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.
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. |
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |