SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
Instrument Science Report COS 2013-03(v1)
Second COS FUV Lifetime Position:
Verification of Aperture and FUV
Spectrum Placement (FENA2)
Charles R. Proffitt1, 2, Steven V. Penton1,3, David J. Sahnow1, Cristina M. Oliveira1,
Derck Massa1, Rachel A. Osten1, Steven N. Osterman3, and Alessandra Aloisi1
1
2
Space Telescope Science Institute, Baltimore, MD
Science Programs, Computer Sciences Corporation
3
University of Colorado, Boulder, CO
8 February 2013
ABSTRACT
CAL/COS proposal 12795 was used to determine the exact updates for the Science
Instrument Aperture File (SIAF) and for the COS aperture block positioning needed
to implement routine science operations at COS FUV Lifetime Position 2. It also
verified that the location of each grating’s spectrum on the FUV XDL detector met
the requirements to deliver good quality science data. The data obtained showed that
a +3.5” offset in the cross-dispersion direction, which corresponds to a +41 pixel
shift on the detector, would provide the desired positioning on the detector. To keep
the COS aperture centered at this location requires shifting the aperture block by −73
steps in the cross-dispersion direction and adjusting the SIAF locations of the
Primary Science Aperture (PSA) at the new position by +3.5” in the cross-dispersion
direction and by −0.05” in the dispersion direction.
Instrument Science Report COS 2013-03(v1) Page 1 Contents
1. Introduction 2 2. Goals and Design of 12795 2.1 Goals, Design, and Execution of Visit 1 2.2 Goals, Design, and Execution of Visit 2 6 6 8 3. Results 3.1 Placement of the New Lifetime Position 3.2 Aperture Centering at the New Lifetime Position 10 10 17 4. Summary 21 Acknowledgements 22 Change History for COS ISR 2013-­‐03 22 References 22 1. Introduction
The gain of the COS/FUV Cross-Delay-Line (XDL) microchannel plate detector
declines with usage, eventually becoming too low to reliably detect and centroid
photons incident on the photocathode. To extend the useful life of the COS FUV
detector, it is necessary to periodically shift the spectrum to another location on the
detector.
In preparation for the first such move, a series of exploratory programs were executed
in mid-2011 to gather the information needed to pick the location and sequencing of
subsequent lifetime positions. By December 2011, the results of these programs led to
the decision that for Lifetime Position 2 the FUV spectra should be shifted by
approximately +3.5” in the cross-dispersion direction (see Oliveira et al. 2013b).
However, before this new lifetime position could be implemented for routine use, a
number of parameters needed to be measured and optimized. To do these a series of
four FUV enablement (FENA) programs were implemented (see Osten et al. 2013 for
an overall description of the lifetime move enablement and calibration programs and
requirements). FENA2 was designed to address two of the specific high-level
requirements of the overall FENA effort:
-
FENA requirement FUV2: The placement of the FUV spectra at the detector
location corresponding to the new lifetime position for the PSA shall be
verified.
FENA requirement FUV3: The PSA aperture shall be located so that the target
is well centered in the aperture when the spectra are placed at the new lifetime
position.
The choice of the new lifetime position was primarily driven by two competing
considerations – maximizing resolution and keeping the spectra at the new position
Instrument Science Report COS 2013-03(v1) Page 2 away from the gain-sagged regions produced by observations at the original position.
The FUV Exploratory program 12678 (Sahnow et al. 2013) had shown that the
spectral resolution degraded with increasing cross-dispersion offset from the original
position, but did so more slowly in the +Y direction. The best possible resolution
would in principle be obtained by shifting the spectrum by ≈ +1.5” in the crossdispersion direction. This behavior was supported by analysis of ray-trace models
(Osterman 2010, private communication). In addition, keeping the 2nd lifetime
position as close as possible to the original one would have the additional benefit of
maximizing the detector area available to use for subsequent lifetime positions.
FUV Exploratory program 12676 (Massa et al. 2013 ) used deuterium lamp spectra to
map the gain and flat field characteristics over the accessible region of the FUV
detector, clearly illustrating the sagged regions that need to be avoided at the new
lifetime position. The most intense illumination and the most rapid microchannel
plate damage results from the geocoronal Lyα illuminating the 2.5” diameter COS
PSA aperture during G130M observations. Each of the 4 FP-POS positions of each of
the 5 original G130M CENWAVE settings illuminates a different portion of the B
segment (FUVB) of the FUV XDL detector, leading to 20 total spots (see Figure 1).
Two of the four similar spots that result on the A segment (FUVA) from the G140L
1105 setting are easily visible in the gain map near the left edge of the segment. Each
of these full-aperture spots spans the cross-dispersion range of well-centered point
source spectral locations for observations with all three gratings. Consequently it is
necessary to ensure that the new location will keep all FUV spectra sufficiently clear
of these previously damaged regions.
Instrument Science Report COS 2013-03(v1) Page 3 Figure 1: Gain maps for FUVA (above) and FUVB (below) of the COS FUV XDL detector based
on internal deuterium lamp spectra obtained in July 2011 as part of CAL/COS program 12676.
The regions of the detector where the gain has dropped as a result of incident photons are
evident as dark bands across the middle of each segment. A number of more deeply sagged
regions are visible on FUVB at the locations where the geocoronal Lyα projects onto the
detector. Two fainter spots from the G140L 1105 geocoronal Lyα can be seen near the left edge
of FUVA near columns 2300 and 2600. Other localized low gain regions are also visible. For the
Instrument Science Report COS 2013-03(v1) Page 4 second lifetime position, the most significant of these is the region near column 8000 on segment
A.
Under the assumption that usage patterns at the new position would match those at the
original position, analysis of the 12676 data concluded that shifting the location by
+35 pixels would be adequate to avoid overlap of the spectra at the new location with
the previously sagged regions. Initial plate scale estimates suggested that this was
equivalent to a 3.5” shift, and the ray-trace models suggested that the decrease in
spectral resolution with this shift would be 10 to 15%, which was judged to be
acceptable. However, there were still a number of complications associated with
translating the desired pixel shift into the required operational parameters.
One issue is uncertainties in the “Y-walk” correction. The apparent position of events
on the COS FUV detector varies with gain due to the geometry of the micro-channel
plate pores (Sahnow et al. 2011). For planning and analysis of this program, it was
assumed that events are shifted downward on the detector below their true position by
0.45 pixels for each unit decrease in the event’s gain. Inaccuracies in the Y-walk
correction may lead to the mis-correction of the position of events. An improved
correction is being developed by Kriss et al. (2013a).
Another difficulty at the time this program was planned was that the effective plate
scales for each grating were not known with sufficient accuracy. The available data
gave inconsistent results, perhaps due to inadequate Y-walk corrections or the
variations in the shape of the spectral cross-dispersion profile with position.
Each of the three COS FUV gratings, (G160M, G130M, and G140L), falls at a
slightly different position in the cross-dispersion direction, and, for consistently
centered point-sources, only the wings of the cross-dispersion profiles overlap. This
had the advantage of spreading the photon accumulation over a wider area of the
detector, and to some extent mitigates the effect of microchannel plate damage
resulting from use of each grating on observations done with the other gratings.
Early in the lifetime of COS, target acquisitions for many of the calibration programs,
including the routine sensitivity monitors, were done using FUV dispersed light target
acquisitions. It was only subsequently realized that the increasing y-walk resulting
from the on-going gain-sag meant that for observations that relied on such target
acquisitions, the location of the spectra on the detector were slowly shifting over time.
For our analysis, we would ideally like to compare our measured spectral locations
with reference spectra taken at the original lifetime position prior to the onset of
significant gain sag. To ensure that the new and the old spectra are consistently
positioned, we will choose reference spectra that used NUV imaging target
acquisitions done with the bright object aperture (BOA); BOA imaging acquisitions
were also done for the visits in this program. This will minimize spurious shifts due to
gain-sag and walk effects. The routine sensitivity monitor observations were switched
to using NUV BOA imaging acquisitions in March of 2010, and so for our
comparison spectra we will select G140L 1280 and G130M 1291 observations from
program 11897 taken on March 3, 2010 and G160M 1600 observations from the same
program taken on March 31, 2010. The cross-dispersion profiles for these
observations, collapsed over the dispersion direction, are shown in Figure 2.
In addition to the external source spectrum, the G130M FUVB 1291 image also
shows a strong full aperture Ly-α emission line that is locally much brighter and
wider than the stellar spectrum. We omitted this geo-coronal emission from the
Instrument Science Report COS 2013-03(v1) Page 5 collapsed cross-dispersion profile for the G130M shown in Figure 2, and
instead.separately show the profile for the core of the geocoronal line to illustrate how
it overlaps with the profiles of the other gratings. The detailed shape of the crossdispersion profile for each grating varies significantly with wavelength and
CENWAVE setting, even within a single segment, but the collapsed profiles used
here will serve as a reference for measuring the overall shift of each grating as a
function of the adopted pointing offset in the cross-dispersion position.
Figure 2: The collapsed cross-dispersion profiles at the original lifetime position for selected
sensitivity monitor observations taken in March 2010. Results for FUVA are shown on the left
and for FUVB on the right. The FUVB G130M cross-dispersion profile shown here excludes the
geocoronal Lyα photons. (For this setting, XCORR values between pixels 9000 and 9170). The
cross-dispersion profile for the core of the Lyα line in this exposure (XCORR values between
9050 and 9125) is shown separately by the dotted line.
2. Goals and Design of 12795
2.1 Goals, Design, and Execution of Visit 1
The primary purpose of visit 1 was to take one spectrum with each grating at the new
lifetime position in order to evaluate whether the placement of the spectra on the
detector was acceptable. It was also desirable to provide estimates of the local plate
scales for the FUV gratings at the new position and to measure the offsets between
the science and wavecal spectra. These latter quantities are needed during execution
of PEAKXD acquisitions to determine how to center the target and these vary
significantly with grating, position, and gain level. Because FENA 2 executed prior to
FENA4, measurements of these quantities from FENA2 were used as initial estimates
for the FENA4 tests.
Exploratory work for choosing the new lifetime position (Oliveira et al 2013b) had
concluded that the new lifetime position should be shifted from the old one by about
35 pixels. Originally it was assumed that this was equivalent to a +3.5” shift, but by
the time that the next COS lifetime position was selected, it was already appreciated
that the nominal FUV cross-dispersion plate scales of 10 pixels/arc-sec that had been
adopted during the initial discussions were not correct. Analysis of data taken at an
offset of +3” as part of the exploratory program 12678 (Sahnow et al. 2013),
suggested that for G130M at least, the actual plate scale was closer to 11.905
Instrument Science Report COS 2013-03(v1) Page 6 pixels/arc-sec. Shifting the spectrum by 35 pixels would then require a spacecraft
pointing offset of only +2.94”, rather than the nominal +3.5” that had been previously
discussed.
However, at the time visit 1 was being planned, not only were there uncertainties in
the geometric and walk corrections that made interpretation of previous data difficult,
but the strategy for managing detector gain at the new position was still under
development, and the initial gain levels and starting HV levels were not yet set. In
addition, only G130M data was available near the new position.
Using as small an offset as possible would improve spectrum resolution and leave
more room for subsequent lifetime positions, but this would allow the data to be more
affected by gain sag from the original position. It was decided that visit 1 should
obtain data for all three gratings with both a +2.94” offset (~35 pixel shift) and a
3.74” offset (~45 pixel shift). A final decision on the exact placement of the lifetime
position could then be deferred until after the visit 1 data were examined and the
effects of gain sag could be directly determined. As will be discussed below,
ultimately it was decided to adopt a +3.5” offset (~41 pixels) for the new lifetime
position, and that value was used for the visit 2 exposures.
For this visit, the DB WD standard star WD0308-565 was selected as it has a flatter
spectral energy distribution than most other WD standards, allowing for more uniform
exposure times and S/N vs. wavelength for the different gratings. The target
acquisition was done using an ACQ/IMAGE exposure with the BOA aperture and
MIRRORA.
At the time of execution, the default HV levels for the COS FUV detector at the
initial lifetime position were 169 steps for FUVA and 175 steps for FUVB. The final
HV values that would eventually be adopted for use at the second lifetime position
had not yet been determined. While the 169/175 settings gave the best results at the
gain sagged region of the detector near the original position, using that setting at the
higher gain regions near the new lifetime position would result in misregistered
events due to the excessively high gain, (Sahnow et al. 2011). To avoid this artifact,
the FUVB voltage was lowered to 167 for all exposures in both visits of this program.
Note that the reference exposures discussed above from program 11897 in March
2010 also used the 169/167 step HV settings for FUVA and FUVB. Note that there
was no need for FENA2 to use the exact voltages that would be adopted for
operations at the second lifetime position. This allowed FENA2 to proceed without
waiting for results from any other enablement programs, such as the HV sweep done
as part of FENA1 (Kriss et al. 2013b).
A single G130M observation was taken at the original position, and then one
exposure with each of the G130M, G160M, and G140L gratings was taken at a crossdispersion offset of +2.94” (targeting a shift of ~ 35 pixels), and then again at +3.78”
(targeting ~ 45 pixels). The local plate scale estimates provided by using each grating
at two positions were intended to not only provide the initial estimates for use by
FENA4, but also to allow fine adjustment of the exact offset.
The inputs for the ALIGN/APER command used to move the aperture block to
nonstandard positions are specified in units of motor steps relative to the default
position. For this program this default aperture position was the standard Lifetime
Position 1 Prime Science Aperture (PSA) position. It was assumed that commanding
an offset of −21 XAPER steps is equivalent to moving the aperture +1” on the sky in
Instrument Science Report COS 2013-03(v1) Page 7 the POSTARG2 direction, while +21 YAPER steps is equivalent to moving the
aperture +1” in the dispersion direction. Note that this aperture coordinate system
uses the detector coordinates that were devised early in instrument development and
these differ from those used in standard CALCOS pipeline products, where the +x is
in the direction of increasing wavelength and +y is in the +POSTARG2 direction.
This change was made to allow the APT interface and the pipeline products for COS
to be consistent with those of STIS and the other HST instruments.
All exposures at nonstandard aperture positions used the special command
ELNOAPMAIN to avoid automatic relocation of the aperture back to its normal PSA
position. To improve the S/N for the measurement of the WCA aperture locations, the
lamp flash times were increased to 36 s for the G130M and G160M and to 21 s for the
G140L. This resulted in the sequence of seven FUV exposures shown in Table 1.
Table 1: FUV exposures in visit 1 of 12795. The target for visit 1 exposures was the DB star WD
0308−0565. The visit executed on February 21, 2012.
Setting
Exposure
Exp. Time
POSTARG2
Flash
XAPER name
(s)
(“)
(s)
G130M 1291 LBX401H9Q 54 0 36 0 G130M 1291 LBX401HBQ 54 +2.94 36 −62 G160M 1600 LBX401HDQ 163 +2.94 36 −62 G140L 1280 LBX401HFQ 40 +2.94 21 −62 G130M 1291 LBX401HHQ 54 +3.78 36 −79 G160M 1600 LBX401HJQ 163 +3.78 36 −79 G140L 1280 LBX401HLQ 40 +3.78 21 −79
2.2 Goals, Design, and Execution of Visit 2
The primary purpose of visit 2 was to verify the aperture block centering at the new
lifetime position in both the dispersion and cross-dispersion direction. This visit also
served to confirm the final revisions to the cross-dispersion offset defining the new
lifetime position to be at +3.5”, (about +41 pixels), that were made after the initial
evaluation of the visit 1 data (see discussion in section 3.1 below).
The aperture center in principle could be determined either by keeping the aperture at
a fixed position and scanning the HST pointing via POSTARGs, or by keeping the
pointing fixed and scanning the COS PSA aperture across the target. Moving the
spacecraft has the disadvantage that the location of the spectrum on the detector
changes at each POSTARG position, and the detected spectrum would then vary due
to changes in PSF shape, shifts with respect to the detector fixed pattern noise, and for
changes in the dispersion direction, variation in the exact wavelength range falling on
each segment. As a result, it was decided to keep the target fixed while moving the
aperture to better isolate the vignetting caused by the aperture edges. During these
aperture scans, POSTARG1 was held at 0 and POSTARG2 at a value of +3.5 arcseconds.
The vignetting of an external target as a function of the target’s position within the
COS aperture was previously studied by Ghavamian et al. 2010. For the G130M
grating, the throughput as a function of centering offset is relatively flat in the central
±0.4” and falls off to about 40% of the central value at the 1.25” aperture radius. For
this program we chose a 13 point pattern of aperture motions extending to ±29 motor
Instrument Science Report COS 2013-03(v1) Page 8 steps or 1.381” from the central positions. Spacings of 4 to 6 motor steps between
exposures were used, with slightly coarser centering being used near the center and in
the far wings. The resulting sequence of exposures is documented in Table 2.
The images of geocoronal lines will move on the detector as the aperture moves, and
these are expected to vary in brightness as the sight line with respect to the earth
changes and as HST moves between orbital day and night. To allow easier removal of
these effects, visit 2 used the sensitivity monitor target WD 0947+0857 as the
relatively broad stellar Lyα line in this DA white dwarf allows easier isolation of the
stellar and geocoronal flux. For these observations the G130M 1309 CENWAVE
setting was used as it puts the brightest of the geocoronal O I lines in the gap between
the detector segments.
Table 2: The FUV Exposures in visit 2 of 12795. The target for visit 2 exposures was the DA star
WD 0947+857. This visit executed on March 12, 2012. For clarity, in this table values repeated
from the previous row are in a grey font. The LAMP column lists which Pt/Ne lamp and current
setting was used for each exposure. XAPER and YAPER give the offsets, for the cross-dispersion
and dispersion directions respectively, of the aperture block in motor steps from their default
values for the first lifetime position. The quantities δ2 and δ1 give the equivalent offset of the
aperture block in arc-seconds, assuming a plate scale of 21 motor steps per arc-second. The sign
convention on δ1 and δ2 is chosen so setting POSTARG1 to δ1 or adding δ2 to the actual
POSTARG2 parameter would give the offset that would have allowed the target to follow the
aperture during the sweep.
XAPER
YAPER
Setting
Exposure
Exp POSTARG2 LAMP
δ2
δ1
counts
(“)
Name
(s)
(“)
(“)
A
G130M 1309 LBX402BGQ 25 0.0 LINE1, MED 0 0 0 0 Cross-­‐dispersion Aperture Scan G130M 1309 LBX402BIQ 50 +3.5 LINE1, MED -­‐102 +1.381 0 0 45836 G130M 1309 LBX402BKQ 50 +3.5 LINE1, MED -­‐96 +1.095 0 0 102432 G130M 1309 LBX402BMQ 50 +3.5 LINE1, MED -­‐91 +0.857 0 0 129156 G130M 1309 LBX403BOQ 50 +3.5 LINE1, MED -­‐87 +0.667 0 0 146464 G130M 1309 LBX402BQQ 50 +3.5 LINE1, MED -­‐83 +0.476 0 0 161434 G130M 1309 LBX402BSQ 50 +3.5 LINE1, MED -­‐79 +0.285 0 0 165072 G130M 1309 LBX402BUQ 50 +3.5 LINE1, MED -­‐73 0 0 0 166609 G130M 1309 LBX402BWQ 50 +3.5 LINE1, MED -­‐67 -­‐0.285 0 0 165094 G130M 1309 LBX402BYQ 50 +3.5 LINE1, MED -­‐63 -­‐0.476 0 0 159852 G130M 1309 LBX402C0Q 50 +3.5 LINE1, MED -­‐59 -­‐0.667 0 0 142244 G130M 1309 LBX402C2Q 50 +3.5 LINE1, MED -­‐55 -­‐0.857 0 0 124864 G130M 1309 LBX402C7Q 50 +3.5 LINE1, MED -­‐50 -­‐1.095 0 0 99958 G130M 1309 LBX402CFQ 50 +3.5 LINE1, MED -­‐44 -­‐1.381 0 0 42629 Along Dispersion Aperture Scan
G130M 1309 LBX402CHQ 50 +3.5 LINE2, LOW -­‐73 0 +29 +1.381 66782 G130M 1309 LBX402CJQ 50 +3.5 LINE2, LOW -­‐73 0 +23 +1.095 114745 G130M 1309 LBX402CLQ 50 +3.5 LINE2, LOW -­‐73 0 +18 +0.857 135332 G130M 1309 LBX402CNQ 50 +3.5 LINE2, LOW -­‐73 0 +14 +0.667 150439 G130M 1309 LBX402CPQ 50 +3.5 LINE2, LOW -­‐73 0 +10 +0.476 163334 G130M 1309 LBX402CRQ 50 +3.5 LINE2, LOW -­‐73 0 +6 +0.285 166219 G130M 1309 LBX402CTQ 50 +3.5 LINE1, MED -­‐73 0 0 0 166936 G130M 1309 LBX402CVQ 50 +3.5 LINE2, LOW -­‐73 0 -­‐6 -­‐0.285 164479 G130M 1309 LBX402CXQ 50 +3.5 LINE2, LOW -­‐73 0 -­‐10 -­‐0.476 154292 G130M 1309 LBX402CZQ 50 +3.5 LINE2, LOW -­‐73 0 -­‐14 -­‐0.667 141511 G130M 1309 LBX402D1Q 50 +3.5 LINE2, LOW -­‐73 0 -­‐18 -­‐0.857 127658 G130M 1309 LBX402D3Q 50 +3.5 LINE2, LOW -­‐73 0 -­‐23 -­‐1.095 103090 G130M 1309 LBX402D5Q 50 +3.5 LINE2, LOW -­‐73 0 -­‐29 -­‐1.381 51280 Instrument Science Report COS 2013-03(v1) Page 9 counts
B 69013 148572 185654 211382 233375 239643 242522 241906 233356 209185 186096 151277 75807
95876 165608 195515 217498 234763 238045 241133 236836 223733 204080 185263 149725 74293
Confirmation of final G160M Position
G160M 1600 LBX402D7Q 50 +3.5 LINE1, MED -­‐73 0 0 0 It has been shown by Oliveira et al. (2013a) that for sufficiently large offsets of the
aperture block in the cross-dispersion direction lamp light can leak through the flatfield calibration aperture (FCA). This can result in excessive illumination from
ordinary wavelength calibration lamp flashes. To guard against the possibility that a
similar light leak might result from offsets of the aperture block in the dispersion
direction, for all exposures with non-zero YAPER values, the LINE2 lamp at a LOW
current setting was used with 8 s lamp flashes with 20 s spacing in place of the
standard LINE1 at the MEDIUM setting. This would reduce the expected count rate,
even in the event of a substantial light leak, to safe levels. These lamp flashes are too
faint to use for ordinary wavelength calibration. For those exposures, the offsets
found for YAPER=0 can be used to define the wavelength scale zero point adjustment
for spectral extraction. Since HST and the COS detector and optics are being held in a
fixed position with respect to the external target, there is no need to adjust this
wavelength zero-point for the motion of the aperture block. In any case, as our
analysis will be done using XCORR and YCORR coordinates, the zero point of the
wavelength scale is unimportant for our purposes. Those visit 2 exposures done with
YAPER=0 (i.e., no dispersion direction offset) used the standard lamp LINE1 and
medium current setting, and, as in visit 1, the flash duration was set to 36 s.
As we will discuss below, the overlap of the G160M spectral location with the gain
sagged regions at the original lifetime position was the critical factor that controlled
the minimum distance between the old and the new position, so a single G160M
spectrum at +3.5” was included in visit 2 as a final verification of this overlap at the
+3.5” lifetime position.
3. Results
3.1 Placement of the New Lifetime Position
Since the COS FUV cross-dispersion profiles can be asymmetric, it is necessary to
clearly define the algorithm used for measuring the profile center. Unless otherwise
noted, we will use the YCORR locations of events that have been corrected for thermal
and geometric distortion and for Y-walk effects, but not for Doppler effects, (which
are small and apply only in the dispersion direction), or corrections determined from
the wavelength calibration exposure. For the external spectrum profiles, we will adopt
a flux weighted centroid as a measure of the overall position in the cross-dispersion.
YC = Σ CYYCORR/ΣCY,
where CY is the total number of counts between YCORR−1/2 pixel and YCORR+1/2 pixel,
and the sum is taken over the region that includes significant contribution from the
spectrum of the external source. As a measure of the upper and lower bounds of a
cross-dispersion profile, we will use the Y positions at the 5th (Y05) and 95th (Y95)
percentile of the cumulative profile in the cross-dispersion direction. The wavecal
spectrum is fainter and has a more symmetrical cross-dispersion than the external
spectrum, so we will define the Y location of the wavecal spectrum, WC, as the center
of the best fit Gaussian.
Instrument Science Report COS 2013-03(v1) Page 10 These position measures are given in Table 3 for those FUV spectral images from this
program where the aperture was centered on the target. This includes all exposures
from visit 1 plus the exposures from visit 2 taken with the aperture block at the
XAPER=-73 and YAPER=0 position. We also include sensitivity monitor
observations from March 2010 taken at the CENWAVE values used in visits 1 and 2.
For each of the exposures from 12795, we also list the shift relative to the location in
the March 2010 reference image for both the centroid of the external spectrum, δYC =
YC(new) – YC(reference) and wavecal location, δWC = YC(new) – YC(reference).
These numbers show that relative to the 2010 exposures, the +2.94” shift moved the
external spectra by about 34 YCORR pixels on the detector, the +3.5” shift by ~ 41
pixels, and +3.78” by ~ 43 pixels. In each case the wavecal spectrum moved by 1 to 3
pixels less than the external spectrum. This discrepancy may be due to errors in the
walk correction, inaccuracies in the geometric distortion, or differences in the
illumination angle of lamp vs. external light. Any miscentering of the external target
in the aperture would also contribute to the offset between the target and wavecal
spectra.
Table 3 Cross-dispersion locations of external spectra (YC) and wavelength calibration spectra
(WC) on the detector. For 12795 visit 2 we only include here exposures taken with the aperture
centered on the target. Grating Seg CENWAVE POSTARG2 YC Y05 Y95 WC (“) (pixels) (pixels) (pixels) (pixels) Reference exposures from program 11897, March 2010, FUVA, target WD0947+0857 G140L A 1230 0 493.501 485.924 499.096 595.412 G130M A 1309 0 485.527 478.589 491.779 587.443 G130M A 1291 0 486.192 477.664 493.898 587.994 G160M A 1600 0 478.788 472.964 483.306 581.381 V01 12795, FUVA, target WD0308−565 G130M A 1291 0 484.963 476.766 492.556 587.823 G130M A 1291 2.94 520.537 512.367 528.187 620.795 G160M A 1600 2.94 512.681 507.766 517.162 613.201 G140L A 1280 2.94 527.309 520.651 532.515 628.005 G130M A 1291 3.78 529.615 521.866 536.898 629.241 G160M A 1600 3.78 521.803 516.733 526.433 622.069 G140L A 1280 3.78 535.846 529.433 541.643 636.424 V02 12795, FUVA, target WD0947+0857 G130M A 1309 0 483.805 477.129 490.133 587.029 G130M A 1309 3.50 526.069 519.348 532.034 625.624 G130M A 1309 3.50 526.530 519.693 532.582 626.099 G160M A 1600 3.50 519.505 514.348 524.487 619.641 Reference exposures from program 11897, March 2010, FUVB, target WD0947+0857 G140L B 1230 0 551.739 540.292 561.489 ... G130M B 1309 0 545.652 538.032 551.908 647.921 G130M B 1291 0 546.239 536.949 553.951 648.270 G160M B 1600 0 537.436 532.433 541.868 640.823 V01 12795, FUVB, target WD0308−565 G130M B 1291 0 546.171 536.354 554.508 647.928 G130M B 1291 2.94 580.856 571.458 589.427 681.176 G160M B 1600 2.94 571.922 566.783 576.101 672.920 G140L B 1280 2.94 585.438 575.396 595.360 ... G130M B 1291 3.78 589.552 581.281 597.435 689.780 G160M B 1600 3.78 581.072 576.094 585.162 681.829 G140L B 1280 3.78 594.211 584.473 605.062 ... V02 12795, FUVB, target WD0947+0857 G130M B 1309 0 544.977 537.315 551.380 647.492 G130M B 1309 3.50 585.952 579.046 591.970 686.565 G130M B 1309 3.50 586.251 579.205 592.497 686.910 G160M B 1600 3.50 578.909 573.957 583.002 679.537 Instrument Science Report COS 2013-03(v1) Page 11 Exposure
Name δYC
(pixels) δWC (pixels) LBB908N5Q LBB908NEQ LBB908NGQ LBB9X1MSQ 0 0 0 0 0 0 0 0 LBX401H9Q LBX401HBQ LBX401HDQ LBX401HFQ LBX401HHQ LBX401HJQ LBX401HLQ −1.23 34.35 33.89 33.81 43.42 43.01 42.35 −0.17 32.80 31.82 32.59 41.24 40.69 41.01 LBX402BGQ LBX402BUQ LBX402CTQ LBX402D7Q −1.72 40.54 41.00 40.72 −0.41 38.18 38.66 38.26 LBB908N5Q LBB908NEQ LBB908NGQ LBB9X1MSQ 0 0 0 0 ... 0 0 0 LBX401H9Q LBX401HBQ LBX401HDQ LBX401HFQ LBX401HHQ LBX401HJQ LBX401HLQ −0.07 34.62 34.49 33.70 43.31 43.64 42.47 −0.34 32.91 32.10 ... 41.51 41.01 ... LBX402BGQ LBX402BUQ LBX402CTQ LBX402D7Q −0.67 40.33 40.60 41.47 −0.43 38.64 38.99 38.71
Collapsed cross-dispersion profiles for the +2.94” and +3.78” shifts of visit 1, and the
+3.5” shift used in visit 2 are compared with the equivalent reference exposures from
March 2010 in Figure 3, while in Figure 4 we show the modal gain values as a
function of the XCORR position for G130M and G160M exposures from visit 1. The
region over which modal gain shown in these plots is measured is determined by
sorting the events within ±50 YCORR pixels of each spectrum’s center by their
XCORR location and dividing these events into bins along the dispersion direction so
that each XCORR bin includes the same number of events. The pulse height
distribution within each bin was then fit with a Gaussian function; the gain at the peak
of the Gaussian fit is taken as the measure of the modal gain.
Figure 3: The collapsed cross-dispersion profiles at each of the offset positions used in 12795
(solid lines) are compared with cross-dispersion profiles at the original lifetime position (broken
lines). Line colors have the same meaning as in Figure 2, the orange lines show the G160M 1600
profile, blue lines show G130M, and green G140L 1280. (No G140L exposures were taken at the
+3.5” position used for visit 2 of 12795). The black line shows the cross-dispersion profile of the
Instrument Science Report COS 2013-03(v1) Page 12 core of the G130M geocoronal Lyα line. Note that the G130M observations in the middle panels
above (+3.5” and the comparison) were taken during visit 2 and used the 1309 CENWAVE
position, while those in the other panels show observations taken during visit 1 that used the 1291
CENWAVE setting for the G130M exposures.
Figure 4: Modal gain of the events observed in the G130M 1291, (upper panels), and G160M
1600 (lower panels) exposures from visit 1 of program 12795 are shown as a function of XCORR
pixel, as is the gain for the comparison exposures from March 2010. Note that the gain of the
G160M spectra on the B segment, (green curve), taken at the +2.94” spectrum are noticeably
impacted at several locations between pixels 8000 and 10000 by the upper edge of the G130M
gain-sag holes from the original position. Dips at the location of the G140L gain sag holes on the
FUVA are also visible in the G160M spectrum at the +2.94” position (green curve) at two
locations near pixels 2000-2500. The very deep feature on FUVA near XCORR = 8000
corresponds to a low gain region that has been present since before launch. This corresponds to
the dark line seen in the middle of the upper part of the upper panel in Figure 1 at the same
XCORR location.
It is apparent that for the +2.94” shift of about 34 pixels, the G160M spectrum is
significantly impacted by the gain sag holes of the original lifetime position on both
the FUVA and FUVB segments. Figure 5 shows a more detailed view of the G160M
modal gain vs. pixel position in the dispersion direction for the spectra taken at the
+2.94” and +3.78” positions used in visit 1, as well as at the +3.50” location used for
visit 2. In Figure 6, we show the locations of these spectra overlaid on images of gain
maps constructed from the 12676 data, and it is again clear that at the +2.94” position
the wings of the old gain sag holes substantially impact the modal gain of the
extracted spectrum. At the +3.5” position, there is still some measurable impact on the
G160M spectra, but only by about 1 PHA bin or less over very localized regions.
Instrument Science Report COS 2013-03(v1) Page 13 Figure 5: Details of the modal gain vs. position along the dispersion direction for the G160M
exposures taken in this program. Note that most, but not quite all, of the impact of the preexisting Lyα holes on the modal gain seen at the +2.94” position, (green curves), can be avoided
by moving to the +3.5” position, (blue curves).
Instrument Science Report COS 2013-03(v1) Page 14 Figure 6: Details of the FUVA (upper) and B (lower) gain maps near the Lyα holes are compared
with the locations spanned by G160M spectra at a variety of offset positions. The solid lines show
the YC central position for POSTARGs of 2.94”, 3.5”, and 3.78” (green, blue, and orange solid
lines respectively), while the short dashed lines show the Y05 positions and the long dashed lines
the Y95 positions that mark the upper and lower bounds containing 90% of the total encircled
energy. This image uses a linear brightness scale running from PHA=0 (black) to PHA=18
(white). Note that the lower edge of the +2.94” POSTARG region is significantly impacted by the
gain sag holes on both FUVA and FUVB.
Initial estimates based on the deuterium lamp flats had suggested that a +35 pixel shift
was adequate to meet the lifetime requirements at the new position. These estimates
had been done by shifting 20 pixel high extraction boxes to a variety of positions
relative to the default spectral positions tabulated in the XTRACTAB reference file.
For G160M on FUVB this tabulated location is about 2 pixels higher on the detector
than we found for the actual location in our March 2010 reference exposures. For a 35
Instrument Science Report COS 2013-03(v1) Page 15 pixel shift, the test extraction region on FUVB would still have spanned YCORR
pixels values from about 564 through 584, which extends below the Y05 lower edge
value of 566.783 found for the G160M 1600 position with a +2.94” POSTARG
(green dashed line in lower panel of Figure 6). So the overlap of the older gain sag
region with the new G160M location at a shift of +35 pixels was properly treated in
the analysis of Massa et al. (2013).
However, this initial analysis had been done assuming that operations would begin at
the 169/167 HV levels for FUVA and FUVB respectively, and that it was only
necessary to keep the modal gain above a value of 3 as long as possible. Subsequently
it was decided that starting at lower initial voltage levels would give substantial
lifetime benefits, and also that noticeable effects on data quality can begin once the
modal gain drops below 5. This made smaller starting gain values less acceptable than
had been initially assumed. After review of the data from visit 1 and detailed
comparison with the gain maps from program 12676, it was decided that a position of
+3.5” (a shift of about 41 pixels in the cross-dispersion direction) provided the best
compromise between avoidance of the gain sag holes while still maximizing spectral
resolution and preserving fresh area near the top of the detector for a subsequent
lifetime position. This position was then adopted as the baseline for visit 2.
The measurements listed in Table 3 can also be used to estimate values for the crossdispersion plate scale. Several measures of this are compared in Table 4. From the
data within visit 1, we can derive a measure of the local plate scale by comparing the
change in the vertical position, δYC, between the two POSTARG2 positions used for
each grating near the 2nd lifetime position. We can also compare these positions with
the positions measured for the reference exposures to obtain an estimate of the plate
scale between the 2nd and 1st lifetime positions. The latter values give significantly
smaller pixel sizes than the former, suggesting that the current geometric distortion
solution does not correct pixels to a uniform spacing on the sky, although changes in
the optical distortion of each grating’s PSF as a function of position will also play a
role.
Table 4: Values for the COS FUV cross-dispersion plate scale are compared. Results given here
are derived from geometrically, thermally, and y-walk corrected positions.
YCORR Plate scale results from this program
Grating/segment Local scale
2.94 – Ref
3.50 – Ref
3.78 – Ref
(3.78 – 2.94)
Visit 1
Visit 2
Visit 1
(“/pixel)
(“/pixel)
(“/pixel)
(“/pixel)
G130M/FUVA
10.81
11.68
11.65
11.49
G160M/FUVA
10.93
11.51
11.63
11.38
G140L/FUVA
10.16
11.50
…
11.20
G130M/FUVB
10.35
11.77
11.56
11.46
G160M/FUVB
10.89
11.73
11.85
11.54
G140L/FUVB
10.44
11.46
…
11.23
Instrument Science Report COS 2013-03(v1) Page 16 Plate scale results from ISR 2010-09 at 1st Lifetime Position
Grating
Published values
Within ±0.5”
(“/pixel)
(“/pixel)
G130M
9.72
11.88
G160M
11.33
11.44
G140L
10.97
11.95
We can also compare with the plate scale measures of Ghavamian et al (2010) for the
original lifetime position. His values differ significantly, especially for the G130M.
These were, however, based on measurements of individual emission lines from an
external source moved using POSTARG commands while the PSA aperture was held
at a fixed position. As the external source approaches the edge of the aperture,
vignetting from the aperture edge will cut off one wing of the point spread function,
leading to a reduced apparent motion of the target on the detector. If we instead
consider only the Y position values from Ghavamian et al. (2010) for positions within
0.5” of the aperture center, results are closer to the estimates from our new data.
The most appropriate plate scale values to use will depend on the application. For the
target acquisition parameters used for PEAKXD exposures, the centroid of the
position needs to be measured in uncorrected RAW coordinates using the same
algorithm that the flight software uses, and the effects of the edge vignetting on the
effective plate scale need to be considered. This is discussed in greater detail by
Penton et al. (2013). Observers interpreting exposures of multiple or extended objects
within the aperture will also need to evaluate how such edge effects will bias their
interpretations. For planning the location of future lifetime positions, the data taken
during the lifetime exploratory program 12678, “COS/FUV Characterization of
Optical Effects”, where the aperture was moved to follow the target, will remain the
best resource.
3.2 Aperture Centering at the New Lifetime Position
As described in section 2.2, for determining the positioning of the source in the PSA
at the new lifetime position, the pointing of HST was held fixed while the aperture
block was scanned across the detector. The relative throughput was then estimated by
summing all counts over the region of the detector containing counts from the source
while taking care to keep the geocoronal Lyα photons and lamp light masked out at
all positions. This was done by using the subarray [1135:1526,45:590] for XCORR
and YCORR values on FUVA, and the two subarrays [800:6980,500:630] and
[7285:15045,500:630] on FUVB. These choices exclude the lamp light at all positions
in the cross-dispersion aperture scans, and the geocoronal Lyα light from the G130M
1309 CENWAVE setting used here at all positions of the along-dispersion scan. The
number of counts measured for each of the exposures in these subarrays are listed in
the last two columns of Table 2, and their relative values, normalized to the counts
measured at the central position, are plotted in Figure 7 and Figure 8. Comparison
with dark exposures taken the same month as these observations shows that detector
background contributions to these subarrays during a 50 s observation should be
between 230 to 415 counts on FUVA, and 160 to 360 counts on FUVB. As this is
Instrument Science Report COS 2013-03(v1) Page 17 never more than about 1% of the observed counts from the external source, this small
background contribution has been neglected.
Figure 7: The relative throughput of the COS PSA aperture as a function of the aperture block
offset in the cross-dispersion direction is shown for observations done as part of visit 2 of
program 12795. The dashed line shows measures for FUVA, and the solid for FUVB. Positions
marking the nominal center, +/-0.4”, and +/-1.25” are marked with vertical dotted lines assuming
a plate scale of 21 XAPER steps per arc-second.
Figure 8: The equivalent of Figure 7 for the aperture block scan in the direction along the
spectral dispersion.
The profile in the cross-dispersion direction appears well centered on the nominal −73
XAPER position relative to the original position (Figure 7). However, the best
Instrument Science Report COS 2013-03(v1) Page 18 centering found by the dispersion direction scan appears to be significantly displaced
in the positive YAPER direction (Figure 8). The definition of the “best” center for an
asymmetric profile is influenced by a number of factors. In addition to simply
maximizing the throughout, it is also desirable to avoid asymmetric clipping of the
line-spread function, and to also ensure consistency between the pointing resulting
from NUV imaging acquisitions, such as was done for this visit, with the results of
dispersed light acquisitions. COS PEAKD acquisitions in particular function by
measuring the flux at a variety of dwell points to find the aperture center, and this is
more sensitive to throughput variations near the aperture edges than to the smaller
throughput variations near the aperture center.
To quantify the centering, we will first calculate the bisectors of the curves shown in
Figure 7 and Figure 8, (that is the mid-point between points of the same height on
opposite sides of the curve). Values between the measured points were linearly
interpolated. We then compare the position of this bisector to the half-width of the
curve at that height (Figure 9). For the cross-dispersion scan, the offsets are mostly
less than 0.02”, show little trend as a function of the profile width, and appear to be of
opposite sign for the different segments. Since 1 motor step of the aperture block
corresponds to about 0.05”, this centering is about as good as could have been
expected. For the scan along the dispersion direction, the results for the two segments
are in close agreement, but there appears to be a somewhat larger offset determined
from the center of the profile than from the wings. Recommended dwell point
spacing for COS PEAKD acquisitions are 1.3” when 3 positions are used and 0.9” for
the preferred 5-position sequence. To allow the centering produced by COS FUV
PEAKXD observations at the new lifetime position to be consistent with the centering
produced by COS NUV acquisitions, an offset of 0.05” was adopted as our best
determination of the aperture centering in the dispersion direction.
Figure 9: Offsets of bisectors of the aperture throughput curves as a function of the half-width of
the throughput curves at the same height as the bisector points are shown for each segment for
the aperture scan perpendicular to the dispersion axis (left) and parallel (right). This shows the
offset in the mid-point found for different sample spacings.
Since the desired offset in the dispersion direction is just about 1 aperture motor step,
the centering at the new lifetime position could have been adjusted by moving the
aperture block. However, this would have required a mechanism motion in the
dispersion direction when switching between NUV and FUV exposures. Currently all
COS aperture positions used operationally differ only in the cross-dispersion direction
and movements in the dispersion direction are only needed for special engineering
Instrument Science Report COS 2013-03(v1) Page 19 tests. To avoid the need for such moves and the associated overheads during routine
operations, it was decided instead to correct for the dispersion offset in the SIAF
definition of the new lifetime position, i.e., by slightly repointing the telescope to
center the target in the dispersion direction.
So for the second COS FUV lifetime position, the FUV aperture positions need to be
shifted from their original position by the equivalent of a −0.05” POSTARG1 in the
dispersion direction and by a +3.5” POSTARG2 in the cross-dispersion direction.
This changes the FUV aperture positions in the V2, V3 coordinate system used in the
Science Instrument Aperture File (SIAF) from the original values of 232.6560,
−237.4450 to new values of +235.0810, −234.9400. The NUV aperture continues to
use the original position.
Note that due to an error in the original analysis, the SIAF update was initially
delivered with the wrong sign for the shift in the dispersion direction (i.e., +0.05,
+3.50 instead of −0.05, +3.50). Fortunately, as it was recognized that sign errors of
this kind can be easy to make and difficult to track down, PEAKD exposure
sequences included in visit 2 of FENA4 (program 12797) were examined to verify the
aperture centering in the dispersion direction. These checks revealed the sign error
and once the source of the error was properly understood, a corrected SIAF file was
delivered and then tested in visit 5 of FENA4 prior to the start of routine operations at
the new lifetime position. In this report we have presented only the corrected analysis.
In Table 5 and Table 6 we list the relative throughput as a function of the equivalent
POSTARG that would move the target towards the same edge of the aperture as did
the ALIGN/APER command. This has the opposite sign from the POSTARG that
would follow the aperture motion. For the dispersion direction case, we include the
0.05” offset found above. Observers may find these tables useful for estimating
throughput losses for mis-centered targets.
Table 5: Fractional throughput as a function of target offset perpendicular to the dispersion
direction. Note that the sign convention given here for the POSTARG value is the reverse of that
given in Table 2, as there we were specifying the POSTARG needed to follow a moving aperture,
while here we are giving the POSTARG necessary to put the target off-center in a fixed aperture.
POSTARG2 (“)
-1.381
-1.095
-0.857
-0.667
-0.476
-0.286
+0.000
+0.286
+0.476
+0.667
+0.857
+1.095
+1.381
XAPER
-29
-23
-18
-14
-10
-6
0
+6
+10
+14
+18
+23
+29
Relative throughput
FUVA
FUVB
0.275
0.285
0.615
0.613
0.775
0.766
0.879
0.872
0.969
0.962
0.991
0.988
1.000
1.000
0.991
0.997
0.959
0.962
0.854
0.863
0.749
0.767
0.600
0.624
0.256
0.313
Instrument Science Report COS 2013-03(v1) Page 20 Table 6: Fractional throughput as a function of target offset along the dispersion direction.
POSTARG1 (“)
-1.331
-1.045
-0.807
-0.617
-0.426
-0.236
+0.050
+0.336
+0.526
+0.717
+0.907
+1.145
+1.431
YAPER
+29
+23
+18
+14
+10
+6
0
-6
-10
-14
-18
-23
-29
Relative Throughput
FUVA
FUVB
0.400
0.398
0.687
0.687
0.811
0.811
0.901
0.902
0.978
0.974
0.996
0.987
1.000
1.000
0.985
0.982
0.924
0.928
0.848
0.846
0.765
0.768
0.618
0.621
0.307
0.308
In Figure 10 we compare our results from the cross-dispersion scan at the new
position with those of the SMOV program 11490 at the original position. The SMOV
program used rather different procedures for measuring the variation in aperture
throughput as a function of target centering. The aperture centering was adjusted
using POSTARGs to adjust the spacecraft pointing rather than by moving the aperture
block, and the target used was an emission line source, which concentrated most of
the detector counts onto a few limited areas of the detector. These differences made
the SMOV program more vulnerable to localized differences in detector throughput
and flat fielding and so it is not surprising that the measured throughput curves from
11490 are not as smooth or as symmetrical as those found here. This is particularly
noticeable for the central +/−0.5”.
Figure 10: Results of the cross-dispersion scan done at the new +3.5” lifetime position (solid lines)
in 12795 are compared with a G130M scan of symbiotic star LIN 139 done in 2009 at the original
lifetime position as part of program 11490 (dashed lines) and reported in Ghavaniam et al. 2010.
This comparison assumes 21 XAPER motor steps are equivalent to a 1” POSTARG. Results for
FUVA are shown on the left and FUVB on the right. The vertical dotted lines mark the +/1.25”radius of the COS PSA aperture.
4. Summary
The results presented here demonstrate that offsetting the COS FUV Lifetime
Position 2 from Lifetime Position 1 by the equivalent of a −0.05, +3.50 POSTARG
Instrument Science Report COS 2013-03(v1) Page 21 move while shifting the aperture block by −73 steps in the XAPER direction moves
the position of FUV spectra on the detector up by about 41 YCORR pixels in the
cross-dispersion direction while keeping the target well centered in the aperture. This
yields adequate separation from the gain-sagged regions at the original position, while
staying close enough to that position to keep the predicted degradation in resolution
within the desired margin of 10 to 15%.
Acknowledgements
Change History for COS ISR 2013-03
Version 1: 03 April 2013 – Original Document
References
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Instrument Science Report COS 2013-03(v1) Page 22