Signal & S/N


The signal we get from a star depend on some adjustable parameters as the slit width or the diaphragm used. But also depend on some others, that not always are accurately known, as actual seeing value, atmosphere transmission, etc ... For this reason, it is not easy to predict in a precise way the number of counts we will get in an exposure.
We show here two sets of observations done with different atmospheric conditions and instrument setup. These can be useful to estimate the exposure time to get a desired signal or S/N ratio.

Table 1.
Observations done with the 200 µ diaphragm, slit width of 120 µ, seeing value aprox. 1.0" and a 15 µ pixel CCD.

V Sp Typ Exp. time Counts Counts/sec S/N
3.19 F6V 70 37000 529 314
5.11 G0V 300 33000 110 290
5.70 F9V 400 17000 43 219
5.97 G2V 1500 53000 35 387
6.45 G2V 1000 30000 30 282
6.51 K0IV 900 36000 40 286
6.58 K0V 1500 58000 39 357
6.78 G5IV 1100 33000 30 274
6.98 dG9 1500 37000 25 292
7.29 G4V 1800 27000 15 272
7.51 K2V 1800 31000 17 248
8.05 K0V 2300 24000 10.4 218
8.59 K0 3000 22000 7.3 210
9.68 G4V 3600 7000 1.9 128


Table 2.
Observations done with the 200 µ diaphragm, slit width of 200 µ, seeing value 1.2"-1.4" and a 15 µ pixel CCD.

V Sp Typ Exp. time Counts Counts/sec S/N
3.40 B8V 120 33200 277 420
3.66 B2IV 120 34500 287 391
3.78 A1V 60 5400 85 212
5.12 B7III 120 6400 53 175
5.62 A0III 120 2200 14 128
6.90 G5 60 390 6.5 41
7.03 G0 110 610 5.5 62
7.11 F8 120 510 4.3 55
7.23 K3V 90 280 3.1 25
7.31 G5 120 350 2.9 41
7.75 G0 120 310 2.6 33
7.83 K3V 120 280 2.3 30
8.59 K4V 300 340 1.1 42
8.73 K0 300 350 1.2 39
8.85 K0 300 300 1.0 33
9.08 K4 300 255 0.85 26
9.15 M3 300 190 0.63 24
9.17 K7 300 180 0.6 20
9.20 G5 300 230 0.77 24
9.76 O 500 550 1.1 38
9.97 M0.5 450 195 0.43 22
10.51 Op 600 350 0.6 25


For both tables, signal and S/N values are referred to the wavelength range 5400-5500 Å.
S/N values are obtained after full data reduction.
Signal it is shown as counts per pixel. If a 24 µ pixel CCD it is used, these values should be increased 2.56 times. S/N ratios remain the sames.
Santos Pedraz Marcos
Last modified: Sun Sep 24 05:04:56 WET DST 2000