Selection of the Infrared Detectors for Wide Field Camera 3 on the
Hubble Space Telescope
M. Robberto1,#, M. Stiavelli1, S. Baggett1, B. Hilbert1, J. W. MacKenty1,
R. A. Kimble2, R. J. Hill2, D. A. Cottingham2, G. Delo2, S. D. Johnson2,%, W. Landsman2, E. M.
Malumuth2, &, E. Polidan2, %, A. M. Russel2, A. Waczynski2, %, Wassel, E.,
Y. Wen2,&, A. Haas3, J. T. Montroy3, E. Piquette3, K. Vural3, C. Cabelli3, and D. N. B. Hall4
1
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218
2
NASA/Goddard Space Flight Center, Greenbelt, MD 20771
3
Rockwell Scientific Company, 5212 Verdugo Way, Camarillo, CA 93012
4
University of Hawaii, Institute for Astronomy, 640 N. Aohoku Place, Hilo, HI 96720
ABSTRACT
Wide Field Camera 3 is a fourth generation instrument for the Hubble Space Telescope (HST), to be installed
during the next HST Servicing Mission 4. For its infrared channel Rockwell Scientific Company has developed a new
type of HgCdTe 1K×1K detector, called WFC3-1R, with cutoff wavelength at 1.7µm and 150K operating temperature.
The WFC3-IR detectors are based on HgCdTe MBE grown on a CdZnTe substrate and use a new type of multiplexer,
the Hawaii-1R MUX. Two flight detectors, a prime and a spare, have been recently selected on the basis of the measures
performed at NASA Goddard Research Center – Detector Characterization Laboratory. These parts show quantum
efficiency higher than 80% at λ=1.6µm and greater than 40% at λ>1.1µm, readout noise of ~25 e- rms with double
correlated sampling, and mean dark current of ~0.04 e/s/pix at 150K. We show that the IR channel of WFC3, equipped
with one of these flight detectors, beats the instrument requirements in all configurations and promises to have a
discovery efficiency significantly higher than NICMOS. In particular, a two-band wide-area, deep survey made with
WFC3 exceeds the discovery efficiency of NICMOS before and after the installation of NCS by a factor of 15 and 10,
respectively.
1. INTRODUCTION
WFC3 is the panchromatic fourth-generation instrument for HST, configured as a two-channel wide-field imager
covering with diffraction limited performance the entire wavelength range between 2000Å and 1.72µm (Cheng et al.
2002; MacKenty et al 2002). It has an optical and an infrared channel, the latter spanning between 0.8µm and 1.72µm.
With this range falls the minimum of background emission at the focal plane of the HST, due to the relatively high
temperature (~300K, Robberto et al. 2000) of the optical assembly. An IR detector optimized for this spectral region can
be operated at relatively high temperatures and still keep the dark current at an acceptably low level, due to the relatively
large bandgap. Moreover, temperatures as low as ~150K can be achieved and maintained indefinitely by thermoelectric
coolers, unlike disposable cryogenic fluids like LN2 that allow short instrument lifetime.
In July 2000, Rockwell Scientific Company, LLC (RSC) was contracted to fabricate the WFC3-1R FPA, a 1024x1024
hybrid HgCdTe device MBE grown on a CdZnTe substrate, with Hg-Cd stoichiometric balance tuned to provide a longwavelength cutoff at ~1.7µm and a T=150K operating temperature. The detector array is indium bonded to the new
#
On Assignment from the Space Telescope Operations Division of the European Space Agency (ESA)
Global Science and Technology, Inc.
&
Science Systems and Applications, Inc.
%
Hawaii-1R multiplexer (MUX), a device largely based on the Hawaii-2 design and on the recent advances for the NGST
project. Main performances requirements for the WFC3-1R detector were:
1. mean dark current <0.2e/s at 150K,
2. quantum efficiency >45% over the entire range 1.0-1.63µm,
3. readout noise <30 e- rms with correlated double sampling.
In three years RSC has produced several lots of detectors that have demonstrated the capability to meet these
requirements (Robberto et al. 2002). In May 2003 the WFC3 Scientific Oversight Committee has selected the two flight
detectors, respectively FPA#64 as prime and FPA#59 as spare. In this paper we report the performance of the two flight
detectors, as result from the extensive test activity carried out at the Detector Characterization Laboratory (DCL) of the
NASA Goddard Space Flight Center (Greenbelt, MD) (Section 2). In Section 3 we briefly discuss the expected
performance of WFC3 IR channel if equipped with the flight detectors.
2. DETECTOR PERFORMANCE
The two detectors, FPA#64 and FPA#59, have been produced in the same lot and show similar performance. Most of the
tests data have been taken in “multiple-read” mode, sampling the discharge of the detector capacitance at uniformly
spaced time intervals.
2.1 Quantum Efficiency (QE)
The reach for the highest possible QE has been one of the main objectives of the development phase. The two flight
parts show a QE that increases nearly linearly with wavelength and peaks at λ ~ 1.6µm, where it reaches QE > 80%
(Figure 1). The short wavelength cutoff, not shown in the figure, is at λ ≅ 0.82 µ m . For both parts, the QE at λ ~ 1µm is
less than the original specifications (QE=0.45), but this is acceptable since this is compensated by a better than expected
throughput of the optics. Moreover, the QE same specification is exceeded by a large factor at longer wavelengths,
where the background is lower and the instrument can reach its ultimate sensitivity. It is also worth noticing that between
λ ~ 0.9µm and λ ~ 1.1µm the absolute QE of these detectors is higher than the QE of the UV-optimized CCD detectors
that equip the optical-ultraviolet channel of WFC3.
Figure 1. Absolute quantum efficiency of the prime WFC3 IR detector, FPA#64 (left) and spare, FPA#59 (right). The data refer to the
averaged over the entire active area. Absolute QE has been estimated by using the average gain (adu/e) of the four quadrants,
measured using standard photon transfer (mean-variance) method.
The cosmetic quality of both parts is excellent, as illustrated by the QE maps at λ = 1.5µm presented in Figure 2. The
associated histograms (Figure 3) show that both detectors have high QE uniformity, with a narrowly peaked distribution
characterized by more than 95% of the pixels within 10% of the mean.
Figure 2. Flat-field images of FPA#64 (left) and FPA#59 (right). Darkening at the corners is due to vignetting in the optical setup,
with the exception of the lower-left corner of FPA#64 where a small number of pixels are electrically unbonded.
Figure 3. Histograms of the absolute QE distribution for the whole array measured at λ = 1.5µm for FPA#64 (left) and FPA#59
(right).
Small variations in the relative concentration of Hg vs. Cd within the detector material are reflected in a spatial
dependency of the QE at the long wavelength cutoff. This is shown in Figure 4, which shows the flat-field response of
FPA#64 and FPA#59 to a monochromatic flux at 1.72µm. The corresponding histograms, presented in Figure 5,
indicate that the QE fall-off is highly uniform. In general, the QE uniformity is restored above 1.70µm. To guarantee the
uniformity of the photometric system across the field of view at the longest wavelengths, the WFC3 Science Oversight
Committee has defined a broad “H-band” filter with red cutoff (5% relative transmission) at 1.70µm.
Figure 4. Flat-field images of FPA#64 (left) and FPA#59 (right) at λ = 1.72µm. The comparison of this images with those presented
in Figure 2 clearly illustrates the QE variations at the long wavelengths cutoff.
Figure 5. Same as Figure 3, relative to the QE data at the long wavelengh cutoff shown in Figure 4.
2.2 Readout Noise
The procurement contract to RSC specified a read out noise less than 30 e- rms for a single correlated double sampling.
Figure 6 shows the histogram of the readout noise for FPA#64 and FPA#59. Our standard readout noise measures are
made by taking a sequence of 16 non destructive reads with almost zero delay (exactly, 62ms) between each frame. The
double correlated readout noise is then estimated by averaging adjacent differences, i.e. frame 2 minus frame 1, frame 4
minus frame 3, and so on. The histograms are relative to the 1014×1014 pixels full-frame (pixels 6:1019,6:1019), i.e.
excluding the reference pixels. The average gain of the four quadrants has been assumed. The readout noise of the prime
detector, FPA#64, peaks at ~24e, whereas FPA#59 shows a 10% higher peak at ~27e. The reference pixels show a ~10e
readout noise, if the same gain of the active pixels is assumed. For the performance of reference pixels see Robberto et
al. (2002).
Figure 6. Readout noise measured on FPA#64 (left) and FPA#59 (right).
The WFC3-1R flight detectors show negligible 1/f noise, and therefore the readout noise remains stable over integration
times a few thousand seconds long, of the order of the longest integration times (~60 min) expected for the instrument.
In Figure 7 (left), we plot the readout noise estimated for differences taken sampling uniformly a 30min long ramp.
The effective reduction of the equivalent readout noise with the multiple reads has been proved for FPA#64. The
experiment was conducted taking 60 ramps of 16 read at the maximum readout speed. The equivalent readout noise at
read n was estimated doing, for each pixel, a linear fit to the images from 1 to n, and then taking the standard deviation
of the slopes. On the left of Figure 7 we show the results obtained for the short exposures. Fitting all reads up to n=16
provides a readout noise, averaged over the entire array, of 15.0 e- . This is down from 24e- for the original correlated
double sampling, in excellent agreement with the theoretical estimate of the readout noise reduction in readout noise
limited regime (Garnett & Forrest, SPIE 1946, 395),
σ ron (n) =
12 ⋅ n σ ron (dcs )
n2 − 1
2
predicting σ ron (n) = 15 for n = 16 .
The same experiment has been repeated on longer exposures, taking 40 ramps of 30min total exposure, still composed
by 16 equally spaced reads. In Figure 7 (right) we show the results obtained from the fit of all 16 reads, after subtracting
the mean dark current shot noise. The asymmetry toward high values is due to the dark current tail, which has not been
subtracted. The mean readout noise remains substantially constant, with a marginal increase at 15.6 e-. Since the
amplifier glow has been practically eliminated in the WFC3-IR devices, an equivalent readout noise of ~15 e- can be
assumed while estimating the instrument performance with FPA#64, as long as 16 uniformly spaced reads are used to
sample the signal.
Figure 7. Histogram of the equivalent readout noise for FPA#64 attained combining 16 read taken at the maximum readout speed
(left), and equally spaced over a 30 minutes ramp (right).
2.3 Dark Current
The average dark current of the WFC3-IR is specified to be less than 0.2 e-/s/pix at 150K. During the development phase
RSC demonstrated that WFC3-IR detectors can reach dark current as low as 0.02e/s/pix. Figure 8 shows the dark current
histograms for FPA#64 and FPA#59. Both detectors present a mean dark current at ~0.04 e-/s/pix, well within the
specifications. The histograms reveal the presence of a tail of hot pixels, but their influence is expected to be marginal as
we estimate the thermal background from the cold enclosure on the detector to be ~0.4 e-/s/pix. The fraction of pixels
operating below this value is 95.7% for FPA#64 and 95.3% for FPA#59.
Figure 8: Dark current for FPA#64. Left: dark image of the whole array. Right: histogram of the dark current as measured from the
image on the left. The histogram contains 96.4% of the pixels.
In Figure 9 we show the corresponding dark current maps. Both detectors present excellent cosmetic also in what
concern dark current, with only a cluster ~50 hot pixels at the top of FPA#59. The amplifer glow that was affecting
previous detectors, in particular NICMOS, is disappeared thanks to improvements in the detector design and to the use
of off-chip amplifiers.
Figure 9: Maps of the dark current for FPA#64 (left) and FPA#59 (right).
3. INSTRUMENT PERFORMANCE
3.1 Assumptions
On the basis of the measured performances of the flight WFC3-IR detectors, we have evaluated (Stiavelli and
Robberto, 2002) the overall sensitivity of the IR channel and compared it with: a) original instrument requirements, and
b) with the refurbished NICMOS. We have restricted our attention to the F110W (J-band) and F160W (H-band) filters,
plus the F126N narrow band filter. This last has been considered because it represents the regime (narrow band filter at
short wavelengths) where the QE of FPA#64 and FPA#59 is below requirements. Although NICMOS does not have
such a filter, we have maintained it in our NICMOS simulation for the sake of comparison.
Our model considers the estimates of Stiavelli (2002) for the on-orbit average sky brightness in the J and H band,
respectively 0.023 and 0.014 photons HST area-1s-1A-1arcsec-2. These values were converted into photons per pixel
adopting a pixel size of 0.128 arcsec for WFC3, 0.075 arcsec for NICMOS Camera 2, and 0.2 arcsec for NICMOS
Camera 3.
For the optical throughput of the telescope we assume 0.744 both for WFC3 and NICMOS. This is biased against
WFC3 since NICMOS has a cold stop misalignment that causes a slight reduction of throughput, the actual value
depending on the channel (Robberto et al. 2000). Instrument throughput, filter transmission and bandpasses come from
the estimates reported on a number of Instrument Science Reports publicly available at
http://www.stsci.edu/hst/HST_overview/instruments
Concerning the Quantum Efficiency, we have adopted the filter averaged values shown in Table 1:
FPA64
CEI Req.
NIC-NCS
F110W
0.419
0.475
0.3
F160W
0.709
0.583
0.43
F126N
0.488
0.5
0.33
Table 1 : Adopted QE values for the three filters.
It must me remarked that also this comparison is biased against WFC3, as we use early specifications for the detector QE
and the most recent NICMOS QE. The former are tighter than those later used for the actual RSC contract, the latter is
significantly increased with respect to that known when WFC3 was planned.
The detector noise parameters are given in Table 2 below.
Dark current (e/pixel/s)
Ron (e rms/pixel)
Table 2: adopted detector noise parameters.
WFC3
0.1
15.6
CEI Req
0.2
10
NIC3-NCS
0.1
22
For NIC2 we have adopted a read out noise of 20 e rms/pixel instead of 22.
For WFC3, we have added the contribution from the internal background using the most recent values for the
temperature of the various components, obtaining 0.08 ph/s/pixel in the F110W and F126N filters and 0.31 ph/s/pixel in
F160W. For NICMOS we have adopted 0.114 and 0.763 ph/s/pixel, respectively, for NIC2 and NIC3 in F160W. We
have not included a thermal contribution for F110W and F126N.
To consistently evaluate the signal-to-noise ratio, we have assumed optimal signal extraction. Under this
assumption, the number of pixels contributing to the noise (“equivalent noise area”) is given by the inverse of the Point
Spread Functions (PSF) sharpness, calculated using optical models for WFC3 and the standard PSF simulator of the
HST PSF, Tiny Tim’s, for NICMOS. We have considered faint targets requiring multi-orbit exposures. We have
assumed that the detector read out once every 2400s. For the broad band images we have considered a point source of
AB mag = 28 in both J and H. This corresponds to 22.91 nJy or 1.3 10-4 and 9.78 10-5 photons HSTarea-1s-1A-1 in J and
H, respectively. For the narrow band filter we have considered an AB mag =25 or 363.1 nJy. This translates into
2.07×10-3 photons HSTarea-1s-1A-1. For the extended source calculations we have considered values of surface brightness
of AB=26 mag arcsec-2 and AB=23 mag arcsec-2, respectively, for the broad and narrow band filters. In all cases we have
required a S/N=10 (either per source or per arcsec2).
3.1 Performance
We compare the point source speed, defined as the inverse of the number of orbits needed to achieve a given signal to
noise ratio (hereafter S/N) on a faint point source. When normalized to a particular detector-optics configuration, this
definition of speed is independent of the particular S/N and source magnitude used as long as they are such that one
remains in a multi-orbit regime.
In figure 10 we compare WFC3 against NICMOS, normalizing both instrument to the original WFC3 requirements. As
built, WFC3 beats in performance the instrument as described in the original requirements. In terms of point source
sensitivity, in the J band and in the narrow band filter WFC3 performs better than NICMOS. In the H band NICMOS
camera 3 beats WFC3 due to the fact that the NICMOS H band filter is broader than that of WFC3, due to the shorter
wavelength cutoff of the WFC3 IR detectors.
Point Source Speed
3.00
WFC3/CEI Req
2.50
NIC3 post-NCS/CEI Req
NIC2 post-NCS/CEI Req
2.00
1.50
1.00
0.50
0.00
F110W
F160W
F126N
Figure 10. Comparison of absolute speed for WFC3 and NICMOS-camera3 and NICMOS-camera2 for point sources.
The same plot for an extended source (assumed 1×1arcsec2 size) is presented in Figure 11. The as-built WFC3 still
outperforms its original requirements and NICMOS-camera 2. NICMOS camera 3 outperforms WFC3 for extended
sources thanks to its much larger pixel size.
Extended Source Speed
3.00
WFC3/CEI Req
2.50
NIC3 post-NCS/CEI Req
NIC2 post-NCS/CEI Req
2.00
1.50
1.00
0.50
0.00
F110W
F160W
F126N
Figure 10. Comparison of absolute speed for WFC3 and NICMOS-camera3 and NICMOS-camera2 for extended sources.
Pure speed considerations do not take into account the larger field of view of WFC3. A large field of view with smaller
pixels is the main strength of WFC3 IR over NICMOS. In order to estimate with a single parameter the effect of both
sensitivity and field of view we have defined the discovery efficiency as the product of the FOV times the point source
speed. This definition provides a direct estimate of the time needed to carry out a wide area deep survey. The results are
presented in Figure 12. We have considered NICMOS-camera 3 before and after the NCS retrofit, since Nicmoscamera2 is not a good choice for such a survey. It is clear that the discovery efficiency of WFC3 with FPA64 exceeds
that of the CEI requirements instrument and by a large factor that of NICMOS. In a combined J plus H metric,
corresponding to a two band wide area, deep, survey WFC3 with FPA64 exceeds the discovery efficiency of NICMOS
by a factor of 10 (15 with respect to NICMOS before the NCS retrofit).
Discovery Efficiency
3.00
FPA64/CEI Req
2.50
NIC3+NCS / CEI Req
NIC3 pre-NCS /CEI Req
2.00
1.50
1.00
0.50
0.00
F110W
F160W
F126N
Figure 10. Comparison of the discovery efficiency for WFC3 and NICMOS-camera3 and NICMOS-camera2 for point sources.
5. CONCLUSION
We have presented the main characteristics of the two flight detectors selected for WFC3. Their performances
meet the contract requirements, with a deficiency in the short wavelength QE compensated by the excellent QE values at
longer wavelengths. We have shown that WFC3 as built beats its original requirements in all configurations. We have
shown that WFC3 IR promises to have a discovery efficiency significantly higher than that of NICMOS. A two band
wide area, deep, survey made with WFC3 exceeds the discovery efficiency of NICMOS before and after the installation
of NCS by a factor of 15 and 10, respectively.
AKNOWLEDGMENTS
This work is supported by NASA’s Office of Space Science through the Hubble Space Telescope Program, Wide-Field
Camera 3 Project.
REFERENCES
Cheng, E. S., Hill, R. J., MacKenty, J. W., Cawley, L., Knezek, P., Kutina, R. E., Lisse, C. M., Lupie, O. L., Robberto,
M., Stiavelli, M., O'Connell, R. W., Balick, B., Bond, H., Calzetti, D., Carollo, M., Disney, M., Dopita, M., Frogel, J.,
Hall, D. N., Hester, J., Holtzman, J., Luppino, G. A., McCarthy, P., Paresce, F., Saha, A., Silk, J., Trauger, J.T., Walker,
A., Whitmore, B., Windhorst, R., Young, E. T. 2000, in UV, Optical, and IR Space Telescopes and Instruments, J. B.
Breckinridge and P. Jakobsen Eds., Proc. SPIE 4013, 367.
Garnett J. D., and Forrest, J. W. 1993, in Infrared Detectors and Instrumentation, A. M. Fowler Ed., Proc. SPIE 1946,
395.
Mackenty J. W., Cheng, E. S., O’Connell R. W., WFC3 Science Integrated Product Team, Science Oversight Committee
2002, in Future EUV-UV and Visible Space Astrophysics Missions and Instrumentations, J. C. Blades and O. H.
Siegmund Eds., Proc. SPIE 4854, [4854-21]
Robberto, M., Sivaramakrishnan, A., Bacinski, J. J., Calzetti, D., Krist, J. E., MacKenty, J. W., Piquero, J., Stiavelli, M.
2000, in UV, Optical, and IR Space Telescopes and Instruments, J. B. Breckinridge and P. Jakobsen Eds., Proc. SPIE
4013, 386.
Robberto, M, Baggett S. M., Hanley C., Hilbert B., MacKenty J., Cheng, E. S., Hill, R. J., Johnson, S. D., Malumuth, E.
M., Polidan, E., Waczynski, A., Wen, Y., Haas, A., Montroy, J. T., Piquette, E., Vural, K., Cabelli, C., and Hall, D. N.
B. 2002, “Infrared Detectors for WFC3 on the Hubble Space Telescope”, Proc. SPIE 4850-121, 2002, available at
http://www.stsci.edu/instruments/wfc3/wfc3-docs.html
Stiavelli, M., “Expected WFC3-IR count rates from zodiacal background”, 2002, Space Telescope Science Institute
Instrument Science Report WFC3 ISR 2002-02
Stiavelli, M., and Robberto, M., 2002 “Performance of the WFC3-IR channel with FPA#64”, 2002, Space Telescope
Science Institute Instrument Science Report WFC3 ISR 2002-05