6.3.1 Count Rates for Stellar Sources

To estimate the number of electrons collected from a point source of apparent visual magnitude V, one can use the equation:

where t is the exposure time in seconds, the QT integral is given in Table 6.1, and AB is given in Table 6.2 as a function of spectral type and wavelength for some example spectral energy distributions. The quantity AB is a color-dependent correction from V magnitude to AB magnitude at frequency . The AB magnitude system is defined as (Oke and Gunn 1983)

where F is the flux in erg cm^-2 s^-1 Hz^-1.

Table 6.2: AB as a Function of Wavelength. AB is defined as a color-dependent correction from V magnitude to AB magnitude at frequency . Wavelength (Å) runs along the top; spectral classes run down the left most column. The second column contains B-V. See Target Count Rates.

	B-V	1500	2000	2500	3000	3500	4000	4500	5000	6000	7000	8000	9000	10000
sky	1.10	2.45	5.46	5.46	3.12	2.00	1.03	0.55	0.18	-0.11	-0.33	-0.55	-0.65	-0.75
B0	-0.31	-1.60	-1.50	-1.20	-0.78	-0.62	-0.46	-0.36	-0.22	0.16	0.46	0.76	0.96	1.17
A0	0.00	2.22	1.35	1.11	1.21	1.00	-0.23	-0.16	-0.09	0.11	0.22	0.33	0.36	0.4
F0	0.27	7.22	4.10	3.11	1.99	1.38	0.29	0.06	0.03	0.03	0.05	0.08	0.09	0.1
G0	0.58	8.9	6.35	4.61	2.46	1.63	0.67	0.26	0.08	-0.04	-0.12	-0.21	-0.23	-0.25
K0III	1.07	13	10.3	8.11	5.46	2.13	1.16	0.46	0.2	-0.24	-0.42	-0.61	-0.66	-0.72
M0III	1.60	15	12.3	9.36	6.21	4.63	2.26	0.96	0.51	-0.46	-0.76	-1.06	-1.12	-1.19
gE	1.00	6.82	6.41	5.43	3.63	2.49	1.40	0.55	0.21	-0.19	-0.52	-0.81	-1.07	-1.29
Sa	0.80	5.40	4.80	4.10	3.00	2.01	1.12	0.44	0.19	-0.17	-0.44	-0.7	-0.95	-1.16
Sbc	0.60	4.03	3.18	2.86	2.46	1.54	0.84	0.34	0.17	-0.14	-0.37	-0.6	-0.84	-1.04
Scd	0.45	2.67	2.29	2.15	1.76	1.35	0.65	0.28	0.13	-0.11	-0.26	-0.39	-0.47	-0.58
Ir I	0.30	1.77	1.40	1.36	1.24	0.94	0.43	0.34	0.17	0.13	-0.04	-0.21	-0.33	-0.45

Equation 6.1 may be trivially rewritten to give the count rate Robject in units of e^- s^-1 pixel^-1 for a target with a stellar spectrum as: