Instrument Science Report NICMOS-001
NICMOS Design Reference Mission and
Ground System Volume Estimate
John W. MacKenty and Ray Kutina
July 25, 1995
ABSTRACT
We construct an estimate of the distribution of NICMOS observing programs and their
frequencies of occurrence in the post- SMOV era. We predict a daily mean volume of 90
prime, 77 internal parallel,172 parallel with other SIs, and 30 calibration observations
for a total of 369 observations. With the estimated distribution of the usage of the NICMOS readout modes, these correspond to a downlink volume of 751 Mbits per day. This
expands to approximately 918 Mbytes per day of CAL class science data ingested in the
Hubble Data Archive. These estimates are particularly sensitive to assumptions regarding
the fraction of the observations obtained with MULTI-ACCUM mode and the fraction of
observations which are background limited (i.e. requiring chopping).
1. Introduction
Goals of this Report
This ISR provides an description of the anticipated usage of the NICMOS Science Instrument (SI). This estimate is intended to guide the design and development of the HST
ground system elements which will receive, process, calibrate, archive, and distribute
NICMOS data. For this purpose we have constructed a realistic scenario which should neither underestimate nor overestimate the usage of NICMOS. We have aimed for a balanced
estimate since an underestimate might leave us unprepared during Cycle 7 while an overestimate might waste resources and lock us into a particular ground system configuration.
This ISR provides estimates of the mean
•
number of NICMOS prime observations per day
•
number of NICMOS internal parallel observations per day
•
number of NICMOS observations parallel with other SIs per day
•
number of NICMOS calibration observations per day
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•
downlink data volume per day
•
data volume generated by OPUS per day (not including the engineering data stream)
•
size of the Calibration Data Base (CDB)
Structure of this Report
Section 2 considers the distribution of usage across the various observing mode and
classes of observations and targets. Section 3 defines the “Design Reference Mission”.
Section 4 provides our estimate of the volume of data which the HST ground system
should be designed to handle. The contents and size of the NICMOS calibration reference
files in CDBS are estimated in Section 5. A bottom line summary is presented in Section
6.
2. Distribution of NICMOS Usage
NICMOS observing modes
The NICMOS SI has four primary observing modes: ACCUM, RAMP, MULTI-ACCUM,
and BRIGHTOBJ. Of these, MULTI-ACCUM mode is of particular interest for estimating
the data volume since it typically returns of order 10 to 20 times the data volume of the
other three modes. RAMP mode returns datasets which are twice the size of ACCUM and
BRIGHTOBJ mode.
Estimated Distribution of NICMOS Science Programs
The NICMOS SI expands HST’s range of observing space into the near Infrared. While
creating new opportunities, this complicates the task of estimating the frequency and distribution of the types of NICMOS observations. Three major issues limit our ability to
predict NICMOS usage. First, it is not certain at what wavelength the HST OTA thermal
emissivity will start to exceed the external background and it is likely that this will not be
determined until on-orbit measurements are obtained during post-SM2 SMOV. Second,
the extent and nature of the parallel program(s) which will be undertaken with NICMOS
depends on policy, TAC allocation (e.g. to a large “key project”), and the successful installation of a Solid State Recorder (SSR) during SM2. For the purposes of this study we have
assumed that parallel observations are aggressively pursued, that the parallel program does
one third of its orbits in the thermally limited IR, and that the SSR is available in Cycle 7.
Last, until the IDT and GO scientific investigations are defined (and TAC selected for the
GOs), it is difficult to estimate the division of orbits between broad band observations of
faint targets and the use of special capabilities within NICMOS (see below). We may
divide NICMOS observations into four broad categories.
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Bright Targets
Observation of Bright Targets fall into two categories. Really bright targets requiring the
use of BRIGHTOBJ mode require a large amount of spacecraft time and will be relatively
rare and therefore have little impact on the overall data volume. However, many “classical” infrared targets (e.g. the Orion molecular cloud) have a large dynamic range. It is
reasonable to assume that such observations will be taken in MULTI-ACCUM mode to
retain the intrinsic dynamic range of the targets or with multiple slightly offset ACCUM
exposures to both retain dynamic range and to improve the flat field correction. Typically
~20 exposures reaches the point where S/N is limited by other systematics (e.g. flat
field). Exposure times will range from <1 minute for broad filters to full orbit for narrow
filters. We will assume that the median time is 1 minute. We expect that such exposures
will represent ~25% of the prime science orbits.
Special Targets and Modes
NICMOS has a number of capabilities beyond its basic imaging function which offer great
advantages over ground based observations. These include access to regions of the near IR
spectrum inaccessible from the ground. A number of narrow band filters are included specifically for this purpose. NICMOS also includes grisms for low resolution spectroscopy,
polarizers, and a coronographic mode. We expect that these classes of observations will
account for ~25% of prime science orbits and we will assume the median exposure time to
be 8 minutes (i.e. five exposures per orbit on average).
Faint Targets
Arguably the greatest strength of the NICMOS SI is its capability to image faint targets.
Combining the background reduction gain from small resolution elements achieved by
WFPC2 and FOC with the complete removal of the atmospheric near IR emission that
limits ground based IR observations, NICMOS will greatly exceed the largest ground
based telescopes in sensitivity at wavelengths where it is not thermally limited by the
OTA. We expect that typical programs will include three broad passbands of which only
one will be thermal background limited (see below). These exposures will require 20 minutes in Cameras 1 and 2; and 20 minutes for short wavelength Camera 3 exposures. Such
exposures will represent ~35% of prime science orbits.
Background Limited Faint Targets
Since the OTA optical surfaces are at ~20C, their thermal emissivity becomes important at
the long wavelength end of the NICMOS spectral range. The limits of this system are not
yet fully understood but it is likely that observations of faint targets corresponding to the
ground based K band (mainly in Camera 3) will require special observing procedures.
Since the internal thermal radiation will have both temporal (and probably significant spa-
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tial) variations, the traditional technique of obtaining a sequence of short exposures
alternating between the target and a “blank” region of sky will be used. We expect such
observations to account for ~15% of prime science orbits.
Parallel Programs
With the improved data handling capabilities of NICMOS compared to the first generation
SI and its unprecedented resolution and sensitivity in the near Infrared, an ambitious parallel science program may be anticipated. The parallel programs fall into two categories.
We expect NICMOS to obtain “internal” parallel observations in the cameras not being
used for the Prime NICMOS observation. Since many targets are sufficiently extended to
be rather interesting in all three cameras, it is reasonable to assume that these programs
will have a comparable mix of filters as the prime observations and that when the prime
observations are chopped, parallel observations will also be obtained. We have assumed
that the internal parallel observations will not generally use MULTI-ACCUM mode but
rather use RAMP mode to moderate their impact on the NICMOS internal processor and
the overall data management task.
We assume that 2/3 of the parallel observations with other SIs will be in broad passbands
which are not thermally limited and that those exposure times will be 1/2 of an orbit in
MULTI-ACCUM mode and the parallel program in Cameras 1 and 2 will be 1/2 orbit
exposures in RAMP mode. The remaining 1/3 of the parallel observations will be conducted only in Camera 3 using the FOM.
3. Design Reference Mission
Table 1 assumes that NICMOS is prime for 33% of the time (5 orbits per day).
Table 1. NICMOS as Prime Instrument
Science Type
Percentage
of Orbits
Camera(s)
Exposures
per Orbit
Orbits
per Day
Datasets
per Day
Images
per Day
Bright Targets
25%
Prime (1,2or,3)
40 ACCUM
1.25
50
50
Parallel Cameras
4 RAMP
5
5
Prime (1,2,or 3)
5 ACCUM
6
6
Parallel Cameras
4 RAMP
5
5
Prime (1,2,or 3)
2 MULTI-A
3.5
70
Parallel Cameras
4 RAMP
7
7
Prime Camera (3)
40 ACCUM
(1 SAM/Exp)
30
30
Parallel Cameras
80 ACCUM
60
60
Special Targets
Faint Targets
Bkgd Limited
Faint Targets
25%
35%
15%
4
1.25
1.75
0.75
Under the assumption of fully using the available parallel time, we assume that 10 orbits
per day of NICMOS parallel observations are obtained as defined in Table 2.
Table 2. NICMOS in Parallel with other SIs
Science Type
Percentage
of Orbits
Camera(s)
Exposures
per Orbit
Orbits
per Day
Datasets
per Day
Images
per Day
67%
Prime (3)
2 MULTI-A
6.7
13.4
268
Parallel Cameras
4 RAMP
27
27
Prime (3)
(No Parallels)
40 ACCUM
(1 FOM/Exp)
132
132
NonBackground
Limited
Background
Limited
33%
3.3
Basic Assumptions
•
NICMOS will receive one third of the available HST time or 5 orbits per day. An orbit
is assumed to be approximately 52 - 12 = 40 minutes.
•
NICMOS will operate in parallel mode for the additional 10 orbits per day.
•
NICMOS will operate all 3 cameras in parallel whenever data is being taken. The one
exception to this assumption is that Cameras 1 and 2 will not obtain parallel exposures
for background limited exposures with Camera 3 in parallel with another SI (i.e. FOM
chopping).
•
It is desirable to divide NICMOS exposures into sub-exposures with the FOV moved
between each exposure. This assumes that the FOM is available whenever NICMOS is
in parallel with WFPC2 or STIS.
•
NICMOS exposures in Camera 3 at thermally limited wavelengths with broad filters
require exposures times of order 1 minute.
Caveats
These are fairly conservative estimates which may be reduced because:
•
Pure parallel observations in Cameras 1 and 2 with other SIs will be of limited value
due to their relatively small fields of view.
•
The Camera 3 parallel observations may choose not to assign 1/3 of the observing time
to thermally limited observations. This is the largest driver in the estimate presented
here.
However, significant increase may result from the inclusion of:
•
Parallel observations in Camera 2 when Camera 3 is obtained FOM chopped parallel
observations with another SI.
•
Additional dithering of observations to reduce detector effects when observations are
not readout noise limited.
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•
Higher than anticipated thermal background which would result in a larger proportion
of the broad band observations of faint targets being chopped.
4. Ground System Volume Estimate
Sizes of NICMOS datasets
The size of NICMOS datasets depends strongly on the mode selected. Table 3 defines the
size for each mode. We will make the arbitrary assumption that each MULTI-ACCUM
dataset returns 20 samples. During calibration processing the total size of each dataset
grows both due to the FITS data structures and due to the retention of multiple versions of
each dataset. Each dataset is saved as an uncalibrated formatted dataset and as a individually calibrated dataset (e.g. post First Stage Calibration processing). If the data are part of
an “association”, one or more datasets containing the “products” of the Second Stage Calibration of the association are produced. Since these correspond typically to one or two
datasets of nominal size 1.05 Mbytes, they have not been included in our estimates since
they should not increase the total Archive volume by more than ~10% (and they have no
effect on the downlinked data volume). Likewise, each observation includes a Standard
Head Packet and Unique Data Log file which is not included in the estimates. The estimates given in this ISR only include CAL class data, the appropriate multipliers should be
applied to estimate the POD and EDT class data.
Table 3. Sizes of NICMOS Datasets
Mode
Downlink
(Mbits)
STSDAS Uncalibrated
(Mbytes)
STSDAS Calibrated
(Mbytes)
ACCUM
1
0.39
0.66
MULTI-ACCUM
21
8.2
22.1
RAMP
2
0.79
1.05
BRIGHTOBJ
1
0.39
0.66
Volume Estimates
In Table 4 the volume estimates are given. As indicated the prime and parallel volumes are
given separately. Note that the Downlink volume is in units of Megabits per day while the
Archive volume is expressed in Megabytes per day. The columns labeled “Prime” indicate
the prime camera within NICMOS. The columns labeled “Int Parallel” indicate the internal parallel observations. Both the case of NICMOS as the Prime SI and the case of
NICMOS operating in parallel with another SI are included in the same columns. The
design reference mission would return a 721 MBits of data per day and delivery 918
Mbytes daily to the HDA.
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Table 4. NICMOS Volume Estimate by Science Type
Downlink
(Mbits per Day)
Science Type
CAL Class Archive
(Mbytes per Day)
Prime
Int Parallel
Prime
Int Parallel
Bright Targets
50
10
53
9.2
Special Targets
6
10
6.3
9.2
Faint Targets
74
14
106
12.9
Bkgd Limited Faint Targets
30
60
31.5
63
Non Background Limited Parallel
281
54
406
50
Background Limited Parallel
132
0
139
0
Totals
573
148
742
144
5. Calibration Database System Storage Estimate
An approximate inventory of the calibration database (CDB) is given in Table 5. This does
not include the synphot database nor a (presently undefined) background patterns database. The projected total size of the NICMOS CDB at initial load is on the order of 400
Mbytes.
Table 5. CDB Contents
Calibration
Reference File
File Size
(Mbytes)
Number
Required
Total Volume
(Mbytes)
Linearity
1.05 * 4
3
12.6
Dark
1.05
3 * 100
315
Flat
1.05
3 * 19
60
6. Summary
The total number of datasets and the data volume are summarized in Table 6. We have
added a very preliminary estimate of the volume of the NICMOS calibration program.
While this is likely to grow in size, it does not appear likely to dominate the total volume.
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Table 6. Total NICMOS Usage
Science
Datasets
per Day
Images
per Day
Downlink
per Day
(Mbit)
Archive (CAL)
per Day
(Mbytes)
Prime
90
156
160
197
Internal Parallel
77
77
94
94
Parallel with other SIs
172
427
467
595
Calibration
30
30
30
32
Total
369
690
751
918
7. Acknowledgments
These estimates rely upon the memo by Chris Skinner estimating background fluxes in the
various filters and cameras. We have received valuable comments from Dave Axon, Chris
Blades, Stefi Baum, Carl Biagetti, Howard Bushouse, Jim Rose, and Chris Skinner.
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