Instrument Science Report NICMOS-016
The WWW NICMOS Exposure Time
Calculator and Units Conversion
Program.
Daniela Calzetti, Chris Skinner, and Doris Daou
December 11, 1996
ABSTRACT
This ISR describes the World Wide Web implementation of the NICMOS Exposure Time
Calculator and of the supporting software tool, the Unit Conversion Program. Special
enphasis is given to the user interface, while the end-programs which constitute the core of
the Exposure Time Calculator are described in the NICMOS ISR-015, and basics of the
Unit Conversion Program are described in the NICMOS ISR-014. Both the Exposure Time
Calculator and the Unit Conversion Program have been available during the HST Phase 1
proposal submission period, and have proven to be very popular. Some statistics on the
usage of the tools during this period are reported.
1. Introduction
NICMOS, the Near-Infrared Camera and Multi-Object Spectrometer, which will be
installed on HST during SM97, will be a new capability available to the astronomical
community. Its wavelength range, from 0.8 to 2.5 microns, is unmatched by other HST
instruments, and expands to the near-infrared the HST wavelength coverage. Because
NICMOS is a new instrument, the development and the implementation of software tools
for the estimate of exposure times were considered an essential component of the support
provided to proposers to assist the design of their observations. To this end, two software
tools were made available at the end of June 1996 (and updated during the first week of
August 1996): the Exposure Time Calculator (ETC) , and an aiding tool, the Unit Conversion Program (UPC), to convert magnitude and flux units commonly used in astronomy
into the flux units used by the ETC and in the NICMOS Handbook (namely Jansky, Jy).
The two programs were made available to the astronomical community through interfaces
with the World Wide Web, which is being increasingly used by astronomers as the high-
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way for fast access to scientific information. The two tools are accessible through the
NICMOS Home Page, under Software Tools, at the following address:
http://www.stsci.edu/ftp/instrument_news/NICMOS/NICMOS_tools/nicmos_tools.html
This document describes the structure of the WWW NICMOS ETC and UCP as available
to proposers during the Cycle 7 Phase 1 Proposal submission period, and is aimed at providing a baseline for future updates and improvements. Section 2 describes the general
structure of the implementation on the WWW; sections 3 and 4 describe in more detail the
required inputs and outputs for the ETC and UCP, respectively.
2. The WWW Implementation
The structure of the WWW implementation is identical for both the ETC and the UCP, and
is formed by three units:
•
The fore-end unit is an HTML Form, which produces the WWW page used by the proposers to input their parameters through a multiple choice menu
(nicmos_etc_form_point.html, nicmos_etc_form_extend.html, and
nicmos_etc_form_grism.html for the ETC; conversion_form.html for the UCP). The
Form creates a temporary output ASCII file containing the list of input parameters.
•
The intermediate unit is a program in C, whose task is to act as a link between the
HTML Form and the ETC or UCP Fortran codes (nicmos_etc_point.c,
nicmos_etc_extend.c, and nicmos_etc_grism.c for the ETC; conversion2.c for the
UCP). The C program performs three basic tasks: 1) translates the inputs from the
Form into inputs for the Fortran program; 2) sends the command to run the Fortran
code; 3) arranges the output files from the code into the Output WWW Page seen by
the user.
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The bottom-end unit is the core Fortran code (ETC or UCP), which performs the
actual calculation (nicsim.f, nicsimx.f, and mossim.f for the ETC; conversion2.f for
the UCP); details of the structure of the two programs can be found in the NICMOS
ISR-015 and ISR-014, respectively, and some description on the general structure is
given below. The programs have been modified since release of the ISRs, to accomodate diverse input continuum and emission line sources.
An additional set of fortran programs is implemented to transform the outputs from the
ETC into SUPERMONGO plots (nicsimplt.f, nicsimxplt.f, mossimplt.f). These programs
are linked to the SUPERMONGO library ($SMLIBS) during compilation. The C programs are linked to the standard library (util.c) during compilation.
The HTML Forms are lodged in subdirectories of the NICMOS WWW tree on the STScI
Web-server Marvel
(/var/ftp/instrument_news/NICMOS/NICMOS_tools/NICMOS_etc_fast_2 for the ETC
and /var/ftp/instrument_news/NICMOS/NICMOS_tools/NICMOS_others_2 for the
UCP), together with the source codes of the Fortran and C programs. The executables of
the Fortran and C programs and all the attached reference files are lodged in the CGI-BIN
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directory (/opt/httpd/cgi-bin/NICMOS). The input files created by the HTML Forms and
the output files created by the Fortran programs get dumped in the temporary directory (/
tmp/NICMOS). Since each execution of the ETC creates eight files in the /tmp directory, a
procedure was set in place during the HST Phase 1 proposal submission period to provide
each file with a lifetime of five hours in order to prevent the /tmp directory from filling
with old files.
3. The Exposure Time Calculator
The NICMOS ETC is an aiding tool for estimating integration times and Signal-to-Noise
ratios (SNR) of a point-like or extended source, for deriving detection limits, and for
checking saturation limits. Three cases are treated separately by the ETC:
•
Imaging of Point Sources (Continuum and Emission Line).
•
Imaging of Extended Sources (Continuum and Emission Line).
•
Grism Observations (Continuum sources only).
Each of the three cases uses a separate HTML Form, C program, Fortran code, and generates separate outputs. The implementation of each case on the WWW is similar, and we
will describe them all alike, except where otherwise indicated.
The User’s Inputs (HTML Form)
The HTML Form provides the user with a multiple-choice menu for selecting the ETC
input parameters. Two major groups of input parameters can be recognized: instrumentspecific and source-specific.
The Instrument-specific parameters are:
•
Camera number: can be Camera 1, 2, or 3 (grisms are available only in Camera 3).
•
Filter name: a multiple-choice menu is provided on a pull-down list. If the polarizers
(POLxxS or POLxxL) are selected, then only 50% of the input flux is assumed to go
through the filter.
•
Number of Reads: the number of reads at the beginning and at the end of an exposure.
The default is 1. The accepted range for the number of reads is 1-25, for ACCUM
readouts. In this case, the read noise is reduced by the square root of the number of
reads. This behaviour will be made more realistic when we better understand the performance of the instrument.
The Source-specific parameters further divided into continuum source parameters and
emission line source parameters.
For Continumm Sources, the parameters are:
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•
Source Flux: is the flux of the source at the center of the selected filter bandpass. If
provided, must be in Jy for point sources or Jy/arcsec2 for extended sources. For grism
calculations, the source flux (in Jy) at the grism central wavelength is requested. This
input is needed only if the user is interested in the SNR versus time for fixed flux
value. If no flux is provided, only the saturation and the sensitivity tables are provided
(see below).
•
Spectrum: the three spectral types available are the Blackbody Spectrum and two
Power-Laws. For the Blackbody Spectrum, the temperature in Kelvin must be provided. For temperatures lower than 1,000 K a warning message is issued, because the
possible presence of out-of-band filter leaks, yet uncalibrated, renders the exposure
time calulations highly uncertain. The two Power-Law Spectra are defined as Flux(ν)
= A * να and Flux(ν) = B* λβ ,where α and β are the spectral indices to be supplied.
The proportionality constants A and B need not to be specified, since the spectrum is
normalized to the Source Flux at the center of the selected filter bandpass.
The Emission Line sources are treated for the Imaging case only (not Grisms) and the
parameters are:
•
Source Flux: the flux in the emission line can be given in Watt/m2 (Watt/m2/arcsec2 for
extended sources) or in erg/s/cm2 (erg/s/cm2/arcsec2 for extended sources). As in the
case of the continuum sources, this input is needed only if the user is interested in the
SNR versus time for a specific flux value.
•
Line Position: the position of the line in the spectrum can be given either as a wavelength (in µm) or a frequency (in Hz).
Description of the Calculator (Fortran Code).
Many of the parameters used in the ETC Fortran code are still preliminary and are based
on the NICMOS group’s best understanding of the instrument performance. A brief
description of the parameters is given below. More details can be found in the NICMOS
ISR-015.
The System Throughput is based on assumptions on the reflectivity of the system optics,
and on the transmission of the dewar window. The adopted values are 0.95 for the NICMOS fore-optics, 0.80 for the telescope mirrors, and 0.9 for the transmission of the dewar
window. For the filter transmissions and the Detector Quantum Efficiency, the values
made available to us by the NICMOS IDT were used.
For the Point Spread Function (PSF), a synthetic FOC PSF, calculated for a wavelength of
1.0 µm, has been adopted, and is scaled linearly with wavelength.
The sources of noise which enter in the SNR estimate are:
•
Standard Poisson Noise.
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•
Background. NICMOS will be likely affected by two main sources of background: the
Zodiacal light, which is spatially dependent, and the thermal emission from the telescope. The model adopted for the Zodiacal light is based on COBE data at about 45
degrees ecliptic latitude. The thermal background is dominant for wavelengths longer
than 1.7 microns, and will have strong effects on the detection limits beyond this
wavelength (see the NICMOS Handbook). For the Exposure Time Calculator, the HST
primary and secondary mirrors have been assumed to be at a temperature of 293 K and
to have 20% emissivity.
•
Dark Current. The best estimates from the IDT as of August 1996 are that it is probably less than 0.1 e-/sec. In the Calculator, the upper limit, 0.1 e-/sec, is used.
•
Read Noise. To the best of present determinations, the read noise is close to about 30
electrons in each of the three Cameras.
For point source calculations (which includes Grism Observations), it is assumed that the
PSF is centered on a pixel.
For polarimetric observations, each set of polarizers (POLxxS or POLxxL) assumes the
same transmission curve at 0, 120, and 240 degrees; in addition, 50% of the input flux is
assumed to go through the polarizer.
For the Grism Observations, it is further assumed that the dispersion is uniform across the
spectrum. This assumption is expected to be approximately true for NICMOS.
The Outputs
There are three types of results given by the ETC, all three always produced: 1) the SNR
of the user-specified source flux as a function of exposure time; 2) the saturation and
detector-limited thresholds, represented as limiting fluxes versus exposure time; 3) the flux
required to reach a fixed value of the SNR as a function of time. The results are given in
seven files: one Input Information file (ASCII) , three Tables (ASCII) , and three Plots
(POSTSCRIPT).
The Input Information file (INPUT INFO) reports (a) the input parameters the user
selected or provided, (b) the names of the three Tables, (c) diagnostics in case of problems
(e.g. the user did NOT select a filter), (d) the characteristics of the selected filter (mean
wavelength, mean filter transmission, and filter bandwidth), and (e) general information
on the observation: source count rate (in e-/s/pix for extended sources and in e-/s in the
brightest pixel for point sources), the background count rate (e-/s/pix), the read-out noise
(e-/pix), and the dark current (e-/s/pix) used in the calculations. For point sources, the
energy fraction contained in one pixel and the area in pixels which encircles 80% of the
energy in the Point Spread Function are also given. In the case of emission lines, the epsilon value, in units of e-/s/W/m2 (as described in the NICMOS Handbook) is reported.
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The Tables contain the actual results in the form of ASCII files, which are retrievable with
the standard tools available on the WWW. The Tables contain a brief header which
explains the meaning of each column.
The Plots are postscript files which show graphically the content of the Tables in a log-log
scale.
For the Imaging of Point Sources and of Extended Sources, the three Tables are:
•
The SNR Table: the SNR obtained for the user-specified flux, as a function of exposure time, in seconds (see Figure 1). If all zeros are returned for the SNR in this Table,
it is likely that the specified flux is too low to produce a significatant SNR even for the
longest available exposure time. If indefinite values (NaN) are returned for the SNR,
then the specified flux is too high, and the program has overflowed;
•
The Saturation and Noise-Limited Table: the fluxes required to saturate the detector
and to be detector-noise dominated, as a function of exposure time (see Figure 2). The
two curves divide the plot into three separate regions: the detector-noise dominated
region (bottom left), the photon noise dominated region (center), and the detector-saturated region (top right).
•
The Sensitivity Table: the flux required to obtain fixed SNR, as a function of exposure
time; the four lines correspond to SNR=10, 25, 50, and 100, respectively (see Figure
3).
For the Grism Observations case, the three Tables contain:
•
the time, in seconds, required to reach fixed values of the SNR, namely SNR=10, 25,
50, 100, as a function of wavelength (in microns);
•
the central wavelength fluxes (in Jy) required to saturate the detector and to be detector-noise dominated, as a function of exposure time; as in the previous case, the two
lines separate three regions: the detector-noise dominated region (bottom left), the
photon noise dominated region (center), and the detector-saturated region (top right);
•
the flux detected at three different wavelengths of the grism spectrum, as a function of
exposure time; the three wavelengths span the range covered by the spectrum; the
fluxes are given for two values of the SNR, namely 10 and 100.
For both the Imaging and the Grism Observations, the SNR refers to the brightest central
pixel in the point-source case; it is per pixel in the extended source case. The fluxes refer
to the entire source, as input by the user.
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Figure 1: Signal-to-Noise Ratio as a function of exposure time, as produced by the Exposure Time Calculator, for an extended source of surface brightness 0.1 Jy/arcsec2, imaged in Camera 2 with the F160W filter.
Figure 2: The two curves mark, as a function of time, the limiting fluxes for which the detector saturates
(top curve) or the observation is read-noise limited (bottom curve).
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Figure 3: The sensitivity plot gives the fluxes required to reach Signal-to-Noise Ratios = 10, 25, 50, 100
(curves from bottom to top) as a function of time.
4. Supporting Tool : the Unit Conversion Program
The Units Conversion Program converts fluxes of continuum sources from a variety of
units which are widely used in astronomy (e.g., magnitude, ergs/sec/cm^2/A, etc.) into the
units used by the ETC, i.e. Jy, and vice versa.
For general reference: 1 Jy = 1.0E-26 W/m2/Hz = 1.0E-23 erg/s/cm2/Hz .
The User’s Inputs (HTML Form)
The user’s inputs on the HTML Form are self-explanatory and involve the specification of:
•
Input Units: one of the many available options from the pull-down menu (Jy, W/m2/
Hz, W/m2/µm, W/cm2/µm, erg/s/cm2/Hz, erg/s/cm2/A, ph/s/m2/µm, ph/s/cm2/A; magnitudes: U, B, V, R, I, J, H, K, L, L’, M, N, Q).
•
Output Units: also one of the options from the pull-down menu (same list as the Input
Units).
•
Input Flux or Magnitude Value: the input value of the flux or magnitude of the source.
•
Input Wavelength: the wavelength at which the input flux is given. If the Input Unit is
a magnitude, the wavelength can be given any value, but NOT left blank.
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•
Output Wavelength: the wavelength at which the output flux is desired. If the input and
output wavelengths differ by more than 5%, the program extrapolates the flux assuming one of the spectral shapes provided. If the Output Unit is a magnitude, the value of
the wavelength does not need to be specified; the field can be given any value, but
NOT left blank.
•
Spectral shape of the continuum source: one of three options can be selected:
1) Blackbody Spectrum: the temperature, in Kelvin, must be supplied.
2) Power-Law Spectrum: the Spectral Index α must be supplied, according to the convention: Flux(ν) = Flux0 * (ν/ν0)α.
3) Power-Law Spectrum: the Spectral Index β must be supplied, according to the convention: Flux(ν) = Flux0 * (λ/λ0)β.
The Fortran Program
The details of the FORTRAN program which handles the unit conversion are explained in
the NICMOS ISR-014. Modifications were added subsequent to the issue of the ISR to
make the program more flexible and to handle multiple choices for the spectral shape of
the input source.
The Outputs
The output is the flux or magnitude in the desired units, and for the selected source spectrum. The input parameters are repeated on the output page.
The Magnitude Zero-Points
Unit conversion between magnitude and fluxes, and vice versa, requires information on
the magnitude zero-point value and on the bandpass central wavelength. The zero-point
magnitudes used in this program are from the CIT system, or, in the case of the L’ band,
from the UKIRT system (cf. ISR-014), and are listed in Table 1 below together with the
filter bandpass central wavelength:
Table 1: Filters zero-points and central wavelength
Magnitude
Zero-Point (Jy)
Central Wavelength (µm)
U
1810
0.36
B
4260
0.44
V
3540
0.55
R
2870
0.70
I
2250
0.90
J
1670
1.25
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Table 1: Filters zero-points and central wavelength
Magnitude
Zero-Point (Jy)
Central Wavelength (µm)
H
980
1.65
K
620
2.20
L
280
3.40
L’
252
3.74
M
150
4.80
N
37
10.10
Q
10
20.00
5. Statistics on the Use of the WWW Tools.
The WWW ETC and UCP have been widely used by the astronomical community, as
shown by the statistics reported in Table 2. The Table lists the number of accesses to the
programs as a function of time: during the three weeks before the HST Phase 1 proposal
deadline there were a total of 7754 accesses, while in the week following the deadline only
56 accesses were counted.
Table 2. Weekly Usage of the WWW NICMOS Software Tools.
Period
Number of Accesses to the Tools
08/25/96--08/31/96
1318
09/01/96--09/07/96
1716
09/08/96--09/14/96
(deadline week)
4720
09/15/96--09/21/96
(1 week after deadline)
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As an additional indication of the utility of the WWW tools, we performed checks on the
exposure times of submitted proposals during the Technical Review in October 1996. In
the vast majority of cases, the numbers declared by the proposers were within 10% of the
numbers resulting from the ETC, showing that most of the proposers did use the WWW
NICMOS software tools.
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Acknowledgements
The authors would like to thank Warren Hack for constant help and many useful suggestions during the WWW implementation of the NICMOS software tools.
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