# GigaCAM: A One Billion Pixel Imager for the SNAP Satellite

**The SNAP Collaboration**: G. Aldering, S. Deustua, W. Edwards, B.
Frye, D. Groom, S. Holland, D. Kasen, R. Knop, R. Lafever, M. Levi, P.
Nugent, S. Perlmutter, K. Robinson (LBNL), D. Curtis, G. Goldhaber, J.
R. Graham, S. Harris, P. Harvey, H. Heetderks, A. Kim, M. Lampton, R.
Lin, D. Pankow, C. Pennypacker, A. Spadafora, G. F. Smoot (UC Berkeley),
P. Astier, J.F. Genat, D. Hardin, J.- M. Levy, R. Pain, K. Schamahneche
(IN2P3), A. Baden, J. Goodman, G. Sullivan (U. Maryland), R. Ellis, M.
R. Metzger (CalTech), D. Huterer (U. Chicago), A. Fruchter (STScI), C.
Bebek (Cornell U.), L. Bergstrom, A. Goobar (U. Stockholm), I. Hook (U.
Edinburgh), C. Lidman (ESO), J. Rich (CEA/DAPNIA), A. Mourao (Inst.
Superior Tecnico,Lisbon).

# Overview

Based on a radiation tolerant, fully-depleted, high-resistivity CCD
technology, a one billion pixel imager is envisioned for the Supernova
Acceleration Probe (SNAP) satellite currently in early planning \[SNAP
2000\]. The imager is comprised of over one hundred large format CCD's
at the focal plane of a \~2-meter telescope. The diffraction-limited
optics is achieved with a three-mirror anastigmat and provides a
1-square degree field of view. With a complimentary small near-IR imager
and visible and near-IR spectrographs, the SNAP mission can discover
over 2000 Type Ia supernovae in a year at redshifts between z=0.1 and
1.7, and follow their evolution with high-signal-to-noise light-curves
and spectra. The resulting data set can determine the cosmological
parameters with precision: mass density Ω~M~ to ±0.02, vacuum energy
density Ω~Λ~ to ±0.05,and curvature Ω~k~ to ±0.06. The data set can test
the nature of the \"dark energy\" that is apparently accelerating the
expansion of the universe. In particular, a cosmological constant can be
differentiated from alternatives such as \"quintessence\" by measuring
the ratio of the dark energy pressure and density to 0.05, and by
studying this ratio\'s time dependence. A large field imager based on
CCD technology fulfills the requirement for supernova detection and
photometry.

# CCD's

We have successfully developed a new type of large-format CCD\'s on
n-type high-resistivity silicon. The back-illuminated CCD\'s are
fabricated on a 300 μm thick substrate and are fully depleted by the
application of an independent voltage through an optically transparent
backside contact (see Figure 1). Multiple science-grade 2k x 2k pixel
(15 μm^2^ pixels) CCD\'s have been fabricated and tested. Furthermore,
devices on these wafers have shown excellent charge transfer efficiency
CTE \>0.999995, read noise of 4.3e^−^, dark current of 0.003
e^−^/pixel/s, and a well depth of 300,000 e^−^. With back illumination,
the QE at 1 μm is 65% (at T=150 K). As shown in Figure 2, commercially
available CCD\'s with proper coatings reach 15% or less at 1000 nm at
this temperature. Early measurements indicate that as expected \[J.
Janesick\] this technology results in significantly improved radiation
tolerance of the CCD. For long duration missions this could prove to be
a major benefit. These are major achievements in the development of
n-type high-resistivity CCD\'s \[Groom 1999; Holland 1996, 1997, 1997,
2000; Stover 1997; 1998\].

The CCD\'s are self-supporting since the devices do not need to be
thinned. This allows for four-sided abutment making these CCD\'s ideal
for a wide-field mosaic array. In a packaging scheme under
consideration, the edges cantilever from a 3-layer aluminum oxide
structure. The first is a thin insulating layer cemented to the front of
the CCD. The second is a ceramic circuit board with edge pads to which
the CCD pads are wire-bonded. The traces go to a center miniature
connector through which wiring is brought out perpendicular to the CCD
package and through the cold plate. The third layer is an additional
insulator that also captures three indexing pins. Screw-on extensions
facilitate installation and removal without hitting adjacent CCD\'s.
Four-side abuttability and a certain amount of assembly jigging are
therefore automatic. This scheme is shown in Figure 3.

A space-based wide-field imager taking advantage of the 4-side abutment
would require much smaller pixel sizes than ground based telescopes for
reasonable cost and weight of the optics. In this context, we currently
have in fabrication large-format, smaller pixel size (10.5 μm and 12 μm)
devices, and we are exploring lightweight, four-side abutment packaging
technologies.

> ![](media/image1.wmf){width="1.6006944444444444in"
> height="1.7569444444444444in"}

**Figure 1**. Cross-section of the back-illuminated, fully-depleted MOS
CCD. A conventional buried channel CCD is fabricated on a
high-resistivity silicon substrate. A bias voltage applied to the
backside contact results in full depletion of the substrate. The CCD
utilizes conventional MOS technology with the major difference being
wafer thickness, 300 μm vs. the industry standard of 550 μm.

> ![](media/image2.wmf){width="2.88125in" height="2.2416666666666667in"}

**Figure 2.** Measured response of the fully-depleted CCD at ‑100°C
(open circles) for an unoptimized two-layer AR coating (the dark curve
through the points is a model calculation). Response is expected to be
significantly higher for optimized AR coatings. For comparison, measured
response of the 2k x 4k deep-depletion back-illuminated MIT/Lincoln chip
on Keck ESI is shown, as are measurements for two different SITe CCD\'s
on SUBARU and CTIO. Relative spectral responses of standard wide-band
filters are shown.

> ![](media/image3.wmf){width="3.078472222222222in" height="1.675in"}

**Figure 3.** Conceptual design for fully-depleted CCD packaging. The
CCD is cemented to a three-layer aluminum oxide substrate. The middle
layer is a circuit board to which the cantilevered CCD pads are
wire-bonded. Connections exit through the cold plate via a miniature
connector in the center of the board. The third layer provides
insulation and captures three indexing and mounting pins}.

## The SNAP Optical Imager, GigaCAM

A key technological innovation in the SNAP instrumentation is a large
one billion pixel camera, GigaCAM. In order to maximize the sensitivity
of SNAP to discover high redshift supernova this camera requires
excellent sensitivity in the I and Z optical bands. The pixel size is
chosen to be as low as attainable in science grade imagers to minimize
the overall size of the device. The spatial sampling of approximately
one pixel per Airy disk FWHM is sufficient to ensure accurate
photometry. The high-resistivity, p-channel CCD technology provides high
quantum efficiency at 1 μm since the fully-depleted devices are 300 μm
thick and back-illuminated. The longest single exposure is set by cosmic
ray contamination, approximately 400 to 1000 sec. Up to 24 multiple
frames would be stacked and cleaned of cosmic rays prior. The longest
aggregated exposure in the imager is four hours. For the parameters
given in Table 1, which assumes a 2 meter primary mirror, the imager
sensitivity is limited only by zodiacal light background. A possible
deployment of CCD's to cover the 1^o^ x 1^o^ field of view is shown in
Figure 4 and the location of the imager in satellite telescope is shown
in Figure 5.

**Table 1**. Optical Imager/Photometer requirements.

  ------------------------------ ----------------------------------------
  Field-of-view                  1^o^ x 1^o^

  Plate scale                    \~10 μm /0.1 arcsec

  Pixelization                   128 CCD mosaic

  Wavelength coverage            350 nm to 1000 nm

  Detector type                  High-resistivity, p-channel CCD\'s

  Detector architecture          3k x 3k, 10.5 micron pixel

  Detector array temperature     150 K

  Quantum efficiency             65% @ 1000nm, 92% @ 900nm, \>85% @
                                 400-800nm

  Photometric accuracy           3% relative

  Read noise                     4 e^−^ \@100kHz

  Exposure time                  1 sec to 1000 sec (single exposures)

  Number of frames               1 to 24

  Dark current                   0.08 e^−^/min/pixel

  Readout time                   20 sec

  Limiting magnitude sensitivity 30th AB magnitude in Z-band

  Exposure control               Mechanical shutter

  Filter wheel                   15 bands (U, V, R, I, Z, 10 special
                                 filters)
  ------------------------------ ----------------------------------------

![](media/image4.wmf){width="2.1034722222222224in"
height="2.6354166666666665in"}

![](media/image5.wmf){width="3.270138888888889in"
height="2.6645833333333333in"}

**Figure 4.** The 1^o^ x 1^o^ imager focal plane populated with 128
3k x 3k, 10.5 μm^2^ CCD's.

**Figure 5.** Conceptual satellite optics design for a three mirror
anastigmat telescope showing the position of the CCD imager at the focal
plane.

Detection of supernovae is accomplished by a repeated comparison of
fixed fields to reference images. The imager would obtain twenty
discovery fields from dark regions around the north and south ecliptic
poles. These discovery fields would have a limiting detection magnitude
of m~AB~(1μm) \< 27. This set of discovery fields would be recorded at
intervals of four days. At six day intervals detection fields would be
obtained to a limiting magnitude of m~AB~(1μm) \< 28. At eight day
intervals detection fields would be obtained to a limiting magnitude of
m~AB~(1μm) \< 28.5. Two of these fields would have a limiting detection
magnitude of m~AB~(1μm) \< 30 taken every eight days. This strategy is
summarized in Table 2 below. For this baseline study we considered a
Z-band search; higher signal-to-noise may be achieved by combining the
data from several filters. These images represent the majority of the
data to be transmitted. We do however expect to use follow-up images as
search images as well, thereby consolidating much of the data taking and
transfer.

**Table 2.** Observation strategy

+----------+----------+----------+----------+-----------+-----------+
| AB mag.  | SN       | Zodiacal | Total    | Filter    | Repeat    |
|          |          |          |          |           |           |
| B-band   | flux     | Light    | Time     | Fields    | Interval  |
|          |          |          |          |           |           |
| r        | \[       | \[       | \[hrs\]  |           | \[days\]  |
| estframe | e^−^/s\] | e^−^/s\] |          |           |           |
+----------+----------+----------+----------+-----------+-----------+
| 27.0     | 0.4      | 0.27     | 0.1      | Z-band    | 4         |
+----------+----------+----------+----------+-----------+-----------+
| 28.0     | 0.16     | 0.27     | 0.2      | Z-band    | 6         |
+----------+----------+----------+----------+-----------+-----------+
| 28.5     | 0.06     | 0.27     | 0.3      | Z-band    | 8         |
+----------+----------+----------+----------+-----------+-----------+
| 30.0     | 0.03     | 0.27     | 4.6      | Z-band    | 8         |
+----------+----------+----------+----------+-----------+-----------+

The follow-up optical photometry is obtained without specific knowledge
of the location of new supernovae. The optical photometer obtains
wide-field frames overlapping the positions of the discovery frames.
Some of the photometry frames may be taken while the satellite is taking
spectra of specific supernovae. The photometer obtains frames in each of
a specified list of redshifted B-band filters with exposures of
sufficient duration to obtain all the data points required to
reconstruct the light curve. These total exposures will be comprised of
numerous shorter exposures. These shorter exposures will be taken in
dithered groups, allowing elimination of cosmic-rays without resampling
of the image, while providing the subpixel offsets desirable for using
resolution enhancing techniques such as DRIZZLE \[Fruchter 1998\] and
averaging over any residual intrapixel sensitivity variations. The
photometry is also obtained at regular intervals in order to obtain data
points that reasonably approximate the required ten photometric points
along the light curve. The B-band photometry is divided into ranges of
redshift for photometry in a specific filter.

## Acknowledgements

This work was supported by the U.S. Department of Energy under contract
No. DE-AC03-76SF00098, by the National Science Foundation (US) under
grant NSF/ATI 9876605, and by the National Aeronautics and Space
Administration under grant NRA-99-01-SPA-040.

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