Instrument Science Report ACS-97-01
HST Reference Mission for Cycle 9 and
Ground System Requirements
M. Stiavelli, R. Kutina, and M. Clampin
October 1997 - Version 1.2
ABSTRACT
We describe the expected usage of the Advanced Camera for Surveys during cycle 9 and
estimate ground system requirements for the full complement of HST instruments available for cycle 9. Assuming parallel observing with all four of the cycle 9 instruments, we
estimate a worst case average daily downlink of 16.5 Gbits/day (science bits excluding
any overheads) if the ACS WFC images are not compressed onboard and 11.0 Gbits/day
with compression of the WFC images. We foresee the potential for peak daily volumes of
22.1 Gbits/day or 13.2 Gbits/day without and with compression, respectively. We recommend that the current ground system throughput capacity be doubled to support an average daily capacity of 12 Gbits/day and a peak capacity of 18 Gbits/day following the third
servicing mission. We estimate the OPUS processing requirements for the ACS pipeline to
be 700 STIS CCD equivalent images/day, compared to the current 200 STIS CCD equivalent images for the cycle 7 instruments. We estimate the total archive requirements for the
full complement of cycle 9 instruments to be 11.4 GBytes/day compared to the 4.5 GBytes/
day currently archived for the cycle 7 instruments.
1. Introduction
This ISR defines the anticipated usage of the Advanced Camera for Surveys, ACS,
during a typical observing cycle as well as part of the overall HST observing program for
cycle 9, i.e., for the first cycle following the installation of ACS and, possibly, of the NICMOS Cryo-cooler (NCS). From this estimate of typical ACS usage (exposures per orbit),
we derive the expected data volume following the third servicing mission.
Section 2 provides an overview of the ACS instrument from an operational and science
perspective and a brief comparison of ACS with other imaging instruments. Section 3
defines the typical ACS observations for GO, GTO and calibration programs as well as the
predicted data volumes for the full compliment of cycle 9 instruments. Section 3 also esti-
1
mates the changes in usage of the other SIs in the presence of ACS. Section 4 predicts the
requirements for science telemetry downlink, science data processing, and data archiving
for ACS and the entire observatory after the third servicing mission.
2. ACS Characteristics and Usage
2.1 The ACS Performance Compared to Previous Instruments
ACS has three optical channels. The Wide Field Camera (WFC) is designed to have
high throughput (36% at 7000 Å), wide field (200’’×204’’), and a sampling similar to that
of the WFPC2 Planetary Camera (50 mas against 46 mas). The WFC employs a mosaic of
two thinned, backside illuminated 2048 × 4096 SITe CCDs. The High Resolution Camera
(HRC) is designed to be critically sampled at 5000 Å and is expected to have a particularly
high throughput in the near-UV (down to 2000 Å). HRC includes a coronograph. The
HRC detector is a 1024 × 1024 thinned, backside illuminated CCD. The Solar Blind Camera (SBC) has a relatively high throughput in the far-UV. The SBC detector is a MAMA
with a CsI photocathode. All cameras include also dispersive elements: a grism in WFC
and prisms in HRC and SBC. A common characteristic of the cameras is the use of very
high throughput filters, with very tight specifications over a large wavelength range so as
to avoid blue and red leaks. The key characteristics of ACS compared to the other imaging
instruments on HST are summarized in Table 1. Since the main purpose of this ISR is not
a complete description of the capabilities of ACS, we will mostly focus on how the ACS
compares to existing instruments and how its characteristics influence the way ACS and
the whole observatory will be operated. Further information on ACS can be found in Ford
et al. (1997) or in the Advanced Camera for Exploration (ACE) proposal.
2.1.1 WFPC2
WFPC2 is a wide field imager with wide spectral response and a large number of filters. ACS WFC was designed to provide very high throughput with better sampling than
WFPC2 over a FOV 2.4 times larger than that of three WF chips on WFPC2 (2.2 times
when all 4 WFPC2 chips are included). Longwards of 3500 Å WFC is expected to outperform WFPC2 by up to a factor 10 in “discovery efficiency” for broad band imaging and
most narrow band applications. “Discovery efficiency” is essentially the product of field of
view times instrument throughput. HRC is designed to provide a very high throughput in
the near UV with a full sampling of the PSF. Since detector noise limited imaging of large
extended targets shortwards of 3500 Å was not a design driver, for this, relatively rare,
kind of observation ACS will not gain significantly compared to WFPC2. In fact, the large
difference in size (on the sky) between WFPC2 WF pixels and HRC pixels and the larger
FOV of WFPC2 compared to HRC tend to offset the much higher efficiency of HRC. For
targets fitting the FOV and/or requiring the better sampling of HRC, HRC will also signif-
2
icantly outperform WFPC2. In a similar way the SBC will have a throughput significantly
higher than that of WFPC2. It is likely that even for extended targets the much higher
throughput compared to WFPC2 and the absence of read-out noise and the very low value
of dark current will make the SBC preferable to WFPC2.
Regardless of throughput, the better sampling of ACS compared to WFPC2, and the
very well defined filters should result in improved photometric precision. ACS will also
make possible more accurate polarization studies, particularly in the WFC camera. Very
accurate astrometric studies require a flat CCD with uniform pixel response and high quality corrective wide field optics. ACS has a larger geometric distortion than WFPC2 and
uses thinned, back-illuminated CCDs rather than the, nominally flatter, front-illuminated
devices of WFPC2. On the other hand, front illuminated CCDs tend to have non uniform
pixel response. It is likely that astrometry will require an extensive calibration program for
ACS which will be justified only in the presence of a significant number of proposals.
Sampling at
5000 Å
Instrument
Maximum
Throughput %
Equivalent
FOV
Polarization
Accuracy
ACS-WFC
36@7000 Ň
200” × 204”
Half
<1%
ACS-HRC
25@6000 Ň
26” × 29”
Full
<5%
ACS-SBC
6.1@1216 Ň
26” × 29”
Half
NA
WFPC2-WF
14@6000 Å
130” × 130”
Quarter
4%
WFPC2-PC
14@6000 Å
35” × 35”
Half
4%
FOC
1.6@1500 Å
7” × 7”
Full
1%
STIS-CCD
22@6000 Å
51” × 51”
Half
NA
STIS-NUV
2.0@2800 Å
25” × 25”
Half
NA
STIS-FUV
4.5@1216 Å
25” × 25”
Half
NA
NICMOS-NIC3
12@10000 Å
51” × 51”
Half
<2%
Table 1. Comparison of ACS with other HST optical-UV imaging instruments. The better
polarization accuracy expected for WFC compared to HRC is due to the properties of the
protected silver coatings of WFC. ‡ indicates a design goal. $ the STIS CCD FOV with filters is 28” × 51".
2.1.2. FOC
The SBC and the HRC are designed to outperform FOC in all far-UV and near-UV/
optical applications, respectively. FOC is fully sampled in the UV while the SBC is only
half-sampled since many of the science goals required surface brightness sensitivity rather
than resolution. The lack of full PSF sampling should not significantly affect most of the
UV science, one possible exception being imaging of stellar disks and of low angular
3
diameter solar system bodies. (FOC will not be available after SM-3 since ACS will be
installed in the bay currently occupied by FOC).
2.1.3 STIS
STIS has two modes offering similar capabilities to ACS. Optical CCD imaging is
offered with [OII], [OIII], and longpass filters, with a reduced FOV, or with a clear aperture over the whole FOV. Near-UV is possible with both the CCD and, especially, with the
near-UV MAMA detector. The STIS CCD also provides a limited coronographic mode.
The STIS FUV imaging capability is similar to the SBC. The main advantages of ACS are
a wide selection of filters, higher throughput, higher FOV for the WFC and better sampling for the HRC compared to the STIS CCD.
2.1.4 NICMOS
NICMOS offers a near-IR imaging capability from 0.8 to 2.5 micron with FOV ranging from 11” × 11” to 51” × 51”. NICMOS also has an apodized coronograph in camera
two. In the region 0.8 to 1.0 microns there is some overlap between NICMOS and ACS
capabilities. In general, CCDs provide superior performance to IR-arrays in this region
and the WFC optimization for the 8000 Å region makes it the preferred instrument at these
wavelengths.
Observation type
Instrument of choice
Optical imaging (λ >3500 Å)
ACS-WFC
Near-UV imaging of small targets (2000 Å < λ < 3500 Å )
ACS-HRC
Far-UV imaging on faint targets (λ < 2000 Å)
ACS-SBC
Near-UV imaging of very extended targets (2000 Å < λ < 3500 Å )
WFPC2
Far-UV imaging of very bright targets (λ < 2000 Å)
WFPC2
Accurate photometry
ACS
Accurate polarimetry
ACS-WFC
Table 2. Schematic summary table of the optimal instrument for various types of observations. By small and extended targets we intend, respectively, targets smaller or comparable
or significantly more extended than the HRC field of view. By faint or bright targets, we
intend, respectively, targets fainter or brighter than the MAMA bright object protection
limit. With imaging we intend both broad and narrow band applications.
2.2 Operating and Science Modes
For science observations, each of the three ACS channels will be operated in a simple
ACCUMulation mode in which the signal accumulated at the detector during the exposure
4
is read-out and stored in the instrument’s science data buffer as an n × m integer array. The
HRC has an onboard target acquisition mode to support coronographic science observations. These support exposures will typically have short exposures times and generate a
small amount of data by using a 100 × 100 or 200 × 200 detector subarray. ACS will not
support any other operating modes, in particular no time-tag mode is available for the
SBC.
The sizes of full frame images are 4146x4136, 1064x1044 and 1024x1024 for the
WFC, HRC and SBC respectively. The sizes available for scientific exposures are, respectively, 4096x4096 and 1024x1024 for WFC and HRC. The additional pixels are physical
and virtual overscan areas. These images require 32.71 MBytes, 2.12 MBytes and 2.00
MBytes of the available 34 Mbytes of science data buffer memory. Hence, the buffer can
store 1 uncompressed WFC frame or 16 HRC or SBC frames before a buffer dump to the
HST SSR is required. The readout time following WFC and HRC exposures for full frame
images is 96 seconds and 26 seconds respectively while the SBC readout is concurrent
with the exposure.
The WFC and HRC are also capable of reading out a subarray of the detector which
reduces the data volume and readout time of the exposure. In the case of WFC a subarray
has to be contained within one of the two WFC chips, so that the maximum subarray is
about half the WFC field of view. The read-out of such a large array would require 4 minutes. Onboard data compression of WFC images is also available (see Section 2.3).
Camera
setup
(min)
read-out time
(min)
buffer dump
time (min)
buffer space
(# frames)
WFC not compressed
1.6
1.6
5.6
1 WFC
WFC compressed
1.6
2.9
3.0
1 WFC (+ 8 HRC/SBC)
HRC
1.6
0.5
0.8
16 HRC
SBC
0.7
0.0
0.8
16 SBC
Table 3. Exposure overheads for the three ACS cameras. The overheads are not yet final.
As we have seen in 2.1 ACS is primarily an imaging instrument with some spectroscopic capabilities. Its science modes are:
•
imaging
•
slitless spectroscopy
•
polarimetry
•
coronography
The slitless spectroscopy and the polarimetry mode can in principle be used in combination with the coronograph to provide spectro-coronographic and corono-polarimetric
capabilities. The performance and scientific merit of such combinations have not been
5
assessed yet. It is likely that spectro-coronography will, at most, be useful for very specialized (and rare) observing proposals. Spectro-polarimetry may be used by a significant
number of GO and GTO proposals.
2.3 Data Compression
A single ACS WFC frame has a size 6.6 times larger than a full WFPC2 frame, due to
the larger number of pixels (16 M rather than 2.4 M). The sheer size of these images led
the ACS IDT to implement on-board, lossless, compression based on the White pair algorithm (see e.g. ISR ACS-97-02). The resulting compression factor is most likely in the
range 2 to 3. According to the current plan, compression will be implemented in the
ground system in the late 98.
When compression is enabled, the output of one of the (typically) four read-out amplifiers of WFC (each of the two WFC chips has two read-out amplifiers) will be compressed
on the fly. The remaining three quarters of the WFC image will be temporarily stored in
the science data buffer for compression after read-out. In the expected four amplifier readout of WFC, data compression requires an additional 79 seconds per frame following
read-out. In the current plan for the ground system implementation, the 79 seconds are an
additional overhead when compression is used, e.g., the following exposure cannot start
until data compression is completed. Since during sub-array operations only one amplifier
is used, compression for sub-arrays does not lead to any additional overhead.
During compression 2048 pixels of data are compressed at a time using a pre-allocated
buffer with size determined by dividing 2048 by the assumed compression factor. Should a
higher compression factor be achieved the buffer would be filled in with pads. Lower
compression factors would lead to loss of data. Thus, it is extremely important to
determine a minimum guaranteed compression factor.
In the following analysis we will consider both the case of no compression and that of
compression with a factor two or three for calibrations and a factor two for science data.
All tests conducted so far suggest that a factor two compression should be easy to achieve
and represents a conservative choice guaranteeing that no science data are lost. Once more
extensive tests of the compression algorithm are carried out we should be able to revise
this estimate as necessary. The factor three compression for calibrations takes into account
that typical biases, darks, and flat fields are obtained by combining tens of frames. Such
frames should be easier to compress than science data and in addition we can afford to
accept for such calibration images a somewhat higher risk factor than for science observations. Clearly this applies only to routine calibrations (either internals or earth flats).
Since it is likely that compression on the fly of a whole WFC image will not be possible, only one WFC frame at a time will in any case be stored in the buffer memory
regardless of compression. Compression simplifies parallel operations by allowing us to
6
store in addition to a WFC frame also some HRC and SBC images and also reduces the
buffer dump time. Given that the amount of time required to perform a buffer dump when
a WFC frame is stored is in the range of 3-6 minutes, we should expect that full frame
exposures shorter than this limit should be relatively rare unless the observer is willing to
accept significant overheads. Compressed sub-array images will be required for observers
intending to obtain several WFC exposures in a single orbit.There is no plan to compress
HRC or SBC frames.
3. ACS Reference Mission
In order to estimate the usage of the various cameras of ACS we rely on a number of
indicators: the existing WFPC2 and FOC archive, the approved cycle 7 STIS observations,
the planned ACS IDT GTO program, and the new capabilities offered by ACS compared
to those of existing instruments.
area
Fractional Num. Exp.
Exp. Time (s)
Clusters of Galaxies
11.9
1456
Galaxies
40.9
658
Interstellar Medium
4.2
654
Solar System
13.1
180
Stars
17.3
505
Stellar Clusters
10.6
445
Other
2.0
680
Table 4. Percentage of the total number of WFPC2 and FOC exposures and mean exposure time as a function of scientific program.
3.1 The existing instruments: UV vs optical imaging
As of July 1st, 1997 the STScI archive contained 16589 WFPC2 GO science frames,
161 post-SM1 FOC GO science frames, 907 WFPC2 GTO science frames, and 4044
WFPC2 snapshot frames. From our analysis we excluded GTO proposals executed on
WFPC2 but not belonging to the WFPC2 IDT, DD, calibration, SMOV, parallel and carryover proposals. Our primary intent was to have a well defined, homogeneous data set representing GO + GTO primary science for WFPC2. When considering the combined set of
21701 images, one finds that about 2.8% were obtained shortwards of 2000 Å (and thus
would have been suitable for the SBC), 10% were between 2000 and 3500 Å (and thus
would have been suitable for the HRC). Table 4 gives the percentage of the total number
of exposures and the mean exposure time for different scientific areas.
7
STIS GO observations in cycle 7 (those with valid phase II by mid august 97) are for
the 66% CCD observations (39% spectroscopy and 27% imaging), 22% FUV MAMA
(20% spectroscopy and 2% imaging) and 12% NUV MAMA (11% spectroscopy and 1%
imaging). Thus, 30% of STIS observations are imaging and 70% spectroscopy. This result
would not be significantly affected by the inclusion of GTO observations (25% imaging,
75% spectroscopy). A lot of these imaging observations will presumably be taken over by
ACS once it is installed.
In our analysis we have chosen not to consider NICMOS cycle 7 observations since
their wavelength range has little or no overlap with the wavelength range of ACS.
3.2 The expected ACS GO program
By considering the characteristics of ACS and the science requirements of the various
areas of WFPC2 and FOC usage (Table 4) one can infer how the same programs would
have been carried out on ACS. It is reasonable to assume that all observations of galaxies,
clusters of galaxies and stellar clusters will benefit from the large field of view of WFC.
We can assume also that 50 per cent of the interstellar medium projects will also use WFC.
By assuming that the remaining projects would be carried out on HRC unless they require
far-UV filters, we obtain the following usage estimate as a percentage of the total number
of ACS exposures: WFC 67 percent, HRC 30 percent and SBC the remaining 3 per cent.
We can determine the mean exposure times for these categories of WFPC2 images, finding that the mean WFC, HRC, and SBC exposures would be 800 s, 400 s, and 950 s,
respectively. Given these exposure times, primary usage as a percentage of ACS primary
orbits would be 78% for WFC, 18% for HRC and 4% for SBC. It is expected that even
when WFC is not the primary instrument it will still be used in parallel mode.
For most kinds of observations we expect that the observers will use exposure times
comparable to those used for WFPC2. Reducing the exposure time would lead to lower
observing efficiencies. By keeping them essentially the same observers will obtain deeper
exposures thanks to the higher sensitivity of ACS. The long buffer dump time may force
observers who need to obtain many short exposures in a single orbit to read only a subarray. This will in turn reduce the total downlink load.
WFPC2 contains a number of narrow and medium band filters, representing,
respectively, 10% and 5% of the filter usage. In addition the quads and ramp filters have a
combined 6.5% GO usage. Filters that are not present in ACS will be replaced by the ramp
filters, whose usage will increase from 5.5% to 13%.
The straight conversion from WFPC2 and FOC to ACS does not take into account the
qualitatively new capabilities of ACS. The new or improved coronographic, spectroscopic
and polarimetric capabilities will presumably attract users thus reducing the fraction of
standard imaging programs. In the case of the coronograph this increases the number of
8
HRC programs compared to WFC by probably a few per cent. The smaller FOV of HRC
compared to WFPC2 will also increase the number of HRC exposures compared to that of
near-UV WFPC2 exposures. A reasonable assumption is that those programs for which a
single HRC FOV does not cover the target (say 50%) may on average need to cover a
square arcmin, i.e. half of the near-UV programs make use of 4 pointings. This would
increase the HRC usage from 18% to 33%. A fraction of HRC and WFC programs using
ramps will also need more than one pointing, but we suspect that the two effects with cancel out once the relative usage of HRC and WFC is considered. The HRC fractional usage
may increase to 35 per cent when the coronograph usage is taken into account.
Figure 1: Histograms showing the cumulative distribution of WFPC2 Exposure Times.
The shaded area is for GO and GTO exposures only. The unshaded histogram includes
SNAPs. The arrow indicates the median exposure time (400 s).
The low noise of MAMA detectors compared to CCDs may lead to shorter exposure
times, even though long exposures on a single target to reach very high sensitivity may be
9
possible. The bright object count rate limits should also contribute to lengthen the mean
exposure times (since short observations will by necessity have low S/N).
Summarizing, the simple transposition of the existing archive of WFPC2 GO and GTO
and FOC GO observations into ACS suggests the following usage of the three ACS channels: WFC 61%, HRC 35%, and SBC 4%. Assuming an average target visibility after
target acquisition and overheads of 40 minutes, a typical WFC orbit will on average consists of 3 exposures, an HRC orbit of 6 exposures and a SBC orbit of 2.5 exposures.The
typical data rates without parallels will respectively be: 900 Mbit/orbit (450 Mbit/orbit
with compression), 100 Mbit/orbit, and 40 Mbit/orbit. The parallel usage of WFC during
SBC and HRC observations will increase the downlink load by 300 Mbit/s (assuming
compression). This compares to a typical WFPC2 downlink of 160 Mbit/orbit. Daily averages and peak data rates will be given in the following sections.
3.3 The ACS GTO Program
The IDT Science Program as described in the ACS DRM (Stockman and Bartko 1996)
can be divided into three areas: cosmology (470 WFC orbits and 20 HRC orbits), AGNs
(90 HRC orbits), and Solar System (5 WFC, 25 HRC, and 20 SBC orbits). This corresponds to a primary usage of 69% for WFC, 27% for HRC and 4% for SBC. The typical
exposures would be 1300 s for WFC, 800 s for HRC and 300 s for SBC. Thus, in terms of
relative usage of the three channels it appears that the GTO program is structured in a similar way to our expected GO usage. The GTO WFC and HRC exposures are longer, and
the SBC exposures shorter, than those we expect from typical GOs. The discrepancy is
easily understandable in the case of HRC and WFC since the IDT team is planning to
carry out deep imaging studies and therefore using longer exposures than typical GOs.
The discrepancy for SBC is somewhat more puzzling. Given that our requirements will be
dominated by WFC usage the results of our analysis should be relatively insensitive to the
details of SBC usage. In the following we will assume the somewhat longer exposures
suggested by the usage pattern of previous instruments.
3.4 Calibration Program
We have attempted to sketch out a schematic ACS calibration program for cycle 9. Our
basic assumption is that SMOV will by then be completed and that we only need to consider regular calibrations and monitoring activities. Other assumptions are that UV
contamination monitoring is done with internal FF exposures before and after decontamination. Photometric calibration is done using fields containing stars of various colors so
that a single field can yield both the calibration and the color terms.The astrometric calibration is checked only for 2 filters in each CCD camera. We have assumed that WFC
filters will also be calibrated for HRC. The expected calibration program for WFC, HRC,
and SBC is illustrated in Tables 5, 6, and 7, respectively.
10
Item
No.
Photom.
Ext. Flats
Int. Flats
Wavecals
Total
Filters
12
4
12
Grism
1
1
1
Ramps
5
12
12
120
Polarizers
12
4
1
60
Astrom. Stab.
3
2
1
9
Photom. Stab.
36
2
2
144
Earth Flats
3
Bias frames
104
104
Dark frames
208
208
FF Stab.
2
216
5
100
26
300
24
48
Table 5. WFC expected calibration program. The total number of frames is 1235 for one
year, for a total of 3.39 frames/day
.
Item
No.
Photom.
Ext. Flats
Int. Flats
Filters
16
4
12
Grism
1
1
1
Ramps
5
4
4
40
Coronogr. Stab.
16
1
12
208
Polarizers
24
4
1
120
Astrom. Stab.
3
2
1
9
Photom. Stab.
36
2
2
144
Earth Flats
6
Bias frames
104
104
Dark frames
208
208
FF Stab.
2
100
Wavecals
Total
288
5
10
600
24
48
Table 6. HRC expected calibration program. The total number of frames is 1779 for one
year, for a total of 4.87 frames/day.
11
Item
No.
Photom.
Ext. Flats
Int. Flats
Filters
6
4
12
Prisms
2
1
1
Photom. Stab.
36
2
2
Dark frames
104
FF Stab.
1
Wavecals
Total
96
5
20
144
104
24
24
Table 7. SBC expected calibration program. The total number of frames is 388 for one
year, for a total of 1.06 frames/day.
3.5 Use of the Observatory in Cycle 9
Let us consider first a scenario where only STIS, ACS and WFPC2 are in operation.
Assuming that essentially all current STIS imaging is carried out by ACS we have a 35%
STIS usage, a 59% ACS usage and a 6% WFPC2 usage. The latter number is determined
by considering those proposals that involve detector-limited UV imaging of extended
sources, particular narrow band observations, and astrometric observations. We expect it
to be slightly overestimated.
Should NICMOS be operational in cycle 9, we would expect about 20% of NICMOS
usage, somewhat reduced compared to the current cycle 7 usage mainly due to the significant capabilities of ACS at 0.9 to 1 µm. Assuming the usage of NICMOS equally impacts
the usage of the other instruments, we would expect 28% STIS, 47% ACS and 5%
WFPC2. In case WFPC2 is not operational in cycle 9 we should expect the expected usage
of WFPC2 to proportionally augment ACS usage.
Note that in all scenarios we expect ACS-WFC, STIS CCD, NICMOS and WFPC2 to
be operated in parallel mode when not the primary instrument. The parallel use of WFC
was in particular seen by the ACS IDT as one of the mainstays of ACS operation.
3.6 Predicted Data Volumes
The predicted data volumes are illustrated in Table 8. We have computed for each of
the above scenarios the number of ACS frames and the downlink data volume with and
without compression.
For WFPC2, STIS and NICMOS usage we have assumed an usage pattern similar to
that indicated by the archive or by the cycle 7 usage. If time-tag does not see an increase in
usage then ACS is the primary driver for the total data volumes. For NICMOS we have
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assumed that MULTIACCUMs are the primary mode (i.e. no significant fraction of
ACCUMs with MIF reads).
When STIS is the primary instrument we have assumed that 3 exposures are taken on
average for each MAMA orbit (the number is artificially high to simulate the effect of the
time tag mode) and 2 for each CCD orbit. When WFPC2 is primary we have assumed 5
exposures per orbit as indicated by the usage in the past cycles. When NICMOS is primary
we have assumed that 200 frames are downlinked for each NICMOS orbit. The validity of
these assumptions has been verified by reproducing within 10 per cent the current usage
(1.3 Gbits/day for NICMOS and 0.9 Gbits/day for STIS).
For parallel exposures we have assumed 2 CCD exposures for each large field CCD
camera (STIS, WFC and WFPC2) and 100 NICMOS frames. These assumptions are
intentionally on the high side in order to test the effect of a massive parallel program.
Finally, we have included the effect of the calibration program by (conservatively)
doubling the numbers of exposures given in 3.4
.
ACS
Frames/day
ACS nc
Gb/day
ACS cmp
Gb/day
Tot.Dwnlnk
nc Gb/day
Tot.Dwnlnk
cmpGb/day
STIS+ACS+WFPC2
w/o parallel ops
57
6.9
3.7
8.2
5.0
STIS+ACS+WFPC2
w. parallel ops
76
11.9
6.2
14.7
9.0
STIS+ACS+WFPC2+
NICMOS w/o parallel ops
49
5.9
3.2
8.1
5.4
STIS+ACS+WFPC2+
NICMOS w parallel ops
70
11.5
6.0
16.5
11.0
STIS+ACS+NICMOS
w/o parallel ops
52
6.3
3.4
8.1
5.2
STIS+ACS+NICMOS
w. parallel ops
73
11.7
6.1
15.1
9.5
Scenario
Table 8. Data volumes in Gbits/day for various usage scenarios during cycle 9. “nc” indicates no WFC compression, ”cmp” indicates WFC compression. Note that these data volumes are for science bits and do not include any of the science telemetry overheads.
It is clear that if all instruments are operated in parallel we should expect a downlink
data volume requirement of up to 16.5 or 11.0 Gbits/day depending on whether the WFC
frames are compressed. These numbers depend on the assumption that all 15 orbits/day
are suitable for parallels. Should the real number of orbits suitable for parallel exposures
be similar to the current one (about 65 per cent) we should expect 13.6 or 9.0 Gbits/day,
13
respectively, for the uncompressed and compressed case. Thus, doubling the current
ground system average throughput capacity from 6 Gbits/day to 12 Gbits/day for cycle 9
will allow for some uncertainty in the compression factor allowed for prime, parallel, and
calibration data.
Clearly the maximum downlink volume in a single day could be much larger than this.
An upper limit to the peak usage can be set from the fact that the maximum rate at which
WFC images can be obtained is about one every 7 minutes, this results in 8 WFC frames/
orbit, i.e. a data volume of 2.1 Gbits/orbit non compressed or 1 Gbit/orbit compressed
(ignoring parallel exposures and internal calibrations taken during occultation). The estimate maximum daily downlink data volume requirement (for ACS alone) is thus 31.5
Gbits/day or 15.7 Gbits/day, non compressed and compressed, respectively. We believe
that such a limit is not completely unrealistic. In fact, about 50 per cent of WFPC2 exposures in the wavelength range of interest for the WFC on ACS have exposures shorter than
400 seconds. Assuming that the WFPC2 usage pattern is representative, we may have a
large number of ACS users trying to pack as many WFC exposures as possible in each
orbit. Even such a very high rate would be close to what is feasible as long as data compression is available. Note that the above rate would be almost doubled if internals during
occultation were also taken (or observations were done in CVZ). To handle such periods
of high peak-usage one may be forced to impose scheduling constraints and/or limits on
the number of WFC internals or parallels.
A more practical upper limit to the downlink rate can be obtained by assuming that
ACS WFC primary observations are done with the shortest exposure time but that the
daily fraction of ACS WFC is that derived from the usage scenarios discussed in 3.5.
Under this assumption the maximum downlink rates are 22.1 Gbits/day without compression and 13.2 Gbits/day with compression for all of the cycle 9 instruments. Thus, a
doubling of the ground system daily maximum throughput from 9 Gbits/day to 18 Gbits/
day is warranted.
4. Ground System Requirements
4.1 Hardware Requirements in OPUS
The ACS pipeline will presumably be similar to the STIS and the WFPC2 pipelines in
terms of computational requirements. HRC and SBC images will require approximately
the same amount of processing as STIS CCD and MAMA images respectively. WFC
images will require approximately 16 times the amount of processing of a single STIS
CCD image. Given the daily rates of section 3.6 we expect that ACS pipeline requirements
in the case of parallel operations will be equivalent to 700 STIS CCD equivalent images/
day. In the absence of parallel operations the total number of STIS CCD equivalent
images/day will drop to about 400.
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4.2 Archive Requirements
WFC data will be uncompressed on the fly during generic conversion. Therefore compression has no impact on the archiving requirements of ACS data. The archive
compresses all data upon ingestion.
According to the usage scenarios considered in section 3.6 we estimate that about 950
STIS CCD equivalent images will be archived each day, including all instruments. ACS
alone will produce about 700 STIS-equivalent CCD images per day. This corresponds to
1.4 GBytes/day if only the raw data are archived, or to 8.4 GBytes/day if both the raw and
the processed data are archived. The total archived data volume including all instruments
should be approximately 1.7 GBytes/day if only the raw data are archived and 11.4
GBytes/day if also the calibrated data are archived, compared to the 4.5 GBytes/day
archived during cycle 7. These volumes have been estimated assuming that for each raw
image we will also archive a calibrated image, an error image., and a data quality image.
4.3 Hardware Requirements in SMO
Typical calibration programs during SMOV will be comparable to the estimated calibration plan. We expect that it will necessary to store on line hundreds of WFC frames for
a number of applications, e.g., preparation of super-bias and super-dark frames or preparation of flat fields from earth-flat exposures. Since 100 calibrated WFC frames occupy 16
GBytes the disk space requirement of each Instrument Scientist or Data Analyst should be
at least as large as this.
Processing of such large amounts of data will require:
1. very significant CPU power. The factor 6.6 in frame size and 13.1 in CCD size of
ACS-WFC compared to WFPC2 make it necessary to use workstation about a factor 10 faster than those adequate for WFPC2 data.
2. very significant RAM memory. The size of WFC frames will similarly demand as
much RAM memory as it is reasonable to obtain (given that a factor 10 improvement is in this case difficult to achieve).
3. very large screens. A typical 1280 × 1024 monitor allows one to display one to one
a single CCD image from WFPC2. The same screen would only allow to display a
small fraction (1 part in 6.4) of a single CCD of WFC. Clearly, any feasible gain in
this respect would be extremely useful.
We expect that SSD instrument scientists and DAs will most likely have similar
requirements.
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5. References
Ford H., et al., 1996, The Advanced Camera for the Hubble Space Telescope, in Space
Telescopes and Instruments IV, Proc. SPIE, Vol. 2807 (Bellingham, WA: SPIE), 184.
Ford et al., 1994, Advanced Camera for Exploration (ACE)
Stockman, P., Bartko, F., 1996, Advanced Camera for Surveys: Design Reference
Mission Document.
6. Acknowledgments
The authors wish to thank Carl Biagetti, Chris Blades, Holland Ford, and Rodger Doxsey for a careful reading of the manuscript, Marc Postman for providing information on
the archive ingestion, Al Holm for information on OPUS processing and Wayne Kinzel for
providing the STIS and NICMOS cycle 7 information.
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