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CFH12K: optimizing 12 MIT/LL CCID20 CCDs for a direct imaging application
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Jean-Charles Cuillandre (1), Gerard Luppino (2), Barry Starr (1), Sidik Isani (1)
1: Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Hwy, Kamuel
a, HI 96743, USA
2: Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
In ``Optical Detectors for Astronomy''
Proceedings of the 4th ESO CCD Workshop held in Garching, Germany
September 13-16, 1999
P. Amico and J.W. Beletic Editors
ASSL Series - Kluwer Academic Publishers
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Abstract.
CFH12K is a 12k by 8k wide field imaging camera for the CFHT prime
focus. The mosaic consists of twelve MIT Lincoln laboratories 2k by 4k
thinned backside illuminated CCID20 devices. The devices' operating
parameters have been optimized to ensure the best data quality for
use in broad and narrow band filter imaging mode.
Adaptation to the CFHT prime focus environment included
modifications to reduce the scattered light seen by the CFH12K.
Science data taken by the camera has proven the success of
CFHT's new capability for 42 by 28 square arcminute imaging
with high resolution subarcsecond seeing.
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1. Introduction
Over the years, CCD cameras used at the CFHT prime focus
have evolved from Loral 2k x 2k introduced in 1991, to the 4k x 4k
mosaic MOCAM in 1994, to the 8k x 8k mosaic UH8K in 1995.
Compared to these previous instruments, this latest 12k x 8k
thinned CCD mosaic not only increases the sensitivity by a
factor of two from 500 nm to 900 nm
but it also re-opens access to the B band around 400 nm.
This article focuses on describing how the CCDs, the mosaic and
the prime focus optics were optimized and arranged
to achieve the highest observing efficiency on the sky.
This includes getting the highest sensitivity from the
combination telescope plus instrument, collecting the
data as fast as possible and ensuring the best quality of these data.
The first section of this article describes the organization,
optimization and characterization of the MIT/LL CCID20 CCDs,
which were all tested at once within the CFH12K cryostat.
The second section describes the efforts made to improve
the instrument environment in the prime focus. Finally
a quick outline of the scientific programs covered by
CFH12K is given, followed by illustrations of the tremendous
scientific capabilities of subarcsecond high resolution
wide-field imaging with CFH12K at CFHT atop Mauna Kea.
2. Device selection
The CFH12K mosaic is composed of twelve MIT Lincoln Laboratories
CCID20 devices. These 2k by 4k thinned backside illuminated CCDs
(Burke et al. 1998),
are the result of funding from a consortium of several ground
based astronomical facilities (ESO, UH, Keck, CFHT, Subaru, AAO) led
by G. Luppino from University of Hawaii (UH).
Lick Observatory and UH were responsible for characterizing
all the consortium CCDs individually to assert their quality and rate
them (Wei & Stover 1998). All members were interested in building
mosaics for imaging or spectroscopy and selected in a defined
order the CCDs that best suited their application.
This characterization was meant only to qualify the CCDs, not
optimize them for a given application. However, the initial
performance derived from these simple tests were highly
representative of what could ultimately be achieved.
The characteristics produced were:
- functional outputs (CCID20 has two outputs)
- cosmetic quality at -120 deg. C.
- charge transfer efficiency (CTE, serial and parallel)
- quantum efficiency and variations in amplitude across the chip
- fringing (using a monochromatic source)
- gain and readout noise (using a SDSU Generation I controller)
- full well.
As described is the next sections of this article, a direct imaging
application sets lower constraints on the cosmetic quality and
detector noise. Hence, during the three picks that took place from
1998 to 1999, CFHT and UH focused on getting the CCDs with the best
quantum efficiency and the best CTE. The first two drafts permitted
going on the sky in January 1999 with a 10k by 8k mosaic, and the third
draft brought four CCDs to fill the mosaic and replace the two
engineering grade devices used in the initial mosaic. The knowledge
built from operations on the sky during the first semester of 1999
provided new information on the initial CCDs to reorganize them
within the focal plane based on the criteria described hereafter.
This focal plane work was conducted in August 1999 in the newly
acquired clean room at the CFHT headquarters in Waimea. CFH12K now
has twelve CCDs that qualify for an imaging application (Figure 1).
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Figure 1:
Left: CFH12K focal plane - twelve 2k x 4k CCID20 MIT/LL CCDs forming
a 12,289 by 8,192 pixel mosaic. Right: A single pre-processed image
of an M 31 field
showing the cosmetic quality of the whole mosaic.
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3. Mosaic organization
Achieving the highest data quality at the center of the field to
benefit from the best optical quality (no optical aberration versus
the edges of the 1 square degree field, CFH12K covering 42 by
28 square arcminutes) naturally places the CCDs with the best
combination of QE, CTE and cosmetic in the central 8k by 8k
mosaic. CFH12K has a central 4k by 8k mosaic nearly perfect
in those terms.
There are two types of CCID20 devices: those made out of
standard epitaxial silicon (EPI) and those made out
of high resistivity bulk silicon (HiRho). The latter
have a higher QE (up to 10%) in the red part of the spectrum
(Figure 4) and produce less fringing than EPI parts due
to their larger thickness. The CFH12K contains both types
of devices. They are grouped within the mosaic (all the HiRho
parts packed together in the lower right corner) in order to
cover large areas on the sky with a similar response (Figure 2).
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Figure 2:
Left: Organization of the mosaic per CCD type.
Right: Organization
of the mosaic per banks (1 bank per controller,
one output type per bank).
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Each of the two banks of 6 CCDs (12k by 4k) is handled by an
SDSU Generation II CCD controller (Leach et al. 1998).
This controller, optimized to handle mosaics of identical
detectors, constrains the organization of the CCD readout
amplifiers (each bank must have the same output per CCD).
A few of the CCDs available for CFH12K only have one of the two
outputs functional, hence one bank had to be read only from
the left outputs and the second one from the right outputs (Figure 2).
4. Mosaic geometry
With a short beam of f/4.2, depth of field is critical on such
a large surface (21cm by 14cm). Emphasis was given
on designing a focal plane structure that would achieve a
flatness of 60 microns, the depth of field at the CFHT prime
focus for a 0.4 arcsecond seeing (pixel size is 15 microns,
providing a sampling of 0.2 arcsecond per pixel). The CCDs
individually are flat within 20 microns. They are mounted on
a package standing on three shims which can be machined to within
2 microns after proper measurement of the CCD's height at its
four corners. With the twelve CCDs mounted on the molly plate, a
flatness of better than 100 microns is measured.
This is not enough to degrade images taken under even
the best seeing conditions available at CFHT:
during first light on the sky in January 1999, image quality
was checked on 0.5 arcsecond seeing data and no image degradation
larger than 0.04 arcsecond could be measured across the field.
The wide-field corrector induces a radial image distortion,
forcing resampling of the data to achieve proper astrometric
properties on the observed field when needed. As a consequence
and due to an increasing complexity, the relative alignment of
the CCDs along the X and Y axis was a low priority. Nonetheless,
proper manual mounting of the CCDs led to an amazingly good
relative alignment of the devices: the gaps between the CCDs
(both in X and Y) range from 30 to 38 pixels (about 500 microns)
and the relative angles between devices range from -0.3 degree
to +0.3 degree.
5. Device optimization
The CFH12K mosaic was optimized and characterized with
all the CCDs integrated in the cryostat. While this speeds up
the optimization process by having all the data from the twelve
CCDs at once, it implied a software effort to develop all the
automatic procedures to reduce the large amount of data produced.
Those tools allowed completion of the optimization and calibration
phase in the CFHT headquarters CCD lab in less than two weeks.
Subsequent knowledge of the detectors and the mosaic was gained
through tests and general use by observers on the sky.
5.1. Readout speed
Achieving the highest observing efficiency was a major
highlight in the definition of the CFH12K. The SDSU
Generation II CCD controller was chosen since it was
a logical continuation of CFHT's involvement with this
type of controller over the past years and most
importantly would result in the camera performance to
being limited by the detector, not the controller.
Running the two controllers unsynchronized in parallel
during the readout results in high pickup noise.
During development of the camera, DSP code was not available
from SDSU to facilitate synchronizing the two controllers.
Since having this capability would reduce the total readout
time by a factor of two, CFHT implemented this code in-house.
A DSP routine allowing a synchronization within 6 ns was
developed. With the serial CTE being the limiting factor on
three CCDs, a readout time of 58 seconds for the whole
mosaic is achieved (6.7 microsecond per pixel, or 150
kilopixels per second per CCD or 1.8 Mbytes per second per
controller). The limit of the data acquisition system
itself (custom interface cards, see Starr et al. in this
proceeding) is reached at a 40 second readout.
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5.2. CTE
CTE must be high enough to avoid degradation
of the image quality (point spread function, PSF) in 0.6
arcsecond seeing conditions along a 2,048 pixels transfer.
With a 0.2 arcsecond per pixel sampling, a serial CTE of at
least 0.99994 has to be achieved such that the PSF
degradation is not larger than 10% after a whole serial
register transfer. To get the highest CTE, CCDs are run
at -89 deg. C and the serial register timing is slowed down.
Three CCDs in CFH12K caused this limitation in readout
speed to the whole mosaic (serial CTE is higher than 0.99999
for all the other CCDs). Parallel CTE is at least 0.99999 for
all devices.
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5.3. QE response
The rather high running temperature of -89 deg. C is
not only motivated by the need for the best CTE. It
also causes the QE to increase in the red part of
the spectrum by up to 10% compared to -110 deg. C.
The high temperature is also motivated by the fact
that it minimizes the peak-to-peak amplitude of the
brick wall pattern (BWP, Wei & Stover 1998), a feature of the
early CCID20 devices resulting from the laser annealing
process (Figure 3). Typical amplitude of the BWP on the
worst CCDs is 10%. See also the "brick wall pattern" paragraph
in this article.
QE stability across the chips and in time is crucial to
achieve perfect photometric stability. The brick wall pattern
being highly dependent on temperature, a fine thermal
regulation of +/- 1 degree is required (see accompanying
paper by Starr et al.).
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5.4. Dark current
In direct imaging application, the sky background
noise quickly dominates the signal noise even with the use
of narrow band filters. Hence, the dark current was not
considered as an important constraint but even the high
running temperature of -89 deg. C actually produces only
around 1 electron per minute per pixel. This is a very low value
that can be neglected in broad band imaging mode and
corrected by a simple constant in narrow band imaging
mode.
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5.5. Linearity
To achieve the photometric accuracy essential to any
astrophysical program, a linearity of within 1% over
the whole range of the 16 bits analog to digital converter
is required. An automatic acquisition and analysis program
was developed to map the output drain and reset drain
voltages over a 3 volts range with steps of 0.2 volts.
The optimal working point was simultaneously computed
for all the CCDs by looking for the point where both
readout noise and linearity residual tend to be minimal
while the gain reaches a plateau.
No influence on the linearity was found when changing
the reset gate and the output gate voltages in a
reasonably low range around the typical values advocated
by MIT/LL.
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5.6. Gain
The natural gain of the CCID20 devices is very high and
to obtain a readout noise sampled on at least two to three
analog to digital units (ADU) to allow proper signal
analysis, the lowest available gain on the CCD controller
had to be selected. The average gain is 1.6 electron per
ADU (1.4 to 2.1 over the whole mosaic).
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5.7. Detector readout noise
A minor constraint due to the rapidly dominating sky
photon noise, the readout noise was derived from
the best optimal working point defined during
the linearity optimization. Readout noise is
very low for such a high readout speed: typically
5 electrons with a correlated double sampling time
up and down of 1 microsecond each.
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5.8. Other noises
As emphasized by Starr et al. in this proceeding,
high attention was given to the general organization
of the whole system. As a result of proper grounding
within the highly noisy telescope environment, no pickup
noise is affecting the camera performance.
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Figure 3:
Left: A B-band flat-field exhibiting the brick wall pattern.
Right: An I-band fringe
frame. HiRho parts are on the lower
right corner while the latest EPI parts with a new
coating
can be seen on the top bank with no fringing.
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5.9. Full well
Full well must be optimized (parallel voltages) to match
at least the 16 bits ADC dynamic based on the gain setting.
Typical full well of the CCID20 is 150,000 electrons while
the gain setting for CFH12K leads to a digital saturation
at 100,000 electrons using manufacturer's standards voltages.
Saturated bright stars in such wide field are actually the
main cause of pixels lost for science, well before
bad cosmetic. Achieving higher full well capacity
by increasing the swing between the low and high
levels of the parallel clocks is the only solution
but comes with a drawback: spurious noise injection
that forces a bias frame correction for each image.
However, CFH12K runs with the MIT/LL standard parallel
voltages since most of the area affected by blooming
lies within the obliterated region of the reflection
halo caused by the bright object (low level reflection
bouncing back from the CCD to the front window then back
to the CCD, a one to two percent effect).
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5.10. Dithering anti-blooming
As experimented during the early phase of CCD wide field
imaging at CFHT (Cuillandre et al. 1996), the dithering
anti-blooming mode developed by J. Janesick (1992) reduces
the blooming contamination caused by saturated bright objects.
It eliminates exceeding charges using the non-perfect nature
of silicon (presence of traps at the Si/SiO2 interface)
but with the progress made in the past decade, silicon
has improved in quality and today it becomes difficult
to use this feature without being heavily affected by the
drawback of this technique: spurious noise. Also,
some CCDs exhibit a large number of pocket pumping
sites (Tonry et al. 1997) that can't be corrected by
standard techniques during the data reduction (they
are basically dead pixels).
Dithering anti-blooming was then abandoned on CFH12K since
the high full well keeps blooming to a reasonable level.
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5.11. Residual image
The CCID20 devices are known to suffer from
residual image (Wei & Stover 1998). Inter-image
contamination has to be as low as possible since
it affects the photometry of any object on top of
a previously over exposed area of the detector. The
residual image appears only on some CCDs to a various
degree and is always associated with a pixel that
reached over the full well storage capacity (it is
not only located to the bright object site but also
the blooming aisles). The residual image is about
20 electrons per pixel on a 5 minutes integration
after a saturation state.
The SDSU Generation II controller does not easily
allow three clocking states, hence setting the CCDs
in full inversion mode is not yet implemented. A full
inversion mode would promptly re-populate all the sites
at the interface and eliminate all the trapped electrons.
In the meantime, the anti-blooming dithering mode proved
to be a way of reducing this amount of charges
by clocking at very high frequency the parallel
clocks during the very short lapse of time available
between two exposures (1 second usually).
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5.12. Crosstalk
A source of intra-image contamination, crosstalk is
a natural concern for multi-readout systems. Crosstalk
could not be measured between two independent video
boards (see Starr et al. for a description of the
CCD controllers), and a negligible value of 8.10E-5
between two channels of a video board was measured.
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6. Mosaic characterization
6.1. Cosmetic
While the best CCDs in terms of QE, CTE and cosmetic
are at the center of the mosaic, the entire CFH12K
exhibits an excellent cosmetic quality. There
are approximately 200 bad columns (bad column is
defined here as an entire or a fraction of a column),
which are actually concentrated in the lower right corner
CCD (Figure 1). The number of bad pixel clusters is
very low and overall the ratio of bad pixels over
the whole mosaic is only 0.4%.
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6.2. Brick wall pattern
While reorganizing the CCDs in August 1999, the early
phases CCDs with bad brick wall pattern have been
pushed out in the corners (Figure 3). These devices
exhibit a peak to valley amplitude up to 5 to 10% (worse
at shorter wavelengths, i.e. the B band). The most recent
CCID20 devices show negligible signature of this effect (1%,
seen at the center of the mosaic).
The QE brick wall pattern is a multiplicative effect corrected
during the flat-fielding operation. Building a superflat from
night sky data is the best solution but is not always possible
due to a low number of frames on "empty" fields. Twilight
flat-fields flatten very well beyond 600 nm but are a bit
marginal below (B and V bands) because the twilight sky
spectrum differs quite significantly at those wavelengths
from the dark sky spectrum. This is however the only way
to collect high signal to noise calibrations, hence
special automatic tools have been developed to optimize the
number of frames acquired in the short amount of time the
sky brightness is within the right range. This is not totally
satisfactory though and CFHT is investigating a dome flat-field
screen with a set of lamps that would reproduce as closely as
possible the shape of the night sky spectrum.
With the dependence of brick wall pattern amplitude versus
wavelength, there was a concern on the color effect on
astronomical sources. Tests on the sky show that no effect is
introduced, proving that the flat-fielding operation corrects
for the variations. The only impact is the detection limit
that slightly differs from peak to valleys but this
can be averaged using a dithering pointing mode during
a set of exposures that will move the field around on
the focal plane at the BWP geometric scale (60 pixels).
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6.3. Fringing
The HiRho parts exhibit a very low fringing of 0.5%
in the I band due to the higher thickness of the CCD:
30 microns vs. 15 microns for the EPI parts which suffer
from a fringing of 5% at worst.
The latest development in coatings (J. Tonry, private
communication) actually brings fringing to a negligible
level on the EPI parts. Figure 3 shows
a CFH12K fringe frame: the three bottom right CCDs are
HiRho parts, the top right CCD is a standard EPI part
while the CCD to its left (from the latest draft) has
the new coating.
The fringing correction relies on obtaining twilight
flat-fields to access the fringe frame from the flat-fielded
dark sky images. Then, the fringe frame has to be scaled on
a per exposure basis and subtracted. The fringing in the Z'
band is worse by a factor of two relative to the I band.
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Figure 4:
Left: CFH12K faces down from the CFHT prime focus onto
the primary mirror. Not
shown on this diagram are the
power supplies attached to the wall of the prime
focus cage.
Rotation of the instrument is not allowed.
Right: QE response of the two types of CCDs (EPI
and HiRho)
and transmission curves of the filters available
for CFH12K.
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7. Collecting more photons for CFH12K
7.1. Prime focus
The twenty year old prime focus cage needed some modifications
to accommodate this extremely sensitive camera. Experience
with previous CFHT wide-field imagers (MOCAM and UH8K, see
Cuillandre et al. 1996) showed that scattered light from
various sources is a critical limitation for deep imaging
(flat-fielding was usually poor). Indeed, the prime focus
and the wide-field corrector (WFC, see figure 4) were
designed for use with photographic plates and none of the
surfaces had been blackened properly, neither were baffling
rings installed inside the corrector. The whole unit has
been revisited (adding highly low reflectance black velvet
everywhere possible and baffling rings) and large scale
flat-fielding of CFH12K data is now very efficient (within
a percent).
To improve the total efficiency of the instrument, new
anti-reflection coatings were installed on the wide field
corrector lenses, eventually bringing the total efficiency
of the primary mirror plus the three lenses from the WFC
to 73%.
Bright stars are an inevitable source of contamination
for such wide field instrument. Proper baffling
significantly reduces the contamination from most of them but
very bright stars on the field (less than the 6th magnitude)
will obliterate a large fraction of the data. A special
warning tools (Telescope Field Monitoring) has been developed
to prepare the observations and also to be used during the
instrument operation.
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7.2. Filters and zero points
The CFH12K interference filters were produced by Barr Associates
(see accompanying paper by Starr et al.) and all have a
transmission higher than 85%. The eight filters currently
available are shown on figure 4. Note that the Z' filter
is open, the high frequency cut down being defined by
the drop of the CCD QE. The HiRho parts definitely are a
benefit in this band.
With a transmission of 99% for the cryostat front window,
the overall efficiency of CFH12K at prime focus at 650 nm
is 57% when one includes all the telescope optics plus CCD.
This is the highest efficiency ever achieved by an instrument
at CFHT. The zero points at zero airmass in electrons per
second for the four main photometric bands are: B=26.2,
V=26.6, R=26.6, I=26.2.
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8. CFH12K on the sky
8.1. Observing efficiency
Not only is the camera efficient in terms of sensitivity and
readout time, but also a set of efficient tools have
been developed to optimize the use of the observing
time. CFH12K can run with scripts that minimize the
human input for standard operations (dithering modes,
focus sequences, twilight flat-fields sequences,...).
With either a graphical interface, a command line
interface and a scripting language, any new observer is
at ease within the first minutes. With typical exposure
times of 5 to 10mn, a global observing efficiency of
90% or more is commonly achieved. This data rate leads
to a typical 20 Gbytes of data per night (200 Mbytes per file).
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Figure 5:
Left: M 31 and NGC 205 in the full field of view of CFH12K.
This image
was obtained from a composition of a set
of images in the B,V and R bands (resp. 40
mn, 20 mn
and 10 mn). Data reduction and composition were
conducted using the
FLIPS package developed at CFHT.
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8.2. Scientific programs
With a subscription rate of 50% of the whole CFHT observing
time, CFH12K is the most popular instrument within the
French, Canadian and Hawaiian astrophysical communities.
This comes as no surprise when one looks at the global
performance of the instrument: covering 42 by 28 arcminutes
at 0.2 arcsecond per pixel with a median seeing of 0.7
arcsecond. Image quality as good as 0.45 arcsecond has
been often achieved on 10 minute exposures.
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Figure 6:
This mosaic of images illustrates the tremendous
power of CFH12K
at the CFHT atop Mauna
Kea. High resolution wide field imaging allows
probing
the Universe on many scales. The
image quality on these 2h30mn exposures (both
in B and I) is 0.7 arcsecond. The last image
(40x40 pixels) of a distant spiral galaxy
beyond
the nearby galaxy NGC\,3486 shows the true
pixelization from the detector.
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The pressure on the instrument is high and emanates from
all the observational astronomical communities. Here are
a few examples:
- Solar system: Kuiper belt objects
- The Galaxy: gravitational tidal effects on stellar clusters
- Nearby galaxies: dust and stellar populations
- Cluster of galaxies: luminosity functions of dwarf galaxies in Virgo and Coma clusters.
- Cosmology: deep wide-field imaging survey &
wide-field weak lensing survey
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8.3. High resolution wide field imaging
The following images on figures 5 and 6 illustrate the power
of wide field imaging associated with high resolution
capabilities, the major strengths of CFH12K.
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9. Conclusion
The CFHT and its location already promote excellent image quality.
CFH12K allows the CFHT to realize the high resolution wide field
imaging potential of its prime focus. The performance of the CCD,
the camera, and data acquisition environment makes CFH12K the
most sensitive and efficient instrument in CFHT's history. In
sky coverage and efficiency, it surpasses the previous CFHT CCD
mosaics and the competing large mosaic arrays around the world.
Despite some troublesome CCDs within the mosaic, they all have
been successfully optimized for use in this direct imaging mode
application. With proper observing techniques and data reduction
tools, the detector signature can be removed and large monolithic
12k by 8k scientific frames can be obtained. As a result, CFH12K
is heavily subscribed by the CFH astronomical community, occupying
about half of the programmed telescope time.
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Cuillandre, J.-C., Mellier, Y., Dupin, J.-P., Tilloles, P., Murowinski, Crampton, D., Wooff, R., Luppino, G., 1996, PASP, 108, 1120
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Janesick, J., 1992, Applied Optics, 31, 4890
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Leach, R., Beale, F., Eriksen, J., 1998, SPIE, 3355, 512
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Luppino, G., Tonry, J., Stubbs, C, 1998, SPIE, 3355, 469
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