO should be used to access STIS images.
Calling Sequence
cs6 input output -t -c a2center -r maxsearch -x
extrsize -b1 bk1size -b2 bk2size -o1 bk1offst -o2
bk2offst -k bktilt -n backord -a xtracalg
where,
cs6 is the name of the CALSTIS-6 executable
input is the name of the input FITS file
output is the name of the output FIT file,
-t controls the printing of OPUS style time stamps.
-c a2center is the nominal AXIS2 coordinate of the center of the spectrum to
be extracted,
-r maxsearch is the cross correlation search range,
-x extrsize is the size of spectrum extraction box,
-b1 bk1size is the size of background extraction box #1,
-b2 bk2size is the size of background extraction box #2,
-o1 bk1offst is the offset of background extraction box #1 from spectrum
extraction box,
-o2 bk2offst is the offset of background extraction box #2 from spectrum
extraction box,
-k bktilt is the angle of background extraction boxes w.r.t. axis2,
14
-n backord is the order of polynomial fit to background, and
-a xtracalg is the extraction algorithm.
Parameters read from the command line override the corresponding values read
from any of the reference tables.
7. Acknowledgments
Special thanks to Don Lindler for many discussions and whose “Space Telescope
Imaging Spectrograph: Science Data Management and Analysis Requirements Document” provided a valuable road map.
8. References
Baum, S. and Phil Hodge. July 1995. “Plans for the STScI STIS Pipeline I: Overview.”
STIS ISR 95-006.
Horne, K. 1986. Pub.A.S.P., 98, 609.
Marsh, T. 1989. Pub.A.S.P., 101, 1032.
Lindler, D. March 1995. “Space Telescope Imaging Spectrograph: Science Data Management and Analysis Requirements Document.” Advanced Computer Concepts,
Inc.
15
9. Calibration Reference Files and Tables
Below are listed the calibration reference tables used by CALSTIS-6. Only those columns used by CALSTIS-6 are included. For more details on the reference tables refer to
ICD-47.
Table 6. DISPTAB– Dispersion Coefficients Table
Column
Name
Data
Type
OPT_ELEM
C*8
CENWAVE
I*2
SPORDER
I*2
spectral order
REF_APER
C*12
reference aperture
A2CENTER
R*4
NCOEFF
I*2
number of coefficients in dispersion solution
COEFF
R*8[10]
dispersion solution coefficients
Units
Description
optical element in use
Angstrom
central wavelength
pixel
nominal axis2 coordinate for center of spectrum
Table 7. SPTRCTAB–1-D Spectrum Trace Table
Column
Name
Data
Type
OPT_ELEM
C*8
CENWAVE
I*2
SPORDER
I*2
spectral order
NELEM
I*2
number of data points in spectrum
A2DISPL
R*4[1024]
pixel
displacement along axis2
A1CENTER
R*4
pixel
nominal axis 1 coordinate of center of spectrum
A2CENTER
R*4
pixel
nominal axis 2 coordinate of center of spectrum
Units
Description
optical element in use
Angstrom
central wavelength
Table 8. INANGTAB–Incidence Angle Correction Table
Column
Name
Data
Type
OPT_ELEM
C*8
CENWAVE
I*2
SPORDER
I*2
spectral order
NCOEFF1
I*2
no. coefficients in IAC solution for first term
Units
Description
optical element in use
Angstrom
central wavelength
16
Column
Name
Data
Type
COEFF1
R*8[8]
incidence angle correction coefficients for first term
NCOEFF2
I*2
no. coefficients in IAC solution for second term
COEFF2
R*8[8]
incidence angle correction coefficients for second term
Units
Description
Table 9. XTRACTAB–1-D Extraction Parameter Table
Column
Name
Data
Type
APERTURE
C*16
aperture in use
OPT_ELEM
C*8
optical element in use
CENWAVE
I*2
SPORDER
I*2
EXTRSIZE
R*4
pixel
size of spectrum extraction box
BK1SIZE
R*4
pixel
size of background extraction box #1
BK2SIZE
R*4
pixel
size of background extraction box #2
BK1OFFST
R*4
pixel
offset of background extraction box #1 from spectrum
extraction box
BK2OFFST
R*4
pixel
offset of background extraction box #2 from spectrum
extraction box
BKTILT
R*4
degrees
angle of background extraction boxes w.r.t. axis2
BACKORD
I*2
order of polynomial fit to background
XTRACALG
C*12
extraction algorithm
MAXSEARCH
I*2
Units
Description
Angstrom
central wavelength
spectral order
pixel
maximum search size for cross correlation
Table 10. MOFFTAB–MAMA Offset Correction Table
Column
Name
Data
Type
OPT_ELEM
C*8
CENWAVE
I*2
SPORDER
I*2
spectral order
NCOEFF1
I*2
no. coefficients in MAMA offset solution for first term
COEFF1
R*8[8]
MAMA offset correction coefficients for first term
NCOEFF2
I*2
no. coefficients in MAMA offset solution for second term
COEFF2
R*8[8]
MAMA offset correction coefficients for second term
Units
Description
optical element in use
Angstrom
central wavelength
17
Table 11. ESPTAB–Echelle Spectrum Tilt Table
Column
Name
Data
Type
OPT_ELEM
C*8
CENWAVE
I*2
SPORDER
I*2
SPTILT
R*4
Units
Description
optical element in use
Angstrom
central wavelength
spectral order
degrees
angle of spectrum w.r.t. axis2
Table 12. APERTAB–Aperture Throughput Table
Column
Name
Data
Type
APERTURE
C*16
aperture in use
NELEM
I*2
number of data points in throughput array
WAVELENGTH
R*8[65535]
THROUGHPUT
R*8[65535]
Units
Description
Angstrom
reference wavelength
total system throughput at each wavelength
Table 13. PHOTTAB–Photometric Conversion Table
Column
Name
Data
Type
OPT_ELEM
C*8
CENWAVE
I*2
SPORDER
I*2
spectral order
NELEM
I*2
number of data points in throughput array
WAVELENGTH
R*8[500]
THROUGHPUT
R*8[500]
total system throughput at each wavelength
ERROR
R*8[500]
error associated with THROUGHPUT
Units
Description
optical element in use
Angstrom
central wavelength
Angstrom
reference wavelength
Table 14. APDESTAB–Aperture Description Table
Column
Name
Data
Type
APERTURE
C*16
OFFSET1
R*4
arcsec
offset from nominal position in axis1
OFFSET2
R*4
arcsec
offset from nominal position in axis2
Units
Description
aperture in use
18