WFPC2 Technical Instrument Report TIR 99-01
WFPC2 Aperture Photometry
Corrections as a Function of Chip Position
S. Gonzaga, C. O’Dea, and B. Whitmore.
March 25, 1999
ABSTRACT
The WFPC2 PSF shape is a function of position in the chip. As the PSF changes, the relative fraction of encircled light inside and outside a given aperture will also change. The
effect is larger for smaller apertures as shown by ISR 97-01 (Suchkov and Casertano).
This report looks at the effect of PSF changes across the field-of-view on photometric
aperture correction as a function of position on the chips by comparing aperture photometry in apertures of different radii.
1. Introduction
This report attempts to characterize the change in photometric aperture corrections as
a function of chip position, an effect caused by focus and PSF changes over the WFPC2
field-of-view. Aperture photometry was performed on a set of stars, using apertures of different radii. The differences in magnitude at various radii were compared to determine
how aperture correction values changed as a function of chip position. The datasets used
were observations of the Omega Cen field, using the the F555W and F814W photometric
filters. (Program 6195 executed on June 29, 1996; the target was placed on a different chip
in each of the 3 orbits.) We determined the magnitudes in apertures of radius 1,2,5, and 10
pixels, then plotted the difference in magnitudes between the various apertures: this was
done by plotting the magnitude difference vs. radius from the chip center (assuming azimuthal symmetry), as well as by fitting a two dimensional surface to the magnitude
differences to look for non-symmetric behaviour.
2. Analysis and Results
The secondary mirror focus position during the time of the observations (June 29,
1996) was +2.1 microns from the nominal position. Three different pointings for the
Omega Cen target were analysed, each being a cr-split pair. Each image set was combined
#
using the crrej task to remove most cosmic rays. Stars were selected using noao.digiphot.apphot.daofind. In addition, stars along the pyramid edges were removed from each
chip because photometry in these vignetted areas is unreliable. Aperture photometry was
then performed using the noao.digiphot.apphot.phot task, with apertures of 1, 2, and 5
pixels radius, with a sky annulus radius of 5 pixels and an annulus width of 5 pixels. Photometry with apertures of 5 and 10 pixels were also done, with a sky annulus radius of 13
pixels, and annulus width of 5 pixels. After all three fields were processed, the magnitudes
were compiled into a single table so they could be compared.
The next step involved taking the magnitude differences between the various apertures. We computed the following magnitude differences:
magrad=1 - magrad=5
magrad=2 - magrad=5
magrad=2 - magrad=10
magrad=5 - magrad=10
The results of these magnitude differences are presented in three ways:
1. For each chip and filter combination, the mean and standard deviation of the difference in magnitude between two apertures was obtained. Tables 1 and 2 shows the
mean and standard deviation of magnitude differences for the apertures listed
above in the F555W and F814W filters respectively (rejected outliers are not
included in the statistics).
Table 1: Magnitude differences for PC, WF2, WF3, and WF4 in F555W.
Chip
Filter
Aperture
Difference
Number
of Stars
Mean
Magnitude
Differences
Stddev of
Magnitude
Differences
PC
F555W
1-5
116
0.887
0.054
PC
F555W
2-5
115
0.275
0.028
PC
F555W
2-10
115
0.401
0.075
PC
F555W
5-10
115
0.106
0.055
WF2
F555W
1-5
558
0.608
0.130
WF2
F555W
2-5
558
0.160
0.085
WF2
F555W
2-10
544
0.310
0.257
WF2
F555W
5-10
548
0.133
0.204
WF3
F555W
1-5
660
0.680
0.133
WF3
F555W
2-5
656
0.188
0.076
WF3
F555W
2-10
649
0.376
0.308
#
Mean
Magnitude
Differences
Stddev of
Magnitude
Differences
Aperture
Difference
Number
of Stars
F555W
5-10
647
0.154
0.233
WF4
F555W
1-5
828
0.672
0.129
WF4
F555W
2-5
831
0.198
0.115
WF4
F555W
2-10
815
0.386
0.350
WF4
F555W
5-10
814
0.160
0.252
Chip
Filter
WF3
Table 2: Magnitude differences for PC, WF2, WF3, and WF4 in F814W.
Chip
Filter
Aperture
Difference
Number
of Stars
Mean
Magnitude
Differences
Stddev of
Magnitude
Differences
PC
F814W
1-5
67
1.096
0.073
PC
F814W
2-5
76
0.436
0.052
PC
F814W
2-10
50
0.595
0.050
PC
F814W
5-10
58
0.109
0.062
WF2
F814W
1-5
429
0.738
0.053
WF2
F814W
2-5
435
0.192
0.036
WF2
F814W
2-10
290
0.290
0.057
WF2
F814W
5-10
377
0.063
0.048
WF3
F814W
1-5
479
0.796
0.040
WF3
F814W
2-5
480
0.225
0.027
WF3
F814W
2-10
394
0.305
0.048
WF3
F814W
5-10
426
0.068
0.040
WF4
F814W
1-5
643
0.783
0.051
WF4
F814W
2-5
654
0.219
0.034
WF4
F814W
2-10
263
0.369
0.070
WF4
F814W
5-10
362
0.113
0.055
2. Radial plots, created by measuring the distance of the star from the chip center versus the aperture correction between two given apertures. This analysis assumes
that the aperture correction changes are symmetrical about the center of the chip,
which may not be true.
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Figures 1 through 8 illustrates these radial plots for each filter/chip combination. (The
unfilled points in the plots represent rejected measurements.) Note that data for the
large apertures (2-10 and 5-10) tend to have more scatter. This is probably due to lower
signal-to-noise that comes with using a large aperture, as well as contamination from
scattered light of nearby stars."
The tables below summarizes the linear fit (y = a + bx) for aperture correction differences for each filter and chip combination at the various apertures. The fit parameters
are as follows:
a:
the aperture correction between two different apertures at the chip center (y-intercept value, in magnitudes).
the slope of the line fit (magnitudes per pixel).
the standard deviation of the slope.
b:
sigma_b:
We also give the column dmr=500 - dmr=0 that shows the change (in magnitudes) for aperture correction values from the edges (radius=500, near corners) to the center of the chip (radius=0).
Table 3: Fit parameters for PC, WF2, WF3, and WF4 in F555W.
Chip
Filter
Aperture
Difference
a
b
sigma_b
dmr=500 - dmr=0
PC
F555W
1-5
0.818
2.41E-4
4.4E-5
0.120
PC
F555W
2-5
0.244
1.13E-4
2.5E-5
0.057
PC
F555W
2-10
0.376
8.4E-5
5.6E-5
0.042
PC
F555W
5-10
0.109
-2.9E-5
3.4E-5
-0.014
WF2
F555W
1-5
0.536
2.59E-4
3.0E-5
0.123
WF2
F555W
2-5
0.115
1.27E-4
1.7E-5
0.063
WF2
F555W
2-10
0.231
9.9E-5
5.6E-5
0.050
WF2
F555W
5-10
0.081
2.0E-5
4.2E-5
0.010
WF3
F555W
1-5
0.658
8.5E-5
2.3E-5
0.042
WF3
F555W
2-5
0.162
6.6E-5
1.4E-5
0.033
WF3
F555W
2-10
0.276
8.6E-5
4.9E-5
0.043
WF3
F555W
5-10
0.090
4.0E-5
3.7E-5
0.020
WF4
F555W
1-5
0.570
2.37E-4
1.7E-5
0.119
WF4
F555W
2-5
0.137
1.17E-4
1.0E-5
0.059
WF4
F555W
2-10
0.228
2.46E-4
5.5E-5
0.123
WF4
F555W
5-10
0.068
1.05E-4
3.9E-5
0.052
#
Table 4: Fit parameters for PC, WF2, WF3, and WF4 in F814W.
Chip
Filter
Aperture
Difference
a
b
sigma_b
dmr=500 - dmr=0
PC
F814W
1-5
1.012
2.73E-4
7.5E-5
0.136
PC
F814W
2-5
0.383
1.95E-4
4.9E-5
0.097
PC
F814W
2-10
0.491
1.73E-4
1.1E-4
0.086
PC
F814W
5-10
0.083
1.22E-5
8.2E-5
0.006
WF2
F814W
1-5
0.683
1.914E-4
2.3E-5
0.095
WF2
F814W
2-5
0.161
9.6E-5
1.3E-5
0.048
WF2
F814W
2-10
0.230
1.65E-4
4.2E-5
0.082
WF2
F814W
5-10
0.056
5.1E-5
3.0E-5
0.025
WF3
F814W
1-5
0.770
8.6E-5
1.6E-5
0.043
WF3
F814W
2-5
0.196
9.1E-5
7.4E-6
0.045
WF3
F814W
2-10
0.280
1.22E-4
3.3E-5
0.061
WF3
F814W
5-10
0.065
2.62E-5
2.3E-5
0.013
WF4
F814W
1-5
0.744
1.39E-4
1.8E-5
0.069
WF4
F814W
2-5
0.184
1.07E-4
8.3E-6
0.053
WF4
F814W
2-10
0.290
1.73E-4
4.7E-5
0.086
WF4
F814W
5-10
0.078
7.82E-5
3.7E-5
0.039
3. Surface plots, created using the task stsdas.toolbox.imgtools.xyztoim
Figures 9 through 16 illustrate the surface plots for the filter/chip combinations. We
encountered difficulty in getting reliable results due to the lack of high quality PSFs
evenly distributed over the field-of-view. These surface plots are presented only to
illustrate the difficulties in obtaining a surface fit, and should not be used for calibration.
For both radial and surface plots, there is significant scatter in the aperture corrections
as a function of chip position. This is due to faint stars and residual cosmic rays contaminating the larger aperture measurements. In the case of the surface plots, there were not
enough stars in the field to get a robust two-dimensional fit of the variations, even though
we combined data from 3 different pointings to increase the number of stars. By doing
this, some of the scatter we see may also be due to breathing. In addition, some of the scat-
#
ter in the “delta vs radius” plots could be due to the fact that the variation is not centered on
the chip center.
The largest scatters were seen for magnitude differences between aperture radii 2-10
pixels and 5-10 pixels. Although attempts were made to remove “bad” stars by four iterations of a 3-sigma rejection, it was not sufficient to reduce the scatter; for those cases, we
were reluctant to apply more stringent rejection parameters for fear of introducing a biased
and artificial selection into the results. The bottom line is that more stars need to be measured at the same focus to get a better picture of focus changes as a function of field-ofview.
There appears to be significant differences in aperture correction for the smaller apertures (magnitude differences seen for measurements of aperture radii 1-5 pixels and 2-5
pixels). This is seen clearly in some of the radial plots that have small sigma_b’s. For
example, there is a variation of about 0.1 magnitudes between the center and edges of the
chips for aperture radii magnitude differences of 1-5 pixels in some chips.
3. Conclusions
Many of the radial plots show a roughly linear dependence for the aperture correction
with respect to the center of the chip. We have not attempted to fit more complex functions
(e.g. parabolic curve).
The surface plots, although not robust enough to define a two-dimensional aperture
correction function, do illustrate the radial dependence in many cases, especially for the
small apertures. Some of the plots indicate that the aperture corrections may not be symmetrical about the center of the chip, but are, instead, offset from the center. More data and
analysis are needed to better characterize the two-dimensional dependence.
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Figure 1
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Figure 9
Contour Plot F555W PC Aperture 1-5
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Contour Plot F555W PC Aperture 2-5
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 15:52:44 24-Feb-99
Contour Plot F555W PC Aperture 2-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 15:32:37 24-Feb-99
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Contour Plot F555W PC Aperture 5-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 15:56:16 24-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 15:59:01 24-Feb-99
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Figure 10
Contour Plot F555W WF2 Aperture 1-5
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Contour Plot F555W WF2 Aperture 2-5
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 16:06:39 24-Feb-99
Contour Plot F555W WF2 Aperture 2-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 16:04:13 24-Feb-99
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Contour Plot F555W WF2 Aperture 5-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 16:09:03 24-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Wed 16:09:59 24-Feb-99
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Figure 11
Contour Plot F555W WF3 Aperture 1-5
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Contour Plot F555W WF3 Aperture 2-5
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:16:16 25-Feb-99
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:23:10 25-Feb-99
Contour Plot F555W WF3 Aperture 2-10
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Contour Plot F555W WF3 Aperture 5-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:25:41 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:34:14 25-Feb-99
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Figure 12
Contour Plot F555W WF4 Aperture 1-5
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Contour Plot F555W WF4 Aperture 2-5
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:28:09 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:29:34 25-Feb-99
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Contour Plot F555W WF4 Aperture 2-10
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Contour Plot F555W WF4 Aperture 5-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:31:21 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:32:40 25-Feb-99
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Figure 13
Contour Plot F814W PC Aperture 1-5
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Contour Plot F814W PC Aperture 2-5
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 14:53:07 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:27:51 25-Feb-99
Contour Plot F814W PC Aperture 2-10
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Contour Plot F814W PC Aperture 5-10
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:29:59 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:33:55 25-Feb-99
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Figure 14
Contour Plot F814W WF2 Aperture 1-5
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Contour Plot F814W WF2 Aperture 2-5
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:42:38 25-Feb-99
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NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:55:23 25-Feb-99
Contour Plot F814W WF2 Aperture 2-10
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Contour Plot F814W WF2 Aperture 5-10
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contoured from 0.2475484 to 0.4425484, interval = 0.005
contoured from 0.0529414 to 0.1529414, interval = 0.005, labels scaled by 1000.
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:56:45 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:57:34 25-Feb-99
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Figure 15
Contour Plot F814W WF3 Aperture 1-5
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Contour Plot F814W WF3 Aperture 2-5
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contoured from 0.2069638 to 0.4019638, interval = 0.005
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:58:56 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 15:59:53 25-Feb-99
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Contour Plot F814W WF3 Aperture 2-10
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contoured from 0.7806593 to 0.9756593, interval = 0.005
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Contour Plot F814W WF3 Aperture 5-10
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contoured from 0.2891198 to 0.4841198, interval = 0.005
contoured from 0.05574542 to 0.1857454, interval = 0.005, labels scaled by 1000.
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 16:01:04 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 16:02:27 25-Feb-99
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Figure 16
Contour Plot F814W WF4 Aperture 1-5
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Contour Plot F814W WF4 Aperture 2-5
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contoured from 0.1950231 to 0.3900231, interval = 0.005
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 16:03:43 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 16:04:31 25-Feb-99
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Contour Plot F814W WF4 Aperture 2-10
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contoured from 0.7419718 to 0.9369718, interval = 0.005
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Contour Plot F814W WF4 Aperture 5-10
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contoured from 0.2970869 to 0.4920869, interval = 0.005
contoured from 0.06265213 to 0.2026521, interval = 0.005, labels scaled by 1000.
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 16:05:38 25-Feb-99
NOAO/IRAF V2.11EXPORT shireen@whimbrel.stsci.edu Thu 16:06:58 25-Feb-99
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