SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
Instrument Science Report STIS 2015-01(v1)
Improved Photometry for G750L
R.C. Bohlin1, C.R. Proffitt1
1
Space Telescope Science Institute, Baltimore, MD
08 June, 2015
ABSTRACT
The repeatability of STIS G750L spectrophotometry at the longer wavelengths is
significantly worse than that at shorter wavelengths or for the STIS G430L grating.
The cross-dispersion width increases beyond the extent of the standard seven-pixel
extraction width and leads to an increased sensitivity to variations in the width of the
spectral profile. This problem is quantified and one solution using an extraction width
of 11 pixels instead of the standard width of seven is explored in detail here. The onesigma repeatability in a 400 Å broad band centered at 9700 Å is improved from 0.9 to
0.5%, the cross-calibration agreement with ACS is improved, and the agreement of
STIS SEDs with stellar models is better. Even for a flux level as low as 1.3x10-16 erg s-1
cm-2 Å-1, the noise level is increased by only 8%, i.e. the S/N is reduced from 5.9 to 5.4;
and even in the limiting case where the signal approaches the background level, the
noise increases by only 25% for the larger width of 11.
Contents
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Introduction (page 2)
Observations (page 2)
Required Corrections for Calculating STIS Flux Distributions (page 3)
Results (page 14)
Conclusions and Recommendations (page 19)
References (page 19)
Operated by the Association of Universities for Research in Astronomy, Inc., for the
National Aeronautics and Space Administration
1. Introduction
Photometric calibration uncertainties are the dominant source of error in current type Ia
supernova dark energy studies and other forefront cosmology efforts, e.g., photoredshifts for weak lensing mass tomography. Modern surveys require a network of
calibration stars with SEDs that are accurate to ~1% in absolute flux. Precision STIS
flux distributions simultaneously determine effective temperature, log g, log z, and
reddening of stars lying within a good model atmosphere grid (e.g. Bohlin et al. 2014).
The best fitting models can be used as flux standards at wavelengths beyond the range
of HST and constitute the gold-standards with which to directly calibrate JWST and
other facilities. The precision HST photometric heritage can be extended to existing and
upcoming surveys, standardizes (spectro)photometry across observatories, and directly
addresses one of the current barriers to understanding the nature of dark energy.
The STIS low and medium dispersion spectra are extracted with default widths of 11
pixels from MAMA images and 7 pixels from CCD images (Leitherer & Bohlin 1997).
These widths (EXTRSIZE) are a compromise between better photometry with wider
extractions that collect signal from the PSF wings and better S/N and spectral purity
with smaller widths. When noise dominates the signal, wider widths add mostly noise.
However for our goal of 1% and requirement of 2% photometric precision, the sevenpixel width is insufficient at the longer wavelengths of G750L. At the expense of
somewhat lower S/N, an EXTRSIZE=11 improves the 1σ repeatability from 0.89% to
0.51% in the 9500-9900 Å G750L band for the 63 observations of the STIS monitor
standard star AGK+81° 266. Stellar SEDs with the larger extraction width agree with
theoretical model atmosphere calculations better at the crucial region below one µm,
where model atmosphere SEDs are fit to measured STIS flux distributions in order to
extend flux estimates beyond the STIS one µm wavelength limit.
2. Observations
2.1 Spectral PSF Width Changes over Time
Observations in the wide 52X2 arcsec slit of the star AGK+81° 266 are used to monitor
changes in the STIS CCD low dispersion sensitivities for G230LB, G430L, and G750L
(Stys, Bohlin, & Goudfrooij 2004). Figure 1 illustrates the change in the width of the
spectrum over the STIS lifetime. The IDL function gaussfit.pro is used to fit each of
the 1024 columns of each of the 63 AGK+81° 266 observations. Outliers are removed
with a five point median filter before computing the mean FWHM of the PSF at short
(columns 50-150) and long (columns 900-1000) wavelengths for each observation.
These means appear in Figure 1, where the short wavelength PSF is constant; but the
long wavelength PSF width grows at a rate of 0.033 px/yr for a total increase of 0.6 px
to date.
If the width increase is due to CTE losses to the trailing pixels of the readout, then
there should be increasing asymmetry over time with larger values of the PSF shape at
lower row numbers. However, there is no obvious asymmetry, which leaves the cause a
mystery. Other possible causes include instrumental deformation that could degrade the
long wavelength focus of G750L and de-lamination of the G750L replica grating,
Instrument Science Report STIS 2015-01(v1) Page 2 which is suspected as the cause of the STIS echelle grating blaze changes. The increase
in scatter after SM4 in 2009 May also argues for an environmental effect, instead of
CTE, which monotonically degrades. For example, degradation of the external thermal
blankets induces stronger thermal transients in the instrumentation.
Figure 1 The STIS PSF FWHM at the short wavelengths (average over columns 50150) below and at long wavelengths (columns 900-1000) above. The approximate
wavelengths for these two regions are 5519-6007 and 9668-10156 Å. The larger widths
and increasing scatter at the long wavelengths require a larger extraction height to
maintain photometric reproducibility. Dashed lines are linear fits to the data points that
are connected by solid lines.
2.2 Extraction Widths
The changes in measured sensitivity vs. time for several wavelength bins are fit with
linear segments for each grating mode; and these corrections are evaluated at the date
of observation and interpolated in wavelength. An example of the longest wavelength
9700 Å bin (9500-9900 Å) of G750L appears in Figure 2 for EXTRSIZE=7, where
three dashed line segments are the piece-wise linear fits to the data points. The rms
1σ scatter of the data points is 0.89% around the fitted dashed lines. Thus, the expected
1σ difference in the residuals between any two points is 1.26%; but the ~4% difference
between the data points for oc4il3060 at 2013.47 and oceil1060 at 2013.91 is more
than 3σ. Figure 3 is the same as Figure 2, except for a wider EXTRSIZE of 11. The
overall rms scatter is reduced to 0.51%, and few residuals exceed 1%. The rms for
Instrument Science Report STIS 2015-01(v1) Page 3 three separate time periods in the 9700 Å bin are 0.43, 0.54, and 0.56%, respectively
for side 1 operations through 2001 May 16, side 2 until its failure in 2004 August, and
for post SM4 after 2009 May. Temperature control was lost with the side 1 electronics
failure, which causes slightly poorer repeatability after 2001 May. When using the
narrower EXTRSIZE of 7, these values become 0.99, 0.79, and 0.89% for the same
time periods. Perhaps, some of the earliest observations were poorly focused.
The reduced noise with EXTRSIZE=11 makes the choice of the piece-wise linear line
segments more obvious; and a new break at 1999.3 has been added for both G430L and
G750. The break in slope at 1993.3 is even more obvious in the plots for G430L with
EXTRSIZE=11, which correspond to Figure 3 and will be discussed in a future ISR.
The ratio of oc4il3060 to oceil1060 as a function of wavelength is illustrated in Figure
4 for three EXTRSIZE values, where the EXTRSIZE=11 ratio is unity within ~1%.
With a separation of only five months, any change in the CTE correction is negligible
compared to the differences between oc4il3060 and oceil1060. Figure 5 compares the
rms scatter about the fits for EXTRSIZEs of 7 and 11 for all 11 wavelength bins of 400
Å width. The ratio for a height of 11 in Figure 4 dips below unity by ~0.6% in the
6500-7500 Å range, which is ~2σ for the difference between these two extreme
spectra.
Table 1 lists the rms scatter of the linear fits vs. time for the 9700 Å bin and four
different EXTRSIZE choices. The corresponding noise in the spectra of two faint WDs
is also tabulated, as discussed in Section 4.4. A new EXTRSIZE=11 is a suitable
compromise choice for the G750L data reduction pipeline. The optimum extraction
height depends on specific scientific goals and data quality. As long as the background
noise is negligible, the S/N improves with larger extraction heights that extract more
signal. Apparently for AGK+81° 266, the height of 15 is picking up more noise than
signal in comparison to EXTRSIZE=11; but the extreme value of 0.51% for the 9700 Å
point in Figure 5 suggest that EXTRSIZE=13 might reduce the rms scatter further for
that one bin. However, EXTRSIZE=11 is sufficient to bring the repeatability within
our 1% goal for photometric precision.
Instrument Science Report STIS 2015-01(v1) Page 4 Figure 2. The change of sensitivity with time for the 9500-9900 Å band of G750L. The diamonds are
the average signal in the band for EXTRSIZE=7 and the three dashed lines are linear piece-wise fits to
the data, where everything is normalized by the fit value at time zero, i.e. 1997.38. STIS failed in 2004
Aug but was repaired during Servicing Mission 4 in 2009 May.
Instrument Science Report STIS 2015-01(v1) Page 5 Figure 3. As in Figure 2 but for EXTRSIZE=11. Notice the reduction of scatter about the fitted
dashed lines. At 2015.0, the correction is 0.935 vs. 0.922 for EXTRSIZE=7 in Figure 2.
Instrument Science Report STIS 2015-01(v1) Page 6 Table 1. rms scatter for 62 AGK+81° 266 Obs. at 9700Å and S/N for 8720-9390Å.
EXTRSIZE
7
9
11
15
rms (%)
0.89
0.61
0.51
0.58
S/N
LDS749B
60
61
62
62
WD1657
5.9
5.7
5.4
4.9
In principle, an optimal extraction (Horne 1986) should be superior to a fixed width
extraction, especially for faint objects. The Optimal Spectrum Extraction Package for
IDL of J. Harrington1 was tried but failed at Step 5 in trying to fit the normalized
spectrum as a function of wavelength with a polynomial. Apparently, the STIS CCD
has manufacturing geometric distortions at the pixel level, which makes smooth fits
impossible.
3. Required Corrections for Calculating STIS Flux Distributions
In addition to sensitivity changes as a function of time, STIS CDD data require a
correction for temperature and for charge transfer losses (CTE) in the aging CCD
(Bohlin & Goudfrooij 2003, Goudfrooij & Bohlin 2006, Goudfrooij et al. 2006). The
correlation of G750L signal (electrons s-1) vs. temperature is 0.12 ±0.02 % C-1 for
EXTRSIZE=11, instead of the 0.06 ±0.02 % C-1 for EXTRSIZE=7. As these correction
coefficients differ by <3σ, the difference could be just statistical. For the best
precision, the CTE correction should also be updated. However, the STIS signal is not
much larger in a EXTRSIZE=11 extraction, as shown in Figure 6; so that the existing
CTE correction algorithm for EXTRSIZE=7 should not have a large error. An
improved pixel-based CTE correction is under development in order to correct entire
STIS CCD images (Lockwood et al. 2013) using the technique developed for ACS
(Anderson & Bedin 2010).
To compute absolute fluxes (erg s-1 cm-2 Å-1), the correction for EXTRSIZE=11 vs.
EXTRSIZE=7 is done via the *pct.fits reference files (Bohlin 1998), which account for
the systematic difference in signal so that the standard absolute sensitivity function for
EXTRSIZE=7 can be applied. The pct corrections are averages of AGK+81° 266
observations in the 52x2 arcsec slit and should be updated, because many more
observations are available. For example, new G750L 52x2 pct corrections are
illustrated in Figure 7 and may be compared to the original figure 8 of Bohlin (1998),
where 12 wavelength nodes were used, instead of the 10 for the new reference file.
Figure 8 compares the original and new corrections for EXTRSIZE=11. While the
original ISR shows 12 wavelength nodes, that fit was revised to 8 nodes in 1999; and
the new G750L corrections utilize a ten-node fit. However, some of the EXTRSIZE
ratios show a systematic trend as a function of time, so that a more straightforward
http://physics.ucf.edu/~jh/ast/software/optspecextr-0.3.1/doc/
1
Instrument Science Report STIS 2015-01(v1) Page 7 approach is adopted to follow the same procedure as used for calculating fluxes for
EXTRSIZE=7, i.e. the EXTRSIZE=11 sensitivity function is the basis of the G750L
absolute flux calibration.
Figure 9 shows examples of average stellar flux changes for the switch from
EXTRSIZE=7 to 11. While Figures 4 and 6 illustrate an extreme difference in flux of
almost 4% for the EXTRSIZE of 7 vs. 11 for a pair of individual spectra, the change is
always within 2.5% of unity for any particular CALSPEC2 standard star. In addition to
changes of order 1% due random variation of the PSF width, there is a systematic flux
decrease in later observations (e.g. HD205905 at 2011.4) relative to earlier
observations (e.g. HD93521 at 1998.2) because of the larger time corrections for
EXTRSIZE=7 vs. 11, as illustrated for the 9700 Å region in Figures 2-3. For example,
Figures 2-3 show a 1/0.925 correction for EXTRSIZE=7 vs. 1/0.937 for
EXTRSIZE=11 at 2014.0 and 9700 Å. The second and third stars in time sequence are
BD+17°4708 at 2002.9 and Vega at 2003.6. The contribution of the new temperature
correction coefficients to flux changes is generally minor, e.g. <0.1% for HD205905 in
Figure 9. An explanation for the more significant changes illustrated in Figure 9 is the
difference in extracted signal relative to the average.
For example, Figure 6 shows a range for the EXTRSIZE=11/7 ratio of >3% at some
wavelengths. Thus, the changes shown in Figure 9 can be envisioned mainly as a
systematic decrease of ~1% at later times superposed on random fluctuations of ~1%.
For the heavily saturated observations of Vega and Sirius (Bohlin & Gilliland 2004,
Bohlin 2014) that require large EXTRSIZEs to collect all the signal, the special pct
calibration files are updated and utilized to convert the extracted signal to values
appropriate for application of the standard sensitivity functions.
2
http://www.stsci.edu/hst/observatory/crds/calspec.html
Instrument Science Report STIS 2015-01(v1) Page 8 Figure 4. The ratio in 400 Å bins of two 432s exposures of AGK+81° 266: oc4il3060 obtained
2013Jun20 and oceil1060 observed 2013Nov27. The pairs of spectra are extracted with EXTRSIZE=7
(dash), =9 (dots), and =11 (solid).
Instrument Science Report STIS 2015-01(v1) Page 9 Figure 5. The rms scatter about the fits for the time changes in the eleven 400 Å bands of G750L
with EXTRSIZE=7 (black) and =11 (red). At the longer wavelengths, the larger EXTRSIZE
produces better photometry with less scatter, because a larger and more stable fraction of the signal
lies within the 11 pixel width.
Figure 6. The increase in extracted signal for EXTRSIZE=11 vs. the standard 7 pixel
width for a wider spectrum (oceil1060 dots) and a better focused narrower case
(oc4il3060 solid).
Instrument Science Report STIS 2015-01(v1) Page 10 Figure 7. The G750L pct corrections for different EXTRSIZEs from 3 to 600 relative
to unity for a EXTRSIZE=7 and the 52x2 arcsec slit. The error bars at many of the
spline nodes (diamonds) are smaller than the symbol size. The curve for
EXTRSIZE=200 pixels is the dotted line, while the highest correction for
EXTRSIZE=600 represents the infinite width required for the diffuse object specific
intensity absolute calibration.
Instrument Science Report STIS 2015-01(v1) Page 11 Figure 8. The pct correction for EXTRSIZE=11 relative to EXTRSIZE=7 for G750L.
The dotted line is the old locus, while the solid line is the newly revised curve.
Instrument Science Report STIS 2015-01(v1) Page 12 Figure 9. Change in flux for the new EXTRSIZE=11 calibration relative to
EXTRSIZE=7 for the largest increase (HD 93521) and largest decrease (HD 205905)
of the CALSPEC stars and also for five other important CALSPEC standard stars.
Instrument Science Report STIS 2015-01(v1) Page 13 4. Results
4.1 Primary WD Residuals
A useful check on the use of EXTRSIZE=11 is to compare the flux residuals of the
primary calibrators for the two different extraction heights. The modeled standard star
flux distributions for the three primary WDs (G191B2B, GD153, and GD71) define the
HST absolute flux scale (Bohlin, et al. 2014). For the 52X2 arcsec slit of STIS, the
average response for 12, 14, and 17 G750L observations of G191B2B, GD153, and
GD71, respectively, define the STIS low dispersion sensitivities, when each star is
weighted equally. After applying the sensitivities to compute fluxes, the average STIS
flux is compared to the defining model flux SED; and the residuals appear in Figures
10-11 for EXTRSIZE=7 and 11, respectively. The worst error in the three demarcated
bands is a positive offset of 0.7% for G191B2B in the 8688-10015 Å band for
EXTRSIZE=7 in Figure 10, while this residual declines to 0.3% for EXTRSIZE=11 in
Figure 11. For the same band in GD71, there is a residual in Figure 11 of -0.3%. Such
low residuals increase confidence that the WD models of Bohlin et al. (2014) are
correct and are a necessary (but not sufficient) requirement for achieving the goal of
STIS SEDs with sub-percent precision.
4.2 Model Fits for O Stars
Bohlin et al. (2014) tabulate preliminary results for model fits to STIS SEDs with a
chi-squared minimization technique. However, one flaw in those preliminary results is
poor fits at the longer STIS wavelengths, especially for the hottest stars. For the three
hottest OB stars, the chi-square residuals for the fits over the 1700-10000 Å range are
reduced by about a factor of 2 with the STIS EXTRSIZE=11 SEDs.
Instrument Science Report STIS 2015-01(v1) Page 14 Figure 10. Residuals for a complete STIS flux calibration using the current default of
EXTRSIZE=7. The vertical dotted lines define three regions (5764-7179, 7227-8556, and 868810015 Å) for which the average level and rms scatter appear at the bottom of each panel. G191 in the
top panel indicates G191B2B.
Instrument Science Report STIS 2015-01(v1) Page 15 Figure 11. Residuals for a complete STIS flux calibration using EXTRSIZE=11, instead of the 7
pixels used for Figure 10.
Instrument Science Report STIS 2015-01(v1) Page 16 4.3 ACS Photometry
For the three longest wavelength ACS filters, Bohlin (2012) found a >3σ discrepancy
of 1-1.6% between ACS/WFC photometry and STIS synthetic photometry for
BD+17°4708, P330E, and KF06T2, where agreement would be improved for lower
STIS fluxes. The reduction shown in Figure 9 by up to 1% in STIS flux over the
relevant wavelength region should reduce most of the discrepancies for these three
stars to <2σ.
4.4 Faint Stars
A disadvantage to using a wider extraction EXTRSIZE is extra noise from the
additional pixels included in the signal, especially when the signal is near the
background level. The larger width increases the enclosed detector background by
57%, which for a vanishingly faint target would increase the noise by about 25%.
Figure 12 quantifies this extra noise for two relatively faint stars LDS749B (1.0x10-15)
and WD1657+343 (1.3x10-16), where their flux (erg s-1 cm-2 Å-1) at 9000 Å is in
parentheses. Each of the four panels show the ratio of the measured STIS flux to their
model fluxes from the CALSPEC database, where the average ratio and rms scatter per
resolution element are written in each panel for three separate bins. At the top of each
panel, the extraction height of 7 or 11 pixels is indicated Only in the two longer
wavelength bins for the fainter WD1657+343 is there any appreciable increase in the
noise level, e.g. from 17.0% for EXTRSIZE=7 to 18.4% for EXTRSIZE=11 in the bin
centered near 9100 Å, i.e. the S/N decreases from 5.9 to 5.4, as compiled in Table 1. At
shorter wavelengths the S/N is much better because of the much higher signal from this
hot star.
For the brighter LDS749B, the S/N actually improves with wider extractions because
of relatively high signal level, while the background noise dominates the small, added
signal for the faint WD1657+343.
Instrument Science Report STIS 2015-01(v1) Page 17 Figure 12. Residuals for STIS fluxes of two WDs in comparison to their models. The
bottom two panels compare the EXTRSIZE=7 and 11 residuals for LDS749B, while
the upper two panels are the same comparison for the fainter WD1657+343. Pairs of
the average ratio and the rms noise in percent are indicated for three bins delineated by
the vertical dotted lines.
Instrument Science Report STIS 2015-01(v1) Page 18 5. Conclustions and Recommendations
Given the improvements discussed above for the absolute flux accuracy and
repeatability, consideration should be given to changing the default pipeline spectral
extraction height for the G750L from 7 to 11 pixels. The EXTRSIZE would change in
the appropriate row of the 1dx EXTRACTAB table, which contains the 1D extraction
parameters; and the temperature correction would change from 0.06 to 0.12 % C-1. The
throughput curve for the G750L requires updating in the PHOTTAB file (Photometric
Conversion Table). The G750L pct corrections for the 52X2 aperture in the PCTAB,
which contains the photometric correction as a function of extraction box height and
wavelength, is significantly improved by considering a greatly expanded dataset of
AGK+81° 266, 52X2 observations. While pct corrections for other apertures might be
similarly updated by analyzing larger archival datasets, there is no known systematic
error in the existing pct tables. To complete the list of required pipeline changes, the
tds corrections for the G750L change in sensitivity vs. time must be updated in the
TDSTAB file per the discussion of Figures 2-3 above.
The accuracy requirements promised in the STIS IHB are 5% for absolute photometry
and 2% for relative photometry for the L modes. Even though the difference between
the two AGK+81° 266 observations shown in Figure 4 reaches almost 4% in violation
of our 2% requirement, the decision might be made that maximizing S/N at low flux
levels is more important than improving the photometric repeatability. For the weakest
signals, an EXTRSIZE of 3-5 would be better than 7. However, if the decision is made
to keep the default pipeline extraction width at 7 pixels, the fundamental flux
calibration should not be based on the wider extraction width of 11 pixels, because the
flux calibration and TDS corrections must be implemented as a pair. The TDS
correction for EXTRSIZE=7 partially compensates for the increasing width of G750L
spectra at the longer wavelengths. However, the improved G750L 52X2 PCTAB
should be implemented in any case.
References
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Bohlin, R. & Goudfrooij P. 2003, Instrument Science Report, STIS 03-03R,
(Baltimore:STScI)
Goudfrooij, P., & Bohlin, R. C. 2006, Instrument Science Report, STIS 2006-03,
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Instrument Science Report STIS 2015-01(v1) Page 20