Space Telescope Science Institute   9.1.1 The ACS Exposure Time Calculator  9.2.1 Imaging

9.2 Determining Count Rates from Sensitivities


In this Chapter, specific formulae appropriate for imaging and spectroscopic modes are provided to calculate the expected count rates and the signal-to-noise ratio from the flux distribution of a source. The formulae are given in terms of sensitivities, but we also provide transformation equations between the throughput (QT) and sensitivity (S) for imaging and spectroscopic modes.

Throughputs are presented in graphical form as a function of wavelength for the prisms and for the imaging modes in Chapter 10. Given your source characteristics and the sensitivity of the ACS configuration, calculating the expected count rate over a given number of pixels is straightforward, since the ACS PSF is well characterized. The additional required information is the encircled energy fraction (f) in the peak pixel, the plate scale, and the dispersions of the grisms and prisms. This information is summarized in Table 9.1, Table 9.2, and Table 9.3.


Table 9.1: Useful quantities for the ACS WFC.
Filter
Pivot (Ċ)
QT d/
AB mag
zero point
S d
encircled
energy
Flux in
central
pixel
Background
sky rate
F435W
4317.4
0.0744
25.67
3.18E18
0.85
0.22
0.0284
F475W
4744.3
0.1070
26.07
5.53E18
0.86
0.21
0.0609
F502N
5023.0
0.0032
22.27
1.88E17
0.86
0.21
0.0020
F550M
5581.2
0.0362
24.89
2.59E18
0.85
0.22
0.0280
F555W
5359.6
0.0775
25.72
5.11E18
0.86
0.21
0.0545
F606W
5917.7
0.1573
26.49
1.26E19
0.85
0.22
0.1358
F625W
6310.5
0.0915
25.90
8.37E18
0.85
0.22
0.0825
F658N
6583.9
0.0050
22.75
5.00E17
0.85
0.22
0.0045
F660N
6599.4
0.0019
21.67
1.87E17
0.85
0.22
0.0017
F775W
7693.0
0.0731
25.65
9.93E18
0.85
0.20
0.0780
F814W
8059.8
0.0948
25.94
1.41E19
0.83
0.19
0.1044
F850LP
9054.8
0.0352
24.86
6.63E18
0.73
0.14
0.0415
F892N
8914.9
0.0036
22.37
6.48E17
0.77
0.15
0.0039
G800L
7480.7
0.1591
26.50
2.04E19
0.86
0.17
0.1526
CLEAR
6269.8
0.3833
27.45
3.46E19
----
0.10
0.3580

Table 9.2: Useful quantities for the ACS HRC.
Filter
Pivot (Ċ)
QT d/
AB mag
zero point
S d
encircled
energy
Flux in
central
pixel
Background
sky rate
F220W
2255.3
0.0106
23.55
1.23E17
0.79
0.17
0.0001
F250W
2715.8
0.0123
23.72
2.09E17
0.82
0.16
0.0003
F330W
3362.7
0.0172
24.08
4.47E17
0.84
0.16
0.0006
F344N
3433.7
0.0017
21.59
4.67E16
0.84
0.16
0.0001
F435W
4311.0
0.0436
25.09
1.86E18
0.86
0.17
0.0043
F475W
4775.7
0.0650
25.53
3.40E18
0.85
0.17
0.0120
F502N
5022.9
0.0021
21.82
1.24E17
0.85
0.17
0.0003
F550M
5579.8
0.0234
24.42
1.67E18
0.85
0.15
0.0048
F555W
5356.0
0.0503
25.25
3.31E18
0.85
0.16
0.0099
F606W
5888.0
0.0983
25.98
7.82E18
0.85
0.14
0.0238
F625W
6295.5
0.0557
25.36
5.07E18
0.84
0.13
0.0148
F658N
6583.7
0.0030
22.18
2.98E17
0.83
0.13
0.0008
F660N
6599.1
0.0011
21.11
1.11E17
0.83
0.13
0.0003
F775W
7665.1
0.0380
24.94
5.13E18
0.78
0.11
0.0111
F814W
8115.3
0.0521
25.29
7.88E18
0.76
0.10
0.0161
F850LP
9145.2
0.0228
24.39
4.37E18
0.68
0.08
0.0075
F892N
8916.1
0.0022
21.87
4.09E17
0.73
0.09
0.0007
G800L
7506.4
0.0924
25.91
1.20E19
0.81
0.07
0.0250
PR200L
5650.2
0.2756
27.10
2.02E19
0.80
0.16
0.0624
CLEAR
5472.0
0.3205
27.26
2.20E19
----
0.12
0.0683

Table 9.3: Useful quantities for the ACS SBC.
Filter
Pivot (Ċ)
QT d/
AB mag
zero point
S d
encircled
energy
Flux in
central
pixel
Background
sky rate
F115LP
1406.1
0.0147
23.92
6.70E16
0.83
0.11
0.0238
F122M
1273.8
0.0010
20.95
3.56E15
0.82
0.09
0.0042
F125LP
1437.5
0.0122
23.71
5.80E16
0.83
0.12
0.0027
F140LP
1527.0
0.0069
23.09
3.68E16
0.84
0.13
0.0000
F150LP
1610.7
0.0038
22.45
2.27E16
0.84
0.14
0.0000
F165LP
1757.9
0.0010
21.02
7.29E15
0.85
0.16
0.0000
PR110L
1429.4
0.0120
23.69
5.62E16
0.86
0.26
0.0129
PR130L
1438.8
0.0120
23.68
5.66E16
0.87
0.28
0.0027

In each Table, the following quantities are listed:

Here, we describe how to determine two quantities:

  1. The counts/second (C) from your source over some selected area of Npix pixels, where a signal of an electron on a CCD is equivalent to one count.
  2. The peak counts/second/pixel (Pcr) from your source, which is useful for avoiding saturated CCD exposures, and for assuring that SBC observations do not exceed the bright-object limits.

We consider the cases of point sources and diffuse sources separately in each of the imaging and spectroscopy sections following.


 9.1.1 The ACS Exposure Time Calculator  9.2.1 Imaging
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