COS Sensitivity
The table, below, compares the COS sensitivities to those of various STIS
spectroscopic modes. The sensitivity depends upon the size of the spectral
resolution element. Thus, for each mode, values for both the native resolution
and binned resolution of the mode are provided to make these comparisons
easier.
| Mode |
 / |
Binning |
S
(1013 cnt s-1 resel-1) /
(erg cm-2 s-1 Å-1) |
| FUV Modes |
=1300Å |
=1600Å |
COS G130M
COS G160M |
20,000 |
--- |
1.0 |
0.69 |
10,000 |
x2 |
2.0 |
1.4 |
2,500 |
x8 |
8.0 |
5.5 |
1,000 |
x20 |
20. |
14. |
STIS E140M |
45,800 |
--- |
0.060 |
0.024 |
20,000 |
x2.3 |
0.14 |
0.055 |
STIS G140M |
10,000 |
--- |
0.66 |
0.13 |
2,500 |
x4 |
2.6 |
0.52 |
COS G140L |
|
|
|
3.4 |
1,000 |
x2.5 |
20. |
8.5 |
| STIS G140L |
1,000 |
--- |
12.6 |
3.0 |
| NUV Modes |
=1800Å |
=2300Å |
=2800Å |
COS G185M
COS G225M
COS G285M |
20,000 |
--- |
0.43 |
1.1 |
1.5 |
10,000 |
x2 |
0.86 |
2.2 |
2.0 |
2,500 |
x8 |
3.4 |
8.8 |
12. |
500 |
x40 |
17. |
44. |
60. |
STIS E230M |
30,000 |
--- |
0.062 |
0.30 |
0.36 |
20,000 |
x1.5 |
0.093 |
0.45 |
0.54 |
STIS G230M |
10,000 |
--- |
0.20 |
|
|
2,500 |
x4 |
0.80 |
3.8 |
4.1 |
COS G230L |
2,500 |
--- |
5.2 |
9.4 |
6.5 |
500 |
x5 |
26. |
47. |
32. |
| STIS G230L |
500 |
--- |
8.8 |
32. |
34. |
The values shown are counts per resel per second per FEFU, and are for point sources. Please note that these data are plotted for display purposes only and that those planning observations should use the ETC to get accurate estimates. Also note that these data are pre-flight measurements that will be verified after COS is installed. An FUV resel is 6 pixels wide, so the sensitivity per pixel is 6 times lower than the value indicated here.
The values shown are in counts per resel per second per unit FEFU, and are for point sources. The data shown are pre-flight measurements from ground tests. An NUV resel is 3 pixels wide, so the sensitivity per pixel is 3 times lower than the value indicated here.
The values shown are in counts per resel per second per unit FEFU, and are for point sources. NUV resels are 3 pixels wide. The data shown are pre-flight measurements from ground tests. An NUV resel is 3 pixels wide, so the sensitivity per pixel is 3 times lower than the value indicated here.
The flux over all wavelengths must be integrated to get the total number of counts per second.
That total count rate is for the entire image; the PSF shown in Figure 6.2 of the COS Instrument Handbook indicates how much ends up in the peak pixel (about 14%).
Sensitivity to second-order spectra
The result from ground tests is that only G225M shows measurable second-order throughput,
but even then the second-order light was suppressed by factors of 3,000 to 10,000. Because most
astrophysical sources decline in flux in going to shorter wavelengths, it is expected that second-order
contamination in COS will be insignificant.
For more information on COS Sensitivity see sections
5.1.7 and 6.3 of the COS Instrument Handbook.