Instrument Science Report NICMOS 2001-01
Performance of the NICMOS ETC
Against Archived Data
M. Sosey
June 19, 2001
ABSTRACT
A robust test of the newest version of the NICMOS Exposure Time Calculator (ETC) was
conducted in order to assess its performance against actual NICMOS datasets in predicting an accurate Signal - to - Noise ratio for observation planning. All available signal
regimes were explored and the results show good agreement with the ETC for all cases.
Introduction
The NICMOS ETC was completely redesigned in Spring 2000 to support the NCS-era
of NICMOS observations. It also introduced new algorithms to correct the estimate of the
thermal background contributions from the telescope. Further information on the redesign
of the ETC and the new thermal library can be found in Sivaramakrishnan et al, 2000. At
the time of its release the code had been checked for accuracy against the original ETC,
but no testing was completed on its agreement with archived NICMOS datasets. Therefore, this project was designed to confirm that the observed signal-to-noise ratio (hereafter
referred to as SNR) was in good agreement with the predicted one and that the performance of the ETC was consistent over the available temperature range.
Data and Analysis
All data used for this analysis were gathered from the HST archive and represent a
broad range of available NICMOS data, spanning the initial operating temperature of the
instrument (61.5 K - 62.5 K). The basic regimes which were tested are as follows:
•
High SNR observations: these datasets test the Poisson noise limited regime.
•
Low SNR observations, where faint objects were observed just above the background,
achieving a typical signal to noise below 20. These datasets test the background and
instrument limited regimes.
Copyright© 2001 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Instrument Science Report NICMOS 2001-01
The above scenarios were tested for point sources and extended objects in each of the
NICMOS cameras, and when available, in several different filters. Unfortunately data
about known faint stars was unavailable in the database, therefore no testing was done in
this part of the PSF regime. The SNR for each image was calculated as:
St
SNR = ---------------------------2
St + ( σt )
where:
S is the total signal for the source in electrons / pixel / second
σ is the rms noise in the background in electrons / pixel / second
t is the total exposure time for the observation
Each dataset was evaluated separately and the respective properties for each object
were entered into the ETC web form to calculate the predicted SNR.
Point Sources
The ETC returns only the SNR for the central pixel of the observed PSF. To achieve
this, the total flux value calculated by SYNPHOT is multiplied by the pixfrac - the fraction
of total energy that falls on the central pixel of the PSF. This assumes that the image is
centered on that pixel. In practice, it’s possible to measure the signal in the peak pixel
using the imexam routine in the IRAF package. Imexam fits a gaussian profile to the pixels surrounding the center of the star and then returns the peak value for that fit. This is
preferable over using imstat to obtain the highest pixel value around the center of the
star because of individual pixel sensitivities and detector cosmetics. The stars were carefully selected to avoid known bad pixels and grot (Sosey et al, 1999). No preference was
given to any quadrant in any of the cameras, object locations were spread across the entire
array.
Each selected dataset was fully processed through the NICMOS pipeline and associated algorithms when necessary (such as pedsky), using the most current reference files,
which in some cases included synthetic darks and flatfields (to account for changes in
detector temperature). Care was taken in images affected by many cosmic ray hits and
other anomalies to get a realistic measurement of the background noise. No extinction values were calculated for any of the point sources.
The NICMOS standard stars were chosen for testing bright, high SNR PSFs. The requisite Kurucz spectra was chosen for the solar analogs, and a black body spectrum at the
appropriate temperature was used to simulate the White Dwarf. The Kurucz spectra are
stored in the CDBS archive and cover the wavelength range from 1000Å - 10 microns.
They are specified by effective temperature, metalicity and gravity and called through the
SYNPHOT routine icat which then interpolates between the spectra in the database which
bracket the specified values. Table 1 lists information about each of the standard stars.
2
Instrument Science Report NICMOS 2001-01
Table 2 details which stellar datasets were chosen and their resulting measured and predicted SNR values.
Table 1. Information for each of the NICMOS standard stars used
Star Name
Type
Mag
Other Info.
P330E
Solar Analog
12.01 (J)
G2V-Kurucz
G191-B2B
White Dwarf
12.6 (J)
Teff ~ 61300
P177D
Solar Analog
12.47 (J)
G2V-Kurucz
Location of the object on the detector is not a provided option, as the ETC does not
attempt to model flat field variance, it merely uses the average DQE at the pivot wavelength of the source. Since some of the standard star observations were well dithered they
also provided a decent check on how much the variation in pixel sensitivity affects the
SNR. Figure 1 shows the ratio of predicted and measured SNR measurements vs. wavelength for P330E. The large range in the Camera 2, F110W measurements illustrates the
effects of varying pixel sensitivity across the array. Data from proposal 7693, the pupil
transfer function, were used for these calculations. It is emphasized again that these are
measurements on the peak flux from the central pixel, not from a full aperture. Aperture
photometry in NIC 2 has been proven at the 2% level (Sosey et al, in preparation). The
observed spread in SNR is too large for intrapixel sensitivity to be the main culprit. Camera 2 has 75 mas pixels which critically sample the PSF at all wavelengths. The same
spread can be seen in cameras 1 and 3 to a similar extent. Figure 2 shows the ratio of predicted and measured SNR for camera 3, P177D, for the F110W and F222M filters. In the
case of Camera 3 observations intrapixel sensitivity plays a larger role since the PSF is
undersampled at all wavelengths, with flux variations of up to 30% for individual images
of a single star observation. For more information on intrapixel sensitivity see Storrs et al,
1999. The average measured SNR for all the stellar datasets taken in a particular camera
and filter combination was used for comparison against the ETC and match the predicted
values from the ETC extremely well (See Table 2).
3
Instrument Science Report NICMOS 2001-01
Figure 1: SNR measurements for P330E in all cameras
Figure 2: SNR Measurements for P177D in camera 3
4
Instrument Science Report NICMOS 2001-01
Table 2. Results for each of the stellar datasets which were examined
Proposal
ID
Camera
Filter
Object
Name
Exptime
(s)
Predicted
SNR
Measured
SNR
%
Different
7904
1
F110W
G191-B2B
4.53
150
154
3
7904
1
F160W
G191-B2B
9.97
100
106
6
7904
2
F110W
G191-B2B
4.53
270
277
3
7904
2
F222M
G191-B2B
47.96
150
138
8
7607
1
F110W
P177D
3.25
120
120
0
7607
1
F160W
P177D
3.25
88
77
12
7607
1
F110W
P330E
3.25
150
151
<1
7607
2
F110W
P177D
1.62
150
150
0
7607
2
F160W
P177D
1.62
110
103
6
7607
2
F207M
P330E
31.69
240
233
3
7902
1
F095N
P330E
159.16
130
127
2
7902
1
F145M
P330E
8.97
150
120
20
7693
1
F160W
P330E
3.02
86
86
0
7693
2
F222M
P330E
9.97
120
119
<1
7693
2
F110W
P330E
2.42
230
244
6
7693
3
F222M
P177D
7.97
180
177
2
7693
3
F110W
P177D
2.12
220
233
6
7696
3
F110W
G191-B2B
5.98
410
396
3
7696
3
F150W
G191-B2B
7.97
439
450
2
7816
3
F222M
G191-B2B
47.96
290
276
5
7816
3
F110W
P177D
4.98
350
334
5
7816
3
F160W
P177D
5.98
360
345
4
7816
3
F215N
P177D
63.96
200
172
14
7816
3
F240M
P177D
13.95
260
240
8
7152
3
F110W
P330E
1.64
250
132
47
7152
3
F166N
P330E
31.97
190
146
23
7152
3
F222M
P330E
1.64
100
86
14
The largest disagreement between the SNR values predicted by the ETC and those
measured from the data can be found in camera 3. The measurement of P330E in the
F110W filter only agreed to 47%. Other images of P177D and G191B2B in the same filter
agreed quite well with the ETC estimates. Closer examination of the P330E image itself
5
Instrument Science Report NICMOS 2001-01
reveals it to be about twice as noisy as the others (See Figure 3). This dataset was taken 30
minutes after a 23 minute long SAA (South Atlantic Anomaly) passage. Other images
which executed in the same proposal (7152) also show increased noise that improves as
the image start time is increasingly farther from the SAA exit time. This is a good example
of how cosmic ray persistence can have adverse affects on the SNR of NICMOS observations. For more information on NICMOS and the SAA see Najita, et al. 1998.
Figure 3: NIC3 comparison of P177d and P330e images at the same stretch
P330E
P177D
Extended Sources
For extended sources, the ETC returns the SNR for a pixel which is fully illuminated
by the observed source. It accepts information about the source as either the surface flux
from the galaxy in Jy per arcsec2 and the central wavelength, or the magnitude in Vega
mags per arcsec2. Only elliptical galaxies were chosen from the database since they have
the most well defined source spectrum (SED) in the ETC (the elliptical galaxy spectrum
was provided by M. Rieke and is unpublished). Each of the datasets were processed
through the NICMOS calibration pipeline and associated routines when necessary (such
as pedsky). The surface flux from each object was measured in a circular radius and an
estimate for the background noise was calculated through the IRAF routine iterstat.
Incomplete color information about each of the sources makes it hard to calculate a reasonable extinction, therefore, none was used to correct the elliptical SED.
6
Instrument Science Report NICMOS 2001-01
Table 3 is an example dataset of extended object measurements. The data is taken from
proposal 7895, a snapshot survey of field galaxies. Both objects were compact and each
was measured using an aperture of 6 pixels. Even in this faint SNR regime, the ETC does
a fairly good job of predicting the actual SNR for both the individual datasets and the average combined measurements. The compiled results for each of the chosen extragalactic
datasets can be found in Table 4.
Table 3. A closer look at proposal 7895, all data are from Camera 2, F160W
Object
Name
Gal-141748+523117
Gal-141729+522738
Surface
Brightness
Jy arcsec-2
Measured
SNR
ETC
SNR
6.7e-5
9.24
9.7
6.7e-5
9.28
9.7
6.8e-5
9.42
9.8
9.7e-5
12.08
13
9.7e-5
12.58
13
8.2e-5
11.12
12
7
Average
Measured SN
Avg. Surface
Brightness
Jy arcsec-2
ETC results
using average
9.31
6.73e-5
9.7
11.93
9.2e-5
13
Instrument Science Report NICMOS 2001-01
Table 4. Results for each of the galaxy datasets that were chosen
Prop
ID
Cam
Filter
Object
Type
Object
Name
Exptime
(sec)
Predicted
SN
Meas.
SN
%
different
7875
1
F160W
Faint Blue
Compact Galaxies
SA68-9640
2816
8.8
6.7
23
7875
1
F160W
Faint Blue
Compact Galaxies
SA68-8846
2816
8.9
6.7
23
7875
1
F160W
Faint Blue
Compact Galaxies
SA68-17418
2816
8.8
6.5
26
7875
1
F160W
Faint Blue
Compact Galaxies
SA57-1501
2816
3.9
2.8
28
7875
1
F160W
Faint Blue
Compact Galaxies
HER1-13925
1280
19
16
28
7875
2
F160W
Faint Blue
Compact Galaxies
SA68-1067
1344
38
33
26
7875
2
F160W
Faint Blue
Compact Galaxies
SA68-3307
1280
20
16.5
28
7875
2
F160W
Faint Blue
Compact Galaxies
SA68-6597
1344
7.7
6.7
13
7454
2
F160W
Faint Radio
Galaxies
3c184
514
5.5
6.4
14
7454
2
F160W
Faint Radio
Galaxies
3c184
1026
11
10.74
2
7454
2
F160W
Faint Radio
Galaxies
3c184
1026
9.3
9.48
2
7454
2
F160W
Faint Radio
Galaxies
3c184
514
6
6.25
2
7454
2
F160W
Faint Radio
Galaxies
3c184
1026
11
7.46
32
7454
2
F165M
Faint Radio
Galaxies
3c266
1026
2.0
2.33
14
7454
2
F165M
Faint Radio
Galaxies
3c266
514
0.77
0.93
17
7454
2
F165M
Faint Radio
Galaxies
3c266
1026
2.0
2.66
24
7454
2
F165M
Faint Radio
Galaxies
3c266
1026
1.6
1.23
23
7280
2
F110W
Faint Galaxies
53W069
514
1.1
0.59
46
7280
2
F110W
Faint Galaxies
53W069
514
1.4
0.92
34
7280
2
F110W
Faint Galaxies
53W091
514
1.4
0.67
52
7328
1
F160W
Bright Seyfert
Galaxy
IRAS 1832-5926
256
280
270
4
8
Instrument Science Report NICMOS 2001-01
Prop
ID
Cam
Filter
Object
Type
Object
Name
Exptime
(sec)
Predicted
SN
Meas.
SN
%
different
7328
2
F160W
Bright Seyfert
Galaxy
IRAS 1833-654
256
170
121
28
7328
2
F160W
Bright Seyfert
Galaxy
IRAS2302-0004
256
180
140
22
7425
3
F160W
Faint Galaxy
Cluster
0026+1653
576
10
8
20
7425
3
F160W
Faint Galaxy
Cluster
0026+1653
576
16
14
12
7425
3
F160W
Faint Galaxy
Cluster
0026+1653
576
13
11
15
7425
3
F160W
Bright Galaxy
Cluster
0026+1653
576
41
36
12
7425
3
F160W
Bright Galaxy
Cluster
0026+1653
576
29
22
24
7459
3
F110W
Faint Galaxy
Cluster
171411+501550
272
9
11
18
7459
3
F110W
Faint Galaxy
Cluster
171411+501550
272
10
12
16
7459
3
F160W
Faint Galaxy
Cluster
171411+501550
272
19
17
10
7459
3
F160W
Faint Galaxy
Cluster
171411+501550
272
20
18
10
7459
3
F110W
Faint Galaxy
Cluster
171411+501550
272
19
22
14
7459
3
F160W
Faint Galaxy
Cluster
171411+501550
272
36
33
8
7817
3
F110W
Bright Galaxy
HDF NIC Field
1344
25
26
4
7817
3
F110W
Bright Galaxy
HDF NIC Field
1408
26
27
4
7817
3
F160W
Bright Galaxy
HDF NIC Field
1408
94
89
5
7817
3
F160W
Bright Galaxy
HDF NIC Field
1408
43
40
7
7817
3
F110W
Faint Galaxy
HDF NIC Field
1408
18
19
5
7817
3
F160W
Bright Galaxy
HDF NIC Field
1408
62
58
6
7817
3
F110W
Bright Galaxy
HDF NIC Field
1408
120
125
4
7817
3
F110W
Faint Galaxy
HDF NIC Field
1408
19
18
5
7458
1
F110M
Bright Galaxy
NGC 1339
256
70
59
16
7458
1
F110M
Bright Galaxy
NGC 1339
256
68
57
16
7458
1
F110M
Bright Galaxy
NGC 1339
256
66
58
12
9
Instrument Science Report NICMOS 2001-01
Conclusions and Changes to the ETC Web Interface
As a general estimate of exposure time, the ETC predicts a very accurate SNR for
bright point sources. Except for some of the medium and narrow band filters, all of the
SNR predictions were within 10% of the measured values. Even for the case of varying
sensitivity across the detector, the average SNR of the measurements was well estimated.
The results from proposal 7693 show that the local SNR can vary by a large amount, so
users should avoid the lowest DQE areas when possible. Although no faint stellar sources
were tested, the results from this report suggest that the ETC would also do a good job in
predicting accurate SNR’s for faint stellar sources.
In general, the ETC did a good job of predicting SNR for both faint and bright
extended objects. The predicted and measured values are generally within 20% of each
other and in some cases better than 10%. Low SNR observations can suffer greatly from
background noise induced by thermal emission from the telescope optics (affecting the
longer wavelength filters) and the typical noise characteristics of the detectors. In these
cases it is hard to accurately predict the final observation characteristics. The results from
the faint galaxy measurements of proposal 7280 show how difficult it is to predict and
measure SNRs around 1. They also show that the ETC did a fairly good job of calculating
an accurate SNR. Note that the large percentage difference between the predicted SNR
and measured SNR is a result of the small numbers that are involved. The ETC also
appeared to report slightly better SNR values for the faint blue galaxies that were measured in F160W. Users observing such objects may wish to slightly overestimate their
exposure times in order to get the desired SNR.
The following items in the operation of the NICMOS ETC graphical user interface
have been changed:
•
Under the HST standard star spectra that are listed, only the standard stars that have
somewhat well defined infrared spectra in CDBS (i.e. are populated past 0.9 microns
by more than a few points) were saved. All other spectra have been removed from the
options box. However, it should be noted that only the NICMOS standard stars (P330E and G191B2B) have spectra which have been obtained using both ground and
space based data as reference. Other spectra contain only general extrapolations into
the infrared from the visible.
•
The “Real Object” templates should include a text description of their spectral classification in the option box, for easy reference. They cannot be extended to include this
because it would interfere with the scripting of the ETC. This information may be
found in Appendix A3 of NICMOS-ISR-00-0001 and there is already a link in the
ETC GUI directly to that ISR on the NICMOS website.
•
The option to use Grism filters for NIC3 has been removed since they are not supported by the ETC.
10
Instrument Science Report NICMOS 2001-01
•
New aperture corrections were computed as part of work being done to improve the
NICMOS photometric calibration. The old values have been updated to agree with the
new aperture corrections (results of the improved photometric calibration will be presented in a future ISR). The current ETC contains a subset of apertures corrections and
filters that do not represent each filter and camera combination. It has a table of values
which are interpolated to find the aperture correction for a given filter. While this correction is fairly linear with wavelength for all cameras, the ETC would be easier to
maintain if a full table of filter and aperture corrections replaced this old table of values.
•
The option to have more than 1 number of reads (nread) has been removed. The
default value is now 1 and no longer a user option.
NICMOS users in cycle 11 and beyond should expect to use this version of the ETC to
aid in calculating exposure times for their programs. The datasets tested for this analysis
spanned the entire range of available temperatures. After NCS is installed NICMOS is
expected to operate at the higher temperature of 78K, this is now the default temperature
in the ETC web interface. The increased temperature will decrease the large range of DQE
values that were seen during the initial lifetime of the instrument. Users should still avoid
the low DQE regions when designing their observations, but the difference between the
lowest and highest DQE regions will be smaller. Work is in progress to integrate all of the
HST instrument exposure time calculators into a more uniform, observatory wide ETC as
part of the Astronomy Proposal Tools package, but the basic functionality of the ETC
should not change.
References
Burstein, D., Heiles, C., ApJ, 225:40-55, 1978
Calzetti, D., Kinney, A., Storchi-Bergmann, ApJ, 429:582, 1994
Rieke, G. H., Lebofsky, M. J., ApJ, 288:618-621, 1985
Najita, J., Dickinson, M., Holfeltz, S., NICMOS ISR-98-001, 1998
Seaton, M. J., MNRAS, 187, p.75, 1979
Sivaramakrishnan, A., Holfeltz, S., Sosey, M., Simon, B., Robberto, M. NICMOS
ISR-00-0001, May 2000
Sosey, M., Bergeron, L. E., NICMOS ISR-99-008, 1999
Storrs, A.,R. Hook, M. Stiavelli, C. Hanley, W. Freudling NICMOS ISR-99-005, 1999
Zombeck, M. V. 1990, Handbook of Astronomy and Astrophysics, Second Edition:
Cambridge University Press)
11
Instrument Science Report NICMOS 2001-01
12