Instrument Science Report ACS 2013–01
Study of the evolution of the ACS/WFC
sensitivity loss
Leonardo Úbeda and Jay Anderson
Space Telescope Science Institute
January 28, 2013
ABSTRACT
We present a study of the sensitivity loss of the ACS/WFC CCDs for one medium-band, eight
broad-band, and three narrow-band filters. This study was done using a calibration field located 6.7 arcmin West of the center of globular cluster 47 Tucanae. For pre-SM4 images, a
comparison of the sensitivity loss rates found in this research with those calculated using standard white dwarf stars by Bohlin, R. et al. (ISR ACS 2011-03) shows excellent agreement
within the uncertainties of the two methods. We found that the sensitivity losses are less than
∼0.0004 mag/year. We also have a baseline of at least three years of post-SM4 observations
of the 47 Tucanae calibration field. Our study shows that, on average, the sensitivity loss postSM4 is negligible. This is a remarkable result considering that ACS is an instrument that has
been in space for over ten years and subject to contamination.
1. Introduction
The Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) consists of two 4096 ×
2048 charge-coupled devices (CCDs), which are butted together along their long dimension to
create an effective 4096 × 4096 array. The spectral sensitivity of the WFC ranges from ∼3500Å
c 2013 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Copyright 1
to ∼11000Å. As part of the yearly calibration program, ACS/WFC observations are taken regularly of flux standard stars and star cluster fields to monitor the photometric performance of
the camera and its filters.
In this Instrument Science Report (ISR) we intend to quantify the photometric change by analyzing ten years of observations of a particular calibration field in the outskirts of globular
cluster 47 Tucanae. We explore here the entire current database of ACS observations in a wide
range of filters, using both science programs and special calibration proposals.
According to Bohlin, R. et al. (ISR ACS 2011-03) two important events in the life of ACS have
introduced discontinuities in the sensitivity evolution. The first event took place on 04 July 2006
and it is the decrease in the ACS operating temperature from −77◦ C to −81◦ C in order to lower
the number of observed hot pixels. The second event is the CCD Electronics Box Replacement
(CEB-R) that took place during Servicing Mission 4 (SM4) in May 2009 in an effort to remedy
the ACS failure in January 2007. To account for these changes, we follow the recommendation
of ISR ACS 2011-03 and separate analyses for time periods before and after the ACS failure
were performed.
In this ISR we report the results of our photometric study of the sensitivity of the ACS/WFC
using 47 Tucanae data and compare them to the sensitivity loss rates presented in ISR ACS
2011-03 using observations of white dwarf standard stars G191B2B, GD153, and GD71. We
also present new measurements of the WFC CCDs sensitivity loss using post-SM4 images.
For a complete study of the ACS High Resolution Channel (HRC) please refer to Bohlin, R. et
al. (ISR ACS 2011-03). Their study shows no obvious differences for the loss rates between
WFC and HRC.
2. Observations
The main calibration field for this study is located about 6.7 arcmin West of the center of globular cluster 47 Tucanae. This calibration field has been observed multiple times through many
ACS/WFC filters for both calibration and science purposes. We downloaded the entire database
of ACS observations from MAST in the following filters: F435W, F475W, F502N, F550M,
F555W, F606W, F625W, F658N, F660N, F775W, F814W, and F850LP.
The sensitivity of the cameras should be independent of the integration time for each exposure;
however, the longer exposures provide much better signal to noise for more stars, so we focus
here on the deeper exposures whenever possible All images with integration time shorter than 30
seconds were not included in this study. Some of the images in the Archive were not successful.
These include several images from HST proposal 9018 (PI: G. de Marchi) which show all stars
as streaks and were not suitable for this study. Most images from proposal 10771 (PI: M.
Sirianni) were also discarded. The main goal of proposal 10771 was to test the ACS/WFC
operating at three different tempertatures: −74◦ C, −77◦ C, and −80◦ C. For this study we only
2
kept the images obtained at an operating temperature of −77◦ C.
The automated calibration pipeline CALACS (version 8.0.6, 18 Jul 2012) takes care of the
basic data reduction (bias, dark, flat–field corrections). We downloaded the flat fielded images
(_FLT files), as well as the CTE corrected images (_FLC files). The _FLC files are data products
generated using the pixel–based CTE correction algorithm (PixelCTE v3.2; Anderson & Bedin,
2010).
Figure 1 shows the ACS/WFC footprints for all the F606W images that were used (exposure
time > 300 sec). The background is an STScI Digitized Sky Survey image depicting the center
of globular cluster 47 Tucanae and its Western field. The footprints are organized according to
year of observation and shown in red.
In Table 1 we list the number of F606W images used for this study (second column) organized
by year of observation, as an example. The third column lists the proposal identification numbers from the STScI observing programs. The fourth column gives the total integration time (in
units of seconds) for all deep observations.
Year
Images
(count)
2002
2003
2004
2005
2006
2009
2010
2011
2012
5
31
24
43
10
19
126
9
2
Proposal IDs
Exposure Time
(seconds)
9018 9656
9433 9648 9656
10043
10368 10569 10737 10771
10730 10737
11397 11880
11677 11887 12385 12389
12389 12730 12734
12734
3735.0
14670.0
9600.0
16045.0
3573.0
6746.0
167114.0
3417.0
678.0
Table 1: F606W observations used in this study. Shown as an example.
3. Method
3.1 Aperture Photometry
The main goal of this project was to compare accurately measured photometry of as many stars
as possible (> 1000 stars) in a given field in different epochs and determine whether there is a
trend that shows some kind of sensitivity loss. Our analysis is statistical in nature.
3
Figure 1: ACS/WFC footprints for the F606W images with exposure time > 300 seconds used for the calculation
of the sensitivity loss. The background is an STScI Digitized Sky Survey image depicting the center of globular
cluster 47 Tucanae and its Western field. North is up and East is left. The footprints are organized according to
year of observation and shown in red. The orange outline represents the location of the sources in the F606W
master initial catalog.
To acomplish this task, positions and fluxes of point sources were accurately measured with the
software img2xym_WFC.09x10 in the FORTRAN library of codes by J. Anderson. This code
includes the best available geometric distortion solution and is documented and described in
detail in Anderson, J. & King, I. (ISR ACS 2006-01). This calibration field is not excessively
crowded which means that aperture photometry should suffice for our purpose. Aperture pho4
tometry was performed on all _FLT and _FLC files for all filters. This task produces an _xym
file for each exposure containing the position and magnitude for each found source. Since astrometry is not critical here, the positions are measured with simple centroids. The fluxes are
measured using an aperture of 3.5 pixels and a sky between 8 and 12 pixels. The positions
are corrected for geometric distortion and the fluxes are corrected for pixel area and converted
into instrumental magnitudes (−2.5 log(flux)) where the flux is given in units of electrons.
See Section 4.2 for comments on changes in the PSF encircled energy due to variations in the
telescope focus.
In July 2006, the temperature of the WFC detector was lowered from −77◦ C to −81◦ C in order to
mitigate the impact of transfer inefficiencies and the production of hot pixels. This temperature
change caused a loss in sensitivity (Mack, J. et al. ISR ACS 2007-02) which we took into
account for all the observations made after the temperature change.
3.2 Initial Master Catalogs
ACS was installed on board HST in March 2002 during SM3B (ACS Instrument Handbook;
Úbeda, L. et al. 2012). This initial epoch is the baseline against which we measure sensitivity
loss. For each filter we built photometric catalogs by combining the magnitude measurements
of a set of images from these original observations. For most filters, observations from program
9018 provide a wide spatial coverage and enough well-measured stars to create the initial epoch
catalogs. In most cases, images from April and May 2002 were used. For narrow band filters
there were no observations available from program 9018 and images from programs 9656 (PI:
G. de Marchi) and/or 9663 (PI: R. Gilliland) were used instead. Unfortunately, several F606W
images from proposal 9018 failed (stars show as short streaks) and therefore, images from
proposals 9433 (PI: G. Bernstein), 9648 (PI: A. Riess), and 9656 had to be considered. This
means that the initial catalog for F606W consists of the combination of measurements obtained
from April 2002 until March 2003.
To cross-identify the stars in each exposure with the master list, we used an in-house FORTRAN
program from Anderson, J. & King, I. (ISR ACS 2006-01). These cross-identified stars define
a linear transformation from the distortion-corrected frame of each exposure into the master
frame. We then used a different in-house program to generate a master list of stars that could
be found in a minimum number of exposures through each filter.
Table 2 shows the number of sources found in each initial master catalog as a function of filter.
Column three lists the proposal identification numbers for the observations used and column
four shows the range of observation dates assumed as initial epoch. The orange outline in
Figure 1 represents the location of the stars in the F606W initial master catalog as an example.
5
Filter
Sources (count)
F850LP
F814W
F775W
F660N
F658N
F625W
F606W
F555W
F550M
F502N
F475W
F435W
20386
30415
20963
7166
6415
10033
19003
14561
15570
5328
8264
9474
Proposal IDs
Date Range
9018
9018
9018
9656 9663
9656
9018 9656
9018 9433 9648 9656
9018
9656 9663
9648 9656
9656
9018
April 2002
April 2002 – May 2002
April 2002 – October 2002
July 2002 – July 2003
October 2002 – August 2003
April 2002 – December 2002
April 2002 – March 2003
May 2002
July 2002 – July 2003
September 2002 – August 2003
October 2002 – August 2003
May 2002
Table 2: Summary of the initial master catalogs. The number of stars in each catalog are listed as well as the range
of observation dates assumed as initial epoch. Whenever possible, images from GO program 9018 were used.
3.3 Position/Magnitude Correlation. The Relative Photometry
Once we had the master catalog of stars for each filter, we went back and cross-identified the
stars in the individual exposures. We included only those stars that were bright but not saturated
by selecting sources with flux (in units of electrons) such that −13.0 < −2.5 log(flux) < −9.0
or 4000 . flux . 150 000 electrons. Although fluxes for saturated stars can be measured
reliably using approaches described in Gilliland, R. L. (ISR ACS 2004-01), we focus here on
the unsaturated stars, since bleeding can take flux out of our apertures. The _xym files contain
many false detections, such as cosmic rays, hot pixels, or features around saturated stars. These
detections are weeded out by our cross-identification procedure.
4. Results
For each exposure, the cross-identification procedure above gives us a list of stars with photometry in the exposure and average photometry from the master list. Figure 2 shows an example of
our typical magnitude residuals for ∼ 1300 matched stars from F775W image j8hm01yrq_flt.fits
observed on 16 March 2003 and the F775W initial master catalog.
This plot clearly shows a larger dispersion for fainter objects as expected. The adopted mean
value was calculated using an iteratively sigma–clipping flux–weighted method. We used
σ = 2.0 to reject possible bad outliers clearly seen in Figure 2. The final mean value is shown as
a red horizontal line. The dotted lines represent the adopted error in the calculated average. This
6
average magnitude difference (∆m) gives us the information that we need to measure whether
a sensitivity trend exists or not.
We analyze the average magnitude difference (∆m) as a function of time for each filter. Since
observations have different exposure times, we scaled the averages to an exposure time of 1000
seconds. We also shifted the plots so that ∆m = 0.0 at the time of the installation of the ACS on
board HST (07 March 2002) during Servicing Mission 3B.
Figure 3 shows the results for broad-band filters F814W, F775W, F625W, and F606W. Figure 4
shows the results for broad-band filters F555W, F475W, and F435W and for medium-band filter
F550M. Figure 5 shows the results for narrow-band filters F660N, F658N, and F502N, and for
wide filter F850LP.
Each plot shows the average magnitude
difference (∆m) as a function of time in
units of years. The plots on the left were
created with the _FLT files as input.
The plots on the right were created with
the _FLC files as input. A dashed line
through ∆m = 0.0 is displayed to aid
the eye. The symbols are represented in
two shades of blue: the light blue symbols represent short exposure observations (exposure time lower that 300 seconds); the dark blue symbols represent
Figure 2: Typical magnitude residual plot before scaling for exlong exposures (exposure time > 300
posure time. It shows the difference in magnitude of ∼ 1300
matched stars from F775W image j8hm01yrq_flt.fits (m) and the seconds).
F775W initial master catalog (mcat ). The horizontal axis represents the average of the two magnitudes. The horizontal red line
represents the adopted average.
All plots span the lifetime of ACS/WFC:
from March 2002 until the present,
through failure in January 2007 and
successful restoration during Servicing
Mission 4 (SM4) in May 2009. Because of the electronics change in May 2009 the time period
before and after the ACS failure must be analyzed separately (ISR ACS 2011–03).
4.1 Analysis and Discussion of pre-SM4 data
Sensitivity changes of pre-SM4 data have already been published and presented in ISR ACS
2011–03. Table 1 in that document lists the rate of sensitivity loss in magnitudes per year
for all the filters that we analyze in this ISR. The losses are less than ∼ 0.003 mag/year, and
were calculated using high precision aperture photometry of three white dwarf primary standard
stars, namely G191B2B, GD153, and GD71. Charge transfer inefficiencies have been taken into
account in this study using a parameterized CTE correction formula from ISR ACS 2011–01.
7
The left panels of Figures 3, 4, and 5 show the results of the study performed on 47 Tuc _FLT
images (no CTE correction applied). The change in sensitivity for each filter is fit with a line
shown in red. The orange lines represent the white dwarf fit from ISR ACS 2011–03. The
estimated slopes (in units of magnitudes/year) and formal errors are shown in Table 3.
The fitting was performed by minimizing the χ2 error statistic. The error assigned to the point
that represents each image is given by 1σ of the distribution. Measurements performed using
short exposures (integration time < 300 seconds) are less reliable and, for that reason, their
errors were arbitrarily doubled. This method assigns a higher weight to measurements done on
long exposures.
The right panels of Figures 3, 4, and 5 show the results of the study performed on 47 Tuc
_FLC images. These images have been corrected using the pixel-based CTE correction. The
estimated slopes (in units of magnitudes/year) and formal errors are shown in Table 3
In general, the 47 Tuc sensitivity loss rates are in agreement with the WD method values within
the errors. This is true specially for filters F435W, F475W, F502N, F550M, F625W, F775W,
and F814W. Narrow-band filters F658N and F660N show slightly worse sensitivity losses. The
study of filter F606W shows a perfect agreement with the WD method if the _FLT files are
considered. If the correction for ineffective CTE is taken into account, the 47 Tuc method
shows that the sensitivity loss is somewhat smaller.
Filter
F435W
F475W
F502N
F550M
F555W
F606W
F625W
F658N
F660N
F775W
F814W
F850LP
WD Study
-0.0028
-0.0028
-0.0028
-0.0029
-0.0029
-0.0028
-0.0027
-0.0025
-0.0025
-0.0014
-0.0009
0.0000
47 Tuc Study
APPHOT FLT
APPHOT FLC
-0.0042 (0.0006)
-0.0064 (0.0018)
-0.0045 (0.0010)
-0.0047 (0.0010)
-0.0049 (0.0012)
-0.0032 (0.0004)
-0.0061 (0.0015)
-0.0059 (0.0018)
-0.0087 (0.0015)
-0.0039 (0.0007)
-0.0052 (0.0012)
-0.0028 (0.0006)
-0.0027 (0.0006)
-0.0034 (0.0018)
-0.0012 (0.0010)
-0.0024 (0.0010)
-0.0018 (0.0012)
-0.0011 (0.0004)
-0.0030 (0.0015)
-0.0039 (0.0018)
-0.0039 (0.0015)
-0.0016 (0.0007)
-0.0019 (0.0012)
-0.0014 (0.0006)
ePSF FLC
-0.0015 (0.0004)
-0.0008 (0.0007)
-0.0022 (0.0012)
Table 3: Estimated pre-SM4 sensitivity losses per year as a function of ACS/WFC filter for _FLT
and _FLC images. The errors are between parentheses. The losses and their errors are expressed in
units of magnitudes per year. The values in the second column are taken from ISR ACS 2011-03.
These are the values of the fourth order polynomial fit and have a formal error of 0.0008 mag/year.
8
Figure 3: Magnitude residuals average as a function of time for filters F814W, F775W, F625W, and F606W. Dark
blue symbols correspond to deep observations (exposure time > 300 sec) and short exposures are shown in light
blue. The orange line represents the pre-SM4 fitting using the standard white dwarf stars from ISR ACS 2011–03.
The red line is the best fit of the 47 Tucanae study.
4.2 Telescope Breathing and Changes in Focus
Having selected an aperture radius of 3.5 WFC pixels (000. 175) for the aperture photometry
means that changes in the PSF encircled energy due to telescope focus variations will be present
in our measurements. In order to quantify this effect and verify our method we repeated the
study for filters F814W and F775W using ePSF photometry (ISR ACS 2006–01) with the flag
PERT activated. This allows the code to find a spatially constant perturbation to the library of
PSFs to account for focus variations that impact the inner 5.0 pixels of the PSF. The final flux
9
Figure 4: Magnitude residuals average as a function of time for filters F555W, F550M, F475W, and F435W. Dark
blue symbols correspond to deep observations (exposure time > 300 sec) and short exposures are shown in light
blue. The orange line represents the pre-SM4 fitting using the standard white dwarf stars from ISR ACS 2011–03.
The red line is the best fit of the 47 Tucanae study.
is calculated with respect to a 10.0 pixel aperture.
Figure 6 shows a comparison between aperture and ePSF photometry for filter F814W. All the
points correspond to deep exposures (integration time > 300 seconds). It is evident (see bottom
panels) that in some cases, observations made on the same day present ∆m variations of up
to 0.01 mag. The same measurements made using ePSF photometry show lower dispersion
becasue the telescope focus variations are taken into account. However, a similar sensitivity
loss is found for both methods. The ePSF method averages out the focus variations before the
linear fitting and the statistical approach presented in this ISR averages out during the χ2 error
10
Figure 5: Magnitude residuals average as a function of time for filters F850LP, F660N, F658N, and F502N. Dark
blue symbols correspond to deep observations (exposure time > 300 sec) and short exposures are shown in light
blue. The orange line represents the pre-SM4 fitting using the standard white dwarf stars from ISR ACS 2011–03.
The red line is the best fit of the 47 Tucanae study. Note that the CTE corrections for narrow-band filters F660N
and F658N are large because of the low backgrounds detected in those images. This is specially important for
post-SM4 data.
statistic fitting.
Figure 7 shows an equivalent study for filter F775W. Note that both deep and short exposures
were considered for this filter. The lower dispersion in the ePSF study is quite evident. The
final fitting of the _FLC images shows virtually the same sensitivity loss. In Table 3 we report
the values of the sensitivity loss estimated using ePSF photometry in three filters.
11
Figure 6: Comparison between aperture (bottom) and ePSF (top) photometry for images obtained with filter
F814W.
Figure 7: Comparison between aperture (bottom) and ePSF (top) photometry for images obtained with filter
F775W. Note that the ePSF photometry shows lower dispersion, because this type of photometry accounts for
telescope-focus variations. See text for more details.
12
4.3 Analysis and Discussion of post-SM4 data
During Servicing Mission 4 (SM4) in May 2009 the CCD Electronics Box Replacement (CEBR) took place and the gain of the new electronics was set so that the measured signal matched
the signal for the same 47 Tucanae field as measured in 2002. Due to the lack of sufficient
data, Bohlin, R. et al. (2011) adopt a slope arbitrarily set to zero for the post-SM4 points in
the [∆m, time] plane.
For most filters we now have at least three years of observations of the calibration field and we
are able to quantify the sensitivity of the WFC post-SM4 and provide information on the current
status of the CCDs.
Filter
47 Tuc Study
FLC
F435W
F475W
F502N
F550M
F555W
F606W
F625W
F658N
F660N
F775W
F814W
F850LP
0.0004 (0.0005)
-0.0004 (0.0006)
0.0002 (0.0003)
0.0004 (0.0004)
-0.0001 (0.0005)
0.0002 (0.0001)
-0.0003 (0.0008)
-0.0001 (0.0006)
0.0008 (0.0004)
0.0000 (0.0005)
0.0002 (0.0003)
0.0000 (0.0004)
Table 4: Estimated sensitivity losses per year as a
function of ACS/WFC filter for _FLC post-SM4
data. The errors from the χ2 error statistic fitting
are between parentheses. The losses and their errors are expressed in units of magnitudes per year.
We adopt the same initial master catalogs that were used for the pre-SM4 data. Figures 3, 4,
and 5 show the results of the study performed on 47 Tuc images. By comparing the left plots
(_FLT images) with the plots on the right (_FLC images), a first impression shows the effect of
the CTE correction as a function of time. The CTE correction is larger for post-SM4 data than
for pre-SM4 data as shown in Úbeda, L. & Anderson, J. (ISR ACS 2012-03). With the current
study we are able to confirm those results.
Moreover, it is very clear that the point distributions post-SM4 are virtually flat for all filters.
The change in sensitivity for each filter was fit with a line by minimizing the χ2 error statistic.
The estimated slopes (in units of magnitudes/year) and formal errors are shown in Table 4.
These results show that the loss in sensitivity post-SM4 is negligible.
13
5 Conclusions
We performed a study of the sensitivity loss of the ACS/WFC CCDs using twelve filters. This
study was done using a calibration field about 6.7 arcmin West of the center of globular cluster
47 Tucanae.
For data obtained before SM4, a comparison of the sensitivity loss rates with those calculated
using standard white dwarf stars by Bohlin, et al. (2011) shows excellent agreement within
the uncertainties of both methods as can be seen in Figure 8. The only large discrepancy corresponds to filter F606W where our study shows a lower sensitivity loss (−0.0011 mag/year
as opposed to −0.0028 mag/year). The use of ePSF photometry leads to the value −0.0015
mag/year. Refer to Table 3.
The advantage of the pixel-based CTE correction is clearly seen in Figures 3, 4, and 5. In
particular, those plots show the CTE dependency with date of observation as well as exposure
time. Our results confirm the study of the CTE evolution in ISR ACS 2012-03.
Figure 8: Pre-SM4 slopes in units of magnitudes per year for the twelve filters that were
analyzed with the 47 Tucanae data. The fourth order fit from Bohlin, R. et al. (2011) is
shown as a black line with the uncertainty shown as dashed lines. Error bars on each point
are taken from Table 3. Most of the error bars cross the fit with the exception of F606W.
We now have a baseline of at least three years of post-SM4 observations of the 47 Tucanae
calibration field. Our study shows that, on average, the magnitude difference (∆m) is stable
over time. This is a remarkable result considering that ACS is an instrument that has been in
space for over ten years and subject to contamination.
A future research using the white dwarf standards on a longer time baseline would be able to
confirm or improve the loss rates presented in this ISR. No corrections to the observed photometry are necessary because the current PHOTFLAM header keywork in the ACS/WFC images is
calculated as a function of time using the formulae provided by Bohlin, R. et al. (2011).
14
Acknowledgments
The authors would like to thank L. Smith (ACS Lead), R. Bohlin, A. Maybhate, V. KozhurinaPlatais, and M. Chiaberge for their comments and revision suggestions that resulted in substantial improvement of this work. The authors acknowledge J. Mack, P-L. Lim, and R. Avila for
their past contributions to this project.
References
Anderson, J. & Bedin, L. R. 2010, PASP, 122, 1035–1064
Anderson, J. & King, I. R., 2006, Instrument Science Report ACS 2006-01 (Baltimore: STScI)
Bohlin, R. & Anderson, J. 2011, Instrument Science Report ACS 2011-01 (Baltimore: STScI)
Bohlin, R. et al. 2011, Instrument Science Report ACS 2011-03 (Baltimore: STScI)
Gilliland, R. L. 2004, Instrument Science Report ACS 2004-01 (Baltimore: STScI)
Mack, J. et al. 2007, Instrument Science Report ACS 2007-02 (Baltimore: STScI)
Úbeda, L. & Anderson, J. 2012, Instrument Science Report ACS 2012-03 (Baltimore: STScI)
Úbeda, L. et al. 2012, ACS Instrument Handbook, Version 12.0 (Baltimore: STScI)
15