Instrument Science Report NICMOS 00-001
The NICMOS Exposure Time
Calculator: Algorithms and
User Interface
A.Sivaramakrishnan, S. Holfeltz, M. Sosey, B. Simon, and M. Robberto
May 17, 2000
ABSTRACT
This report describes the latest version of the NICMOS Exposure Time Calculator (ETC)
developed to support NCS-era HST NICMOS planning. We added a thermal optics library
to the ETC, and model various detector behaviors seen during the NICMOS end-of-life
period. There are no ad-hoc parameter values in the thermal optics library describing the
telescope and the NICMOS camera. This ETC supports imaging through all filters except
the grisms.
Exposure Time Calculators (ETCs) are used to estimate expected signal to noise ratios
given a source brightness and size, and an expected sky background. In the infrared, a
knowledge of the thermal properties of the optics is also required. There are two older
NICMOS ETCs: a private one by Thompson (1996) and an STScI-supported one by Skinner (1996).
The previous STScI-maintained version, written in FORTRAN by C. J. Skinner, and
modified by L. Colina, D. Calzetti and D. Daou, uses pre-launch characteristics of the
NICMOS camera, and its own collection of throughput and quantum efficiency data. In
consequence, it will be hard to maintain when the NICMOS instrument is revived by the
NCS cryocooler. Furthermore, instrument and telescope parameters in this ETC were
tuned to match observations, especially in the thermal infrared.
This replacement ETC uses measured values of all quantities except for Point Spread
Function (PSF) information. In particular, the measured emissivities of the HST primary
and secondary are used. These result in good agreement with observations (Robberto et
al., 2000). Here we describe the user interface of this new NICMOS ETC, which uses
Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
instrument throughput and data files maintained and updated by the Space Telescope Science Institute (STScI) and data from NICMOS end-of-life observations (Boeker et al.,
1999). The NICMOS detector showed an unexpected bump in dark current during warmup: it is not known whether this bump will recur if the instrument is cooled down again.
The bump peaks at around 7e per second per pixel at about 82K. Thus cycle 10 proposals
should use the ‘dark current bump present’ case when planning observations, as this is the
worst-case scenario. The observer can also look at the more optimistic ‘bump absent’ case
if they wish. Offsetting this predicted increased dark current is the fact that the DQE gets
better with higher temperatures. The expected operating temperature for the detector in the
cryo-cooler era is about 75K (cycle 7 temperatures were between 61 and 64 K).
This version of the ETC will be available for planning cycle 10 observations with NICMOS. We also describe the core algorithms used by the ETC. For software details see the
ISR devoted to maintenance, support and installation (Sivaramakrishnan et al., 2000a): all
the source code is made available to anyone interested in inspecting or modifying the
implementation details on their own, though STScI does not provide development support
for such activity.
Description of the Calculator
This version of the NICMOS ETC uses throughput data in the Calibration Data Base
System (CDBS) at STScI instead of separately maintained private data files. It uses the
STSDAS package SYNPHOT as its throughput calculator. SYNPHOT uses the CDBS instrument data files, and thus brings NICMOS exposure time calculations into line with other
HST instruments (notably STIS, ACS and WFC3). It is hoped than this unification will
make for easier data management. All the code used is non-proprietary, and can be distributed without concern for licensing issues. Copies of the CDBS data base are also freely
available (eg., through STScI’s Starview software package).
While this NICMOS ETC derives its high level software architecture and execution
thread from a STIS ETC (Hack et al., 1996, with subsequent modifications and redesign
by B. Simon), it differs from the STIS and other optical ETCs in that it has to estimate a
noise component due to thermal contributions from the telescope and the NICMOS foreoptics. SYNPHOT is not capable of handling internal sources of radiation yet. In order to
accomplish this a library of NICMOS-specific thermal and optical transfer routines were
added to the ETC. The NICMOS thermal model implemented in this library differs from
the one described in the NICMOS ISR-14 in that it contains no ad-hoc adjustment factors
to match observations, and it models various components of the HST pupil as seen from
NICMOS, following the work of Krist et al. (1999).
Background from sky is presumed to be zodiacal in origin. A model of zodiacal light
from the previous STScI NICMOS ETC is used (cf. the one described in NICMOS ISR14). This is an average of zodiacal light at 45 degrees from ecliptic. Table 1 indicates the
values used in photons per second per steradian per unit area per unit wavelength interval.
2
Point source signal is estimated in apertures of 1, 1 and 2 arc seconds diameter in the
NIC 1, NIC 2 and NIC 3 cameras respectively. Signal for point sources is the number of
photons (or electrons --- the numbers are equal for HgCdTe FPA’s) detected in the brightest central pixel, assuming the image is centered on that pixel. The Tiny Tim PSF
simulator is used to calculate the point spread function. COUNTRATE provides the pivot
wavelength for the source/filter/detector combination, which is parsed out of the SYNPHOT
results on the source. The aperture correction is estimated using a grid of aperture corrections obtained from Tiny Tim simulations. These simulations reside in the SCIDATA area
of STSDAS (Bushouse, 1998).
Table 1. Zodiacal background as a function of wavelength
Wavelength
microns
Zodiacal Background
(number s-1cm-2sr-1¨-1)
1.0
5.0e+08
1.1
4.3e+08
1.2
3.7e+08
1.3
3.2e+08
1.4
2.8e+08
1.5
2.5e+08
1.6
2.3e+08
1.7
2.0e+08
1.8
1.9e+08
1.9
1.7e+08
2.0
1.6e+08
2.1
1.4e+08
2.2
1.3e+08
2.3
1.2e+08
2.4
1.1e+08
2.5
1.1e+08
2.6
1.0e+08
Signal for an extended source is the number of electrons detected in a pixel fully illuminated by the source. The noise considered here is due to sky brightness, thermal
radiation from telescope and instrument optics, dark current inherent in the HgCdTe detector, and Gaussian noise characteristic of the readout electronics.
Like the STIS ETC, the controlling executable is compiled from a C program that
parses name-value pairs typical of an HTML post. That is, a web GUI interface can create
3
the appropriate input file, and post the information to the ETC executable, which resides in
a world-accessible cgi-bin directory. This executable program then calls the STSDAS.SYNPHOT.COUNTRATE task with the appropriate parameters, and calculates the expected signal
to noise ratio (SNR). Two calls to COUNTRATE are made by the ETC: one for the target and
one for the sky background.
COUNTRATE output is the number of source and sky background photons detected per
second. These numbers are post-processed by the high level ETC code to account for DQE
variations with temperature before signal and noise are estimated. The DQE at the pivot
wavelength of the source is used in this post-COUNTRATE processing. Given the size of the
pixel-to-pixel variation in the DQE, this approach is considered sufficient.
Output from the ETC executable is in the form of HTML, which is displayed on the
browser in the normal operating mode. Use of the ETC from the UNIX command line is
possible, will be described in a companion ISR (Sivaramakrishnan et al., 2000).
Theory
Signal
ns
electron / pixel / second
Sky
nsky
electrons / pixel / second
Dark
nd
electrons / pixel / second
Thermal
nt
electrons / pixel / second
Read noise
rn
Double-correlated sampling read noise standard
deviation in electrons
The NICMOS focal plane array (FPA) produces one electron per detected photon. For
an exposure time of t seconds the target signal is n t [electrons/sec/pixel] and the total
s
signal is nt [electrons/sec/pixel], where n = ( n + n
s
sky + n d + n t ) . Assuming Poisson statistics apply, the standard deviation of the total signal is σ =
deviation of the total noise is
nt . The standard
2
nt + σ rn , yielding a signal to noise ratio of
ns t
S = -------------------------,
2
nt + σ rn
where σ
rn , the standard deviation of the double-correlated sampling read noise, is a
function of the number of non-destructive reads performed. Solving for the exposure time
t yields
4
22
2
2
n s t =  nt + σ rn ⋅ S


2
2 2
2 2 2
nS +  n S + 4n s S σ rn


t = ----------------------------------------------------------------------2
2n s
The thermal optics library
At the time of development of this ETC SYNPHOT does not handle thermal radiation by
components of the optical train. Each optic in the train has an area, a temperature and an
emissivity. Usually the emissivity is the quantity with the greatest uncertainty. Dust on an
optical surface can also be a source of thermal radiation: on HST the dust appears to be
well-coupled to the mirror surfaces, and is assumed to be at the same temperature as the
optical element itself.
Consider an optical train with n elements. Each optic has a throughput factor
0 ≤ t i ( λ ) ≤ 1 (reflectance or transmittance), and an emissivity ε i ( λ ) , where λ is the
wavelength of the radiation passing though the relay. If there is any scattering,
t i + εi < 1 .
We use a quantity known as the étendue, which is an invariant of the optical train, to
calculate the thermal contribution of each optic to the detector. Regardless of intermediate
magnifications and beam f-ratios, the étendue, or the product of the area of optic and the
solid angle of the detector as seen by the optic, is preserved. The étendue can be calculated
by forming the product of the area of the entrance pupil, A, and the solid angle of the pixel
on the sky, ω . The entrance pupil area for NICMOS is the HST primary’s visible area as
seen from the detector). Thompson (1996) gives the NICMOS 1,2 and 3 camera pixel
étendues as:
NIC1 = 1.75e
NIC2 = 5.29e
NIC3 = 3.61e
– 13
– 13
– 12
2
m sr
2
m sr
2
m sr
5
The first optic in the train, M1 (the HST primary) contributes
n
B ( λ, T 1 )
[ DQE ] Aω ---------------------- ε 1 ∏ t i
hυ
i=2
es
–1
pixel
–1 –1
λ
counts to each detector pixel at the final image because of it thermal radiation. Similarly,
M2, the secondary, adds
n
B ( λ, T 2 )
[ DQE ] Aω ---------------------- ε 2 ∏ t i
hυ
i=3
es
–1
pixel
–1 –1
λ
counts to the thermal background. Each optic down the relay is treated similarly. We
present the values of the various optics’ emissivities and temperatures used in appendices
A1 and A2.
As is well-known, HST’s NICMOS is not well-aligned with the telescope optics.
There are several warm opaque surfaces visible from the NICMOS detectors. The bestcharacterized misalignment is that of camera 2. To calculate the thermal contribution from
the primary and its surrounding opaque, warm structure, geometric details of the entrance
pupil (Krist et al., 1998), and representative structure temperatures (Bacinski, 1999) were
used to calculate the effective contribution from the misaligned HST pupil as seen from
NICMOS 2 in the thermal library as the M1 contribution to thermal background. Camera 3
has more warm obstructions in its field of view. These are not treated yet --- the NIC2
pupil description is used for all three cameras at present. Upgrading the values for NIC3
could be a future development task for the ETC. We present the various areas and their
temperatures in appendix A1. However, we note that the dominant sources of thermal
background are still the large, warm, reflective mirror surfaces of the telescope.
In the cases when the optical surface is composed of various areas at different temperatures and emissivities (e.g. the primary ‘optic’ being the circle around the outer edge,
with baffles, pads, spiders and cold stops intruding into this area), we weight the corresponding Planck function (evaluated at the temperature of the appropriate pupil
component) by the filling factor of the area at that temperature, and its emissivity, to calculate its contribution to the thermal background.
The thermal library needs data files with the products of all filter transmissions and
detector quantum efficiencies (tabulated as a function of wavelength) in order to calculate
the internal optics’ thermal contribution. To this end, SYNPHOT’s CALCBAND task is used to
create all 57 possible ‘filter times DQE’ throughput tables. These tables are read in from a
file specified in the call to the public functions offered by this library, and the thermal
background for the instrument is returned.
6
World Wide Web Implementation and Usage
The ETC tool can be found on the NICMOS web site at http://www.stsci.edu/instruments/nicmos/topnicmos.html. The GUI will evolve in time as more features are added.
Figure 1 through Figure 4 below show the current graphical user interface.
Figure 1: ETC Web interface
Section 1: Select a detector and available filter.
Section 2: Specify wether you want your observation parameters to be calculated for a
given exposure time or a required signal to noise limit.
7
Figure 2: ETC web interface
Section 3: Specify the parameters for the source you wish to observe.
•
user supplied spectrum: This is accomplished by placing the user input spectrum in an
ftp staging area, which the program will load for the simulation. This "staging area" is
the anonymous ftp directory:
ftp.stsci.edu:/outside-access/in.coming.
•
Kurucz Model: These model spectra are calculated from the Kurucz database (Dr. R.
Kurucz, CD-ROM No. 13, GSFC) which have been installed in the Calibration Database System (CDBS).
•
HST Standard Star Spectra: These spectra are available in CDBS and were chosen
from the paper Turnshek, et al., 1990, An Atlas of HST Photometric, Spectrophotometric, and Polarimetric Calibration Objects.
•
Real Object Templates: Observed object spectra that are on-line.
8
Figure 3: ETC web interface
Section 4: Normalizing the source flux. Whether supplying your own spectra or using
one of the supplied model spectra, the source’s continuum flux needs to be normalized at
some wavelength. This wavelength needs to be within the wavelength range of the input
spectrum. The ETC will use it only for normalization and calculate the appropriate flux
values for the wavelength range of the observations. If your object is point-like, it can be
normalized to a magnitude at a particular Johnson band, or it can be normalized to a flux
[in Janskys] value at a given wavelength.
If you supply your own spectrum or use one of the HST calibration sources, you can
either normalize this spectrum to a fixed value, or you can use the "Do not renormalize"
option on the form. For an extended source, you must specify the surface brightness.
E(B-V): The flux is normalized after the extinction is taken into account so that it
always corresponds to the observed flux. The ETC supports two different extinction laws:
-An average Galactic extinction law taken from Seaton (MNRAS, 187, 73p, 1979)
-An LMC extinction Law taken from Koorneef & Code (ApJ, 247, 860, 1981).
9
Figure 4: ETC web interface
Section 5: Specify the expected background levels. Currently only one model is provided, regardless of choices made here: a zodiacal light level corresponding to a 45 degree
elevation above the ecliptic. In future more choices (low and high zodiacal levels and
earthshine) may be added to the calculator.
Sections 6 and 7: The detector temperature option was added to allow users to estimate exposure times in the NCS era of NICMOS operations. During end of life, a large
rise and decline in the dark current (known as “the bump”) was observed over the predicted temperature range of NICMOS operation in cycle 10. Since it is currently unknown
whether this event will repeat itself after NCS is installed, the user should choose to
include its effect in the exposure time calculation for phase 2 of the proposals. A detector
temperature of 75K should be used for Cycle 10 proposals.
10
Figure 5 shows an example output page that is returned to the user. It contains the suggested exposure time, target signal to noise and the chosen observation parameters.
Figure 5: Returned output to the user
Acknowledgements
Thanks are due to J. Bacinski, L. Bergeron, T. Boeker, H. Bushouse, C. Hanley, P.
Knezek, J. Krist, and R. Shaw from STScI, R. Thompson of Steward Observatory and M.
A. Pahre of the Harvard-Smithsonian Center for Astrophysics for detailed and helpful discussions. The observed elliptical galaxy spectrum was provided by M. Rieke. The cool
star/brown dwarf spectra were obtained from I. N Reid and B. R. Oppenheimer.
11
References
Boeker, T., Bacinski, J., Bergeron, E., Gilmore, D., Holfeltz, S., Monroe, B. and Sosey,
M. NICMOS ISR 001-1999 (revision)
Bushouse, H. 1998. SYNPHOT Users’ Guide, STScI.
Bergeron, E. 1999, private communication
Hack,W.,Sahu, K., Kinney, E. and Bohlin, R. 1996, STIS ISR-020
Krist, J. E., Golimowski, D. A., Schroeder, D. J. and Henry, T. J. 1998, PASP 110,
1046-1058
Robberto, M., Sivaramakrishnan, A., Bacinski, J., Calzetti, D., Krist J. E., MacKenty,
J. M., Piquero, J., and Stiavelli, M., Simon, B., 2000, SPIE Conference proceedings v4013 (Munich)
Simon, B. 1998, STIS ETC C and HTML software
Sivaramakrishnan, A., Holfeltz, S. T., Sosey, M. 2000, In preparation
Skinner, C. J., NICMOS ISR 14-1996, and associated software
Thompson, R. I. 1996 NICMOS Performance Predictor IDL software
A1: HST physical values used
All values are in SI units unless the units are explicitly stated in the variable names.
9.520e-01
3.000e-02
2.400e+00
4.524e+00
288.0
hst->primary_reflectivity
hst->primary_emissivity
hst->primary_diameter
hst->primary_area
hst->primary_temperature
9.520e-01
3.000e-02
7.904e-01
4.907e-01
290.0
hst->secondary_reflectivity
hst->secondary_emissivity
hst->secondary_diameter
hst->secondary_area
hst->secondary_temperature <under review>
3.036e+00
7.600e-01
3.580e+00
hst->primary_visible_area
hst->secondary_clear_fraction
hst->RC_platescale_as_mm
256.0
1.000e+00
hst->spider_temperature
hst->spider_emissivity
290.0
1.000e+00
hst->pad_temperature
hst->pad_emissivity
272.6
9.500e-01
hst->hole_baffle_temperature
hst->hole_baffle_emissivity
253.0
hst->edge_baffle_temperature
12
9.500e-01
hst->edge_baffle_emissivity
Note that if some of the dust particles are smaller than a wavelength in size, the emissivity and reflectivity do not have to add to unity
Currently the following values are used for all three NICMOS cameras, though they
were measured for NIC2. Areas are also given as percentages of the primary area. See
Krist et al. (1998) for details.
4.520e-02
2.220e-02
4.700e-02
1.603e-01
1.108e+00
hst->nic_hot_pad_area[NIC2]
1.00%
hst->nic_hot_spider_area[NIC2]
0.49%
hst->nic_hole_baffle_lune[NIC2]
1.04%
hst->nic_edge_baffle_lune[NIC2]
3.54%
hst->nic_pupil_coldmask_area[NIC2] 24.49%
Varying the temperatures of the primary and secondary, and assuming an emissivity of
3%, we obtain the following thermal background contributions (in electrons per second
per pixel) from the longest wavelength filters. These mirror temperatures are in line with
the range of measured temperatures of the mirrors. The following three tables show the
thermal background count rates in electrons per second in a pixel in two longer
wavelength filters of NIC2.
Table 2: Primary Temperature 283K
Secondary Temp.
280
285
290
F222M
6.3
6.8
7.5
F237M
26
28
31
Table 3: Primary Temperature 288K
Secondary Temp.
280
285
290
F222M
6.9
7.4
8.1
F237M
28
30
33
Table 4: Primary Temperature 293K
Secondary Temp.
280
285
290
F222M
7.7
8.2
8.9
F237M
31
33
36
13
Table 5: Previous thermal background estimates in electrons/sec. pixel
NICMOS Handbook v.II
NICMOS Handbook v.III
STScI Recommendation
F222M
8.0
9.26
8.8
F237M
31.0
35.6
34.0
The primary is assumed to have a temperature of 288K, and the secondary 290K in the
this ETC. The mirror temperatures are known from the engineering telemetry data from
HST. The secondary temperature can vary somewhat (depending on attitude): the ETC
attempts to model the telescope’s average thermal behavior. The previous STScI-maintained ETC assumed emissivities of 4.8%, and primary and secondary temperatures of
291K and 288.5K respectively, though the calculation of the secondary mirror’s thermal
contribution underestimated the flux reaching the detector by a factor of a few. We find
that both HST mirrors have similar contributions to the thermal background. Recent work
(Robberto et al., 2000) indicates that the 3% emissivity figure is within the expected range
of mirror emissivity given the history of HST.
A2: NICMOS physical values used
nic->darkcurrent_bump
nic->detector_temperature
nic->foreoptics_temperature
nic->bend1_mirror_reflectivity
nic->reimaging_mirror_reflectivity
nic->pupil_mirror_reflectivity
nic->image_mirror_reflectivity
nic->paraboloid1_mirror_reflectivity
nic->paraboloid2_mirror_reflectivity
nic->bend2_mirror_reflectivity
nic->dewar_window_transmission
nic->pixel_size_um
nic->pixel_size_as[NIC1]
nic->pixel_size_as[NIC2]
nic->pixel_size_as[NIC3]
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
BUMP_PRESENT;
70.0;
270.0;
0.985;
0.985;
0.985;
0.985;
0.985;
0.985;
0.985;
0.93;
40.0;
0.043;
0.075;
0.20;
Relative DQE is measured at two wavelengths, lrefmu[0] and lrefmu[1], for each
detector, at various temperatures. A quadratic fit (in temperature) to these data (Bergeron,
private communication) produces the three coefficients a[0, 1, 2]. The relative DQE at the
pivot wavelength of the source/filter under consideration is found by linear extrapolation
at the particular detector temperature being used. For more detail see Boeker et al., 1999.
/* at lrefmu[0] microns: */
measured_rdqe[0] = a0[0] +
a1[0] * T
+
a2[0] * T * T;
/* at lrefmu[1] microns: */
measured_rdqe[1] = a0[1] +
a1[1] * T
+
a2[1] * T * T;
nic->rel_dqe_l_mu[NIC1][0]
= 1.1;
14
nic->rel_dqe_l_mu[NIC1][1]
nic->rel_dqe_a0[NIC1][0]
nic->rel_dqe_a1[NIC1][0]
nic->rel_dqe_a2[NIC1][0]
nic->rel_dqe_a0[NIC1][1]
nic->rel_dqe_a1[NIC1][1]
nic->rel_dqe_a2[NIC1][1]
=
=
=
=
=
=
=
1.6;
-4.1412421;
0.11773110;
-0.00057135511;
-3.4798412;
0.10636465;
-0.00055803535;
nic->rel_dqe_l_mu[NIC2][0]
nic->rel_dqe_l_mu[NIC2][1]
nic->rel_dqe_a0[NIC2][0]
nic->rel_dqe_a1[NIC2][0]
nic->rel_dqe_a2[NIC2][0]
nic->rel_dqe_a0[NIC2][1]
nic->rel_dqe_a1[NIC2][1]
nic->rel_dqe_a2[NIC2][1]
=
=
=
=
=
=
=
=
1.1;
1.6;
-3.1131409;
0.093230650;
-0.00044176987;
-2.5612221;;
0.083302061;
-0.00042354937;
nic->rel_dqe_l_mu[NIC3][0]
nic->rel_dqe_l_mu[NIC3][1]
nic->rel_dqe_a0[NIC3][0]
nic->rel_dqe_a1[NIC3][0]
nic->rel_dqe_a2[NIC3][0]
nic->rel_dqe_a0[NIC3][1]
nic->rel_dqe_a1[NIC3][1]
nic->rel_dqe_a2[NIC3][1]
=
=
=
=
=
=
=
=
1.1;
2.2;
-5.1221150;
0.14595018;
-0.00077193987;
-1.8900719;
0.072383888;
-0.00041987179;
nic->aomega[NIC1]
nic->aomega[NIC2]
nic->aomega[NIC3]
= 1.75e-09 * 1.0e-4;
= 5.29e-09 * 1.0e-4;
= 3.61e-08 * 1.0e-4;
Our étendue include opaque obstructions within the outer perimeter of the primary.
Dark currents are recalculated on the fly using NICMOS end of life warmup data. See
Boeker et al. 1999 for details.
A3: Real object templates
Elliptical -- unpublished elliptical galaxy spectrum, M. Rieke
Gl 229B -- Cool brown dwarf spectrum, Oppenheimer et al., 1998, ApJ 502, 932
GD 165B -- Brown dwarf spectrum, Kirkpatrick et al., 199N, ApJ 519, 834
Gl 752B -- M8 spectrum, H. Jones
Gl 406 -- M6 spectrum, H. Jones
Gl 411 -- M2 spectrum, H. Jones
Other objects will be offered in the future.
A4: The high level routine imgston() pseudocode
Get the
get
set
set
detector name from input
filtername
camera number
camera-dependent ’qetfactor_key’
(for temp dependent QE multiplicative factor)
set camera-dependent ’dark_key’
15
(for temp-dependent dark current)
Get
Get
Get
Get
Get
Get
the point source / extended source switch value from input
read noise for double-correlated sampling from ’mode file’
pixel scale from ’mode’ file
pivot wavelength of source from synphot output
detector temperature from input
dark current bump key (yes/no switch) from input
Calculate sky background from synphot output on background and pixel
scale
**
*
*
*
*
*
*
Calculate thermal background from optics:
initialize the NICMOS and HST objects
set_nic_physical(...detector temperature)
set_nic_physical(...presence/absence of dark current bump)
create directory name where dqe*filter files exist from
#defines DATADIR and QEFILTSUBDIR
(in config.h)
obtain counts per sec in a pixel from thermal radiation
from HST and NICMOS optics
get_nic_physical(...relative dqe at pivot wavelength, detector T)
scale
scale
scale
scale
synphot source counts
synphot background counts
sky counts
thermal background counts
(*=
(*=
(*=
(*=
rdqe)
rdqe)
rdqe)
rdqe)
For point source request:
obtain camera- and pivot-wavelength-dependent
aperture correction (1", 1", 2" dia apertures)
scale source counts by aperture correction
(for ’counts in std aperture’ info only)
obtain camera- and filter-dependent central-pixel-fraction
For extended source request:
calculate target area from input target diameter
calculate counts per pixel from synphot source counts,
target area and pixel scale
Calculate SNR or Exposure time (as requested in input)
A5: Filter Summary
The following tables show the three cameras’ filters’ properties. The definitions of the all
columns except the last (Fraction) are those of the STSDAS.SYNPHOT.BANDPAR task.
These definitions are summarized here. For further details please see the on-line help on
BANDPAR. Here ‘INT’ refers to an integral over wavelength, and ‘THRU’ is the throughput of the filter at that wavelength. ‘LAM’ is wavelength.
Central Wavelength = bandpar.pivwv
defined as PIVWV = SQRT(INT(THRU * LAM) / INT (THRU / LAM))
Mean Wavelength = bandpar.avgwv
defined as AVGWV = INT(THRU * LAM) / INT(THRU)
Peak Wavelength = bandpar.wpeak
16
defined as wavelength at peak throughput
FWHM = bandpar.fwhm
defined as FWHM = SQRT(8 * LOG(2)) * BANDW
Max Transmission = maximum transmission OF THE FILTER ONLY
(no other optical components) as measured pre-launch in the lab
Max Throughput = bandpar.tpeak
defined as peak bandpass throughput
The right most column, Fraction, is the fraction of light falling on the central
pixel (assuming the point source PSF is centered on that pixel as estimated
by Tiny Tim PSF simulations.
All data in these tables (except for the PSF-related ‘Fraction’) are generated from the
STScI CDBS tables, which are updated as more accurate information is obtained. The
observer is strongly advised to regenerate these tables from the most up-to-date data in
CDBS if the exact values are critical to planning observations. The SYNPHOT user’s
guide (Bushouse, 1998) is the best guide on how to do this.
Table 6: NIC 1 Filters
Filter
Central
Mean
Peak
FWHM
Range
Trans
Thru
F090M
0.9041
0.9058
0.9176
0.1318
0.8 - 1.0
79.64
4.6174
0.150
F095N
0.9537
0.9537
0.9564
0.0088
1%
66.31
3.8254
0.136
F097N
0.9716
0.9716
0.9739
0.0104
1%
73.51
4.5612
0.131
F108N
1.0816
1.0816
1.0790
0.0090
1%
81.94
5.6077
0.111
F110M
1.1026
1.1042
1.1294
0.1390
1.0-1.2
90.49
7.2642
0.108
F110W
1.1292
1.1412
1.3430
0.3894
0.8-1.35
95.11
9.6072
0.113
F113N
1.1298
1.1298
1.1312
0.0111
1%
86.24
6.6160
0.102
F140W
1.4399
1.4593
1.7775
0.5592
0.8-1.8
95.50
16.0600
0.0766
F145M
1.4557
1.4569
1.5115
0.1384
1.35-1.55
94.03
12.0850
0.0640
F160W
1.6071
1.6114
1.7730
0.2755
1.35-1.75
96.37
16.1670
0.0562
F164N
1.6460
1.6461
1.6476
0.0142
1%
93.41
13.5890
0.0534
F165M
1.6489
1.6500
1.7122
0.1396
1.55-1.75
94.78
15.6350
0.0521
F166N
1.6607
1.6607
1.6624
0.0145
1%
90.12
13.8580
0.0505
F170M
1.7067
1.7078
1.7888
0.1410
1.6-1.8
95.55
16.3380
0.0481
F187N
1.8748
1.8748
1.8756
0.0158
1%
88.93
16.2170
0.0411
F190N
1.8986
1.8986
1.8942
0.0169
1%
93.05
16.6510
0.0401
17
Fraction
Table 6: NIC 1 Filters
Filter
Central
Mean
Peak
FWHM
POL0S
1.0602
1.0684
1.1435
0.3102
Range
0.8-1.3
Trans
Thru
77.60
3.4483
Fraction
0.048
Table 7: NIC 2 Filters
Filter
Central
Mean
Peak
FWHM
F110W
1.1285
1.1402
1.3402
0.3844
F160W
1.6060
1.6103
1.7722
F165M
1.6516
1.6527
F171M
1.7212
F180M
Range
Trans
Thru
Fraction
0.8-1.4
94.90
11.7710
0.288
0.2772
1.4-1.8
96.59
18.5650
0.159
1.7136
0.1395
1.55-1.75
95.36
18.4150
0.149
1.7214
1.7259
0.0584
1.68-1.75
95.15
17.7640
0.138
1.7971
1.7973
1.8131
0.0563
1.765-1.835
93.40
18.4280
0.128
F187N
1.8740
1.8740
1.8746
0.0159
1%
88.91
18.3620
0.119
F187W
1.8718
1.8732
1.8906
0.1707
1.75-2.0
87.97
18.0140
0.117
F190N
1.9003
1.9004
1.9005
0.0147
1%
93.22
19.2570
0.116
F204M
2.0355
2.0357
2.0634
0.0799
1.99-2.09
91.77
21.9010
0.104
F205W
2.0714
2.0795
2.3171
0.4314
1.75-2.35
98.14
29.5350
0.107
F207M
2.0824
2.0829
2.0683
0.1069
2.0-2.15
96.40
20.7120
0.0976
F212N
2.1213
2.1213
2.1228
0.0206
1%
90.90
21.4380
0.0953
F215N
2.1487
2.1487
2.1562
0.0187
1%
85.91
21.0440
0.0933
F216N
2.1641
2.1641
2.1668
0.0186
1%
90.07
21.8620
0.0915
F222M
2.2182
2.2188
2.1866
0.1205
2.15-2.3
89.90
22.2510
0.0881
F237M
2.3696
2.3701
2.4338
0.1115
2.3-2.45
92.38
28.6910
0.0797
POL0L
2.0007
2.0017
2.0750
0.1490
1.9-2.1
96.67
12.0740
0.33
Table 8: NIC 3 Filters
Filter
Central
Mean
Peak
FWHM
F108N
1.0800
1.0800
1.0777
0.0112
F110W
1.1264
1.1388
1.3402
F113N
1.1284
1.1284
F150W
1.5484
F160W
1.6078
Trans
Thru
1%
75.82
6.1366
0.582
0.3964
0.8-1.4
94.90
10.3110
0.590
1.1315
0.0106
1%
84.81
7.3788
0.579
1.5658
1.7685
0.5522
1.1-1.9
97.67
18.6900
0.534
1.6120
1.7730
0.2761
1.4-1.8
96.59
17.8900
0.524
18
Range
Fraction
Table 8: NIC 3 Filters
Filter
Central
Mean
Peak
FWHM
F164N
1.6460
1.6461
1.6476
0.0142
F166N
1.6583
1.6583
1.6602
F175W
1.8360
1.8635
F187N
1.8748
F190N
Trans
Thru
1%
86.27
14.9580
0.523
0.0139
1%
87.24
15.1480
0.513
2.2843
0.7605
1.2-2.3
96.58
28.7950
0.486
1.8748
1.8756
0.0158
1%
88.91
17.8240
0.471
1.9004
1.9004
1.9005
0.0147
1%
93.22
19.2900
0.466
F196N
1.9639
1.9639
1.9697
0.0177
1%
93.74
20.2760
0.450
F200N
1.9975
1.9975
2.0022
0.0196
1%
91.75
19.7980
0.445
F212N
2.1213
2.1213
2.1228
0.0206
1%
90.90
21.1640
0.418
F215N
2.1487
2.1487
2.1562
0.0187
1%
85.91
20.7940
0.413
F222M
2.2184
2.2190
2.1866
0.1204
2.15-2.3
89.9
22.0990
0.397
F240M
2.3969
2.3976
2.4162
0.1384
2.3-2.5
92.43
28.5600
0.363
19
Range
Fraction