Instrument Science Report NICMOS-98-006
Observed Camera 3 Performance
A. D. Storrs
May 6, 1998
ABSTRACT
This report describes the sensitivity of NICMOS camera 3 (NIC3) to point sources both
during the January 1998 “NIC3 Campaign”, and outside the campaign. The basic results
are given in table 1. Detectability of point sources is increased by 15%±5% at J (1.1 µm)
to 25%±5% at H (1.6 µm) but no significant change is seen at long wavelengths (2.2 µm).
Because of the undersampled point-spread function, we strongly recommend that observers dither their observations, to increase the chances of faint point sources being observed
at least once near the center of a pixel.
1. Introduction
We have analyzed data from point sources in NICMOS camera 3 (NIC3) images made
both immediately before and during the “NIC3 Campaign” of January, 1998. During this
period, the HST secondary was moved so that NIC3 was in focus. During normal operation, the NICMOS dewar anomaly prevents exact focus of NIC3.
2. Data and Analysis
The data analyzed for this report were almost entirely taken in January, 1998-- either
immediately before the campaign, or during it. The images were visually inspected for
point sources, which were then analyzed to determine the percentage of the flux in the
peak pixel, the full width at half maximum (FWHM) and the percentage of the flux to the
first airy minimum. Standard IRAF analysis tools were used: the imstat routine for the
photometry, and the pprofile routine for the FWHM and flux to first minimum criterion.
Subjective visual selection of the faintest point sources may include some slightly
extended sources in the analysis (e.g. compact planetary nebulae or AGN, depending on
the field). These would tend to have lower flux in the peak pixel and larger FWHM than
equivalent true point sources. These sources are generally so faint that no significant flux
is observed in adjacent pixels, so comparison to theoretical PSFs can’t easily distinguish
them from true point sources. Thus the ensemble of measurements were evaluated as a
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function of signal strength, as is shown in Figures 1-3. The ensemble results do not vary
significantly from the results obtained from only the bright sources. The larger population
used for the ensemble results gives smaller errors, so these results are reported here.
After the point sources were selected, their flux was determined by measuring the total
counts in two concentric square boxes, centered on the sources. The inner box was chosen
to contain all of the source that was visible (including the diffractions spikes) and the outer
box was four pixels larger. The sources were chosen so that no other source was contained
in the outer box. Thus for faint sources (which showed no diffraction spikes) the inner box
was typically 7 pixels (1.4 arcsec) on a side, and the outer box therefore 2.2 arcsec on a
side. For brighter sources larger boxes, up to 4.2 arcsec on a side, were used.
If the inner box has area a and total counts A, while the outer box has area b and total
counts B, two equations can be formed:
B = S + bK
A = S + aK
where S = the total counts from the point source, and K is the background rate per
pixel. Subtracting these two equations allows one to find the background rate:
B–A
K = ------------b–a
and once you have the background rate you can easily find S. The imstat routine conveniently returns the flux in the peak pixel, so the ratio of this number to S gives the
fraction of the flux in the peak pixel.
The pprofile routine was developed by Robert Jedrzejewski for analysis of well oversampled FOC images. The NIC3 pixels subtend 0.2 arcsec and so undersample the point
spread function (PSF) even in the red. In addition, the first minimum in the PSF is within
one pixel of the peak at all but the reddest wavelengths. Thus image subsections centered
on each point source were replicated (using the blkrep routine) four times in each dimension, to give images with pixels subtending 0.05 arcsec. The pprofile routine was able to
successfully centroid on these images and plot the azimuthally averaged profile and encircled energy. These plots were analyzed to determine the FWHM and energy to the first
minimum in the PSF. This extra manipulation leads to some concern about the precision of
these results. As seen in the next section, however, these track the more robust determination of the percentage flux in the peak pixel.
3. Results
The mean percentage of the flux in the peak pixel at J (F110W), H (F160W), and K
(mostly F215N) before and during the January NIC3 campaign are shown in Table 1.
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Table 2 shows the FWHM for the same wavelengths, inside and outside the campaign, and
Table 3 shows the encircled energy within the first PSF minimum.
Table 1. Percent flux in peak pixel.
J
H
K
before
34%±1.3%
30%±1.8%
32%±1.3%
during
40%±1.7%
40%±1.4%
34%±1.2%
J
H
K
before
1.34±0.03
1.29±0.03
1.35±0.03
during
1.33±0.03
1.26±0.02
1.31±0.03
J
H
K
before
39%±1.1%
41%±1.8%
55%±0.8%
during
43%±1.1%
49%±1.1%
58%±0.8%
Table 2. Full width at half maximum (pixels).
Table 3. Percent flux to first PSF minimum.
Mean values determined for the bright stars do not differ significantly from those
determined from the whole ensemble. Ensemble values are reported here.
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Figure 1: Histogram of % flux in peak pixel at J (1.1 µm), H (1.6 µm), and K (2.2 µm).
Hatched histogram is bright stars only. Upper, outside, and lower, during campaign.
Dashed line is the mean for the whole ensemble.
NOAO/IRAF V2.11EXPORT storrs@polar.stsci.edu Mon 13:35:24 20-Apr-98
J band, outside of campaign
J
8
H
NOAO/IRAF V2.11EXPORT storrs@polar.stsci.edu Mon 13:33:37 20-Apr-98
H band, outside of campaign
5
4
6
3
out
4
2
2
1
0
0
20
20
40
60
80
NOAO/IRAF V2.11EXPORT storrs@polar.stsci.edu Mon 13:35:25 20-Apr-98
J band, during campaign
10
40
60
80
NOAO/IRAF V2.11EXPORT storrs@polar.stsci.edu Mon 13:33:38 20-Apr-98
H band, during campaign
12.5
10
7.5
in
7.5
5
5
2.5
2.5
0
0
20
40
60
K
out
80
20
40
60
80
NOAO/IRAF V2.11EXPORT storrs@polar.stsci.edu Mon 08:21:12 27-Apr-98
K band, outside of campaign
3
2
1
0
20
40
60
80
NOAO/IRAF V2.11EXPORT storrs@polar.stsci.edu Mon 08:21:13 27-Apr-98
K band, during campaign
8
in
6
4
2
0
20
40
60
80
The histograms in Fig. 1 are much wider than the variations in Tables 1-3. This is
because the large NIC3 pixels dramatically undersample the PSF. The percentage flux in
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the peak pixel depends on where the PSF falls within the pixel- if the PSF lands in the
middle, most of the flux is recorded there, but if near the edge or the corner the flux is
shared with adjacent pixels. Pointing alone can explain the breadth of the distribution seen
in the histograms. Note that the histograms get narrower as the wavelength increases. This
is expected since at longer wavelengths the PSF is less undersampled and the above effects
are less pronounced.
From the histograms in Fig. 1, the detectability of a star can be seen to improve by
about a factor of 2 if centered, and in the mean (Table 1) by as much as 30%. Note the tail
in the histograms of in-campaign data. This tail is especially prominent at the shorter
wavelengths. This is the result of the PSF being sharply focused: as mentioned above, the
fraction of the energy in the peak pixel is a strong function of the position of the PSF
within the pixel when the peak of the PSF is much smaller than the pixel. Outside of a
refocus campaign, even the shorter wavelength PSFs are about the size of a pixel and you
do not get this high-detectability tail.
Histograms of the other two figures of merit (FWHM and % energy to first minimum)
show much smaller differences in and out of focus. This is expected, because the effect of
the defocus is to move energy from the core of the psf to the wings, while not changing the
shape of the core dramatically. Note that the expected monotonic wavelength dependence
is not seen in tables 1-3. This may be due to the interaction of the first airy ring of the psf
and the edge of the pixel at J, but a detailed investigation is beyond the scope of this ISR.
The total flux from a given point source can vary noticeably between images of the
same field, made with small dithers. This variation can be 10% or more for single observations at short wavelengths. This variation falls below 5% at K. This is due to the intrapixel
sensitivity variation, and dead space between pixels. It is noticeably worse for in focus
images and at shorter wavelengths, since more of the flux in the PSF is concentrated in a
portion of the pixel that experiences these effects.
4. Conclusions
Table 1 shows that at J and H (1.1 and 1.6 microns) the detectability of point sources
can be enhanced by 20-30% by shifting HST’s secondary to put NIC3 in focus. The gain is
less at longer wavelengths-- NIC3 is effectively in focus at K (2.2 microns) with the standard HST focus. The visibility of extended targets is probably not enhanced by as great a
factor. “Typical” science observations (e.g. faint, high redshift galaxies in the HDF) however, will subtend less than the 0.2” pixel size and so are effectively point sources.
Observers of crowded and/or faint fields in the 1.0-1.8 micron wavelength region will
benefit most from scheduling in a campaign-- faint point sources will be enhanced 20-30%
over normal observations. Such observations will greatly benefit from subpixel dithering.
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