Space Telescope Science Institute   6.5.1 Point Sources -- PSF Fitting  6.5.3 Extended Sources

6.5.2 Point Sources -- Aperture Photometry


When aperture photometry is used, one must consider the fraction of the object counts encircled by the aperture, as well the background noise in the aperture. In the bright target limit the SNR is given by

where S is the number of detected photons, f(r) is the fraction of the total counts encircled by the aperture with radius r, and Robject is target count rate. Representative values of f(r) are given in Table 6.7; values for other aperture sizes and filters can be estimated from Figure 5.3, or Figure 5.4.

In the faint target limit the noise contributed by background counts determines the SNR

where B represents the effective background counts per pixel, and r is the aperture radius in pixels.

In the generalized case the SNR per exposure for aperture photometry is given approximately by:

where the parameters are summarized in Table 6.8.

Table 6.7: Encircled Energy for Representative Filters. Encircled energy values are normalized to unity at large radius.
CCD
Aperture
Radius
(r)
Encircled Energy f(r)
F218W
F555W
F814W
PC1
0.1"
0.60
0.67
0.53

0.2"
0.73
0.85
0.78

0.5"
0.84
0.96
0.87

1.0"
0.92
1.00
0.92
WF3
0.1"
0.40
0.46
0.44

0.2"
0.69
0.76
0.74

0.5"
0.85
0.90
0.91

1.0"
0.94
0.94
0.96


Table 6.8: Parameters for Point Source SNR Estimation - Aperture Photometry.
Parameter
Description
Units
Approx. Value
Better Value
Robject
object count rate
e- s-1

Equation 6.1, 6.2, or 6.3
Pdark
dark count rate
e- s-1 pixel-1
0.004
Table 4.2; Eqn 4.1 on page 90
Psky
sky count rate
e- s-1 pixel-1
Pbackground
count rate from background light (if any)
e- s-1 pixel-1

readnoise

e-
ATD-GAIN=7 use 5.31
ATD-GAIN=15 use 7.5
f(r)
encircled energy

r
aperture radius
pixels


t
exposure time
s


1ATD-GAIN defaults to 7 unless otherwise specified on Phase II proposal.

 6.5.1 Point Sources -- PSF Fitting  6.5.3 Extended Sources
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