ALFOSC UBVRI Zero Point Monitoring

In August 2000 we decided to start monitoring the UBVRI zero points for ALFOSC on each possible technical time occasion at the NOT in collaboration with the GL team at CUO (PI: Jens Hjorth). A data reduction pipe-line was made by Andreas Jaunsen who kindly has performed the photometric analysis.

The photometric standard cluster positions and exposure times we use.
Name	RA (2000)	DEC (2000)	U#7	B#74	V#75	R#76	i#12
M92	17:17:03.82	43:02:03.2	60	40	30	20	20
NGC2419	07:38:16.00	38:55:32.0	120	60	20	20	20
NGC7790	23:58:27.70	61:13:01.0	30	30	10	10	10

Three photometric clusters were selected in order to cover visibility at an airmass < 2 at any time. They are all among the Stetson Photometric Standard Fields . M92 is preferred, though, because it has good U-band standards (from L. Davis, NOAO). Observations should be performed at 3 different airmasses in photometric conditions. Online catalogue is zpmon.cat. Sky flat fields and bias frames are needed, as well.

Results

Find below one table per filter with date of observations, cluster observed, and the derived zero point magnitude (zp) which is the magnitude for 1 ADU per second, the colour term (ct), and the atmospheric extinction coefficient (ext), all given with errors. Note that in some cases the extinction coefficients were not determined from the observations on that night, but fixed extinction values were used. The fitted zp, ct, and ext values were obtained with the following equations:

U = U_zp + u + U_ct * (U-B) + U_ext * X 
B = B_zp + b + B_ct * (B-V) + B_ext * X 
V = V_zp + v + V_ct * (V-I) + V_ext * X 
R = R_zp + r + R_ct * (R-I) + R_ext * X 
I = I_zp + i + I_ct * (V-I) + I_ext * X

U-filter.
Date	Field	U_zp	U_ct	U_ext	U_rms	Comment
2000-Sep-11	NGC7790					no U-band standards.
2000-Oct-16	NGC7790					no U-band standards.
2001-Jan-14	NGC2419, M92	23.65 ± 0.02	0.09 ± 0.04	-0.462	0.026	fixed extinction
2001-Jan-14	NGC2419, M92	23.4 ± 0.2	0.09 ± 0.04	-0.3 ± 0.1	0.020	free fit (10 stars)
2001-Feb-08	NGC2419					no U-band standards
2001-Mar-08	NGC2419					no U-band standards
2001-Apr-07	M92	23.61 ± 0.01	0.10 ± 0.04	-0.462	0.031	fixed extinction
2001-Oct-04	NGC7790					no U-band standards

B-filter.
Date	Field	B_zp	B_ct	B_ext	B_rms	Comment
2000-Sep-11	NGC7790	25.704 ± 0.002	0.053 ± 0.002	-0.292	0.017	fixed extinction
2001-Oct-16	NGC7790	25.712 ± 0.002	0.062 ± 0.002	-0.292	0.019	fixed extinction
2001-Jan-14	NGC2419, M92	25.710 ± 0.006	0.032 ± 0.008	-0.292	0.037	fixed extinction
2001-Jan-14	NGC2419, M92	25.53 ± 0.02	0.050 ± 0.009	-0.18 ± 0.01	0.041	free fit
2001-Feb-08	NGC2419	25.60 ± 0.01	0.06 ± 0.02	-0.220 ± 0.007	0.023	free fit
2001-Feb-08	NGC2419	25.68 ± 0.02	0.08 ± 0.03	-0.292	0.040	fixed extinction
2001-Mar-08	NGC2419	25.62 ± 0.02	0.06 ± 0.02	-0.280 ± 0.006	0.030	free fit, 3-4" seeing
2001-Apr-07	M92	25.651 ± 0.005	0.057 ± 0.009	-0.292	0.027	fixed extinction
2001-Oct-04	NGC7790	25.423 ± 0.007	0.055 ± 0.002	-0.214 ± 0.005	0.018	free fit

V-filter.
Date	Field	V_zp	V_ct	V_ext	V_rms	Comment
2000-Sep-11	NGC7790	25.660 ± 0.002	-0.059 ± 0.002	-0.179	0.019	fixed extinction
2000-Oct-16	NGC7790	25.684 ± 0.002	-0.061 ± 0.002	-0.179	0.025	fixed extinction
2001-Jan-14	NGC2419, M92	25.672 ± 0.005	-0.074 ± 0.005	-0.179	0.037	fixed extinction
2001-Jan-14	NGC2419, M92	25.54 ± 0.01	-0.050 ± 0.004	-0.107 ± 0.007	0.032	free fit
2001-Feb-08	NGC2419	25.62 ± 0.01	-0.061 ± 0.006	-0.135 ± 0.005	0.025	free fit
2001-Feb-08	NGC2419	25.68 ± 0.01	-0.060 ± 0.009	-0.179	0.035	fixed extinction
2001-Mar-08	NGC2419	25.62 ± 0.01	-0.062 ± 0.008	-0.164 ± 0.006	0.037	free fit, 3-4" seeing
2001-Apr-07	M92	25.624 ± 0.004	-0.046 ± 0.005	-0.179	0.022	fixed extinction
2001-Oct-04	NGC7790	25.44 ± 0.01	-0.063 ± 0.002	-0.108 ± 0.007	0.023	free fit

R-filter.
Date	Field	R_zp	R_ct	R_ext	R_rms	Comment
2000-Sep-11	NGC7790	25.493 ± 0.004	-0.047 ± 0.007	-0.136	0.030	fixed extinction
2000-Oct-16	NGC7790	25.515 ± 0.004	-0.067 ± 0.006	-0.136	0.033	fixed extinction
2001-Jan-14	NGC2419, M92	25.499 ± 0.004	-0.089 ± 0.009	-0.136	0.036	fixed extinction
2001-Jan-14	NGC2419, M92	25.41 ± 0.01	-0.06 ± 0.01	-0.088 ± 0.008	0.035	free fit
2001-Feb-08	NGC2419	25.48 ± 0.01	-0.09 ± 0.02	-0.086 ± 0.005	0.033	free fit
2001-Feb-08	NGC2419	25.55 ± 0.01	-0.09 ± 0.02	-0.136	0.043	fixed extinction
2001-Mar-08	NGC2419	25.47 ± 0.02	-0.04 ± 0.02	-0.140 ± 0.010	0.043	free fit, 3-4" seeing
2001-Apr-07	M92	25.497 ± 0.005	-0.09 ± 0.01	-0.136	0.027	fixed extinction
2001-Oct-04	NGC7790	25.33 ± 0.01	-0.071 ± 0.005	-0.095 ± 0.008	0.025	free fit

I-filter.
Date	Field	I_zp	I_ct	I_ext	I_rms	Comment
2000-Sep-11	NGC7790	24.866 ± 0.003	-0.033 ± 0.003	-0.079	0.027	fixed extinction
2000-Oct-16	NGC7790	24.850 ± 0.003	-0.042 ± 0.003	-0.079	0.033	fixed extinction
2001-Jan-14	NGC2419, M92	24.930 ± 0.005	-0.109 ± 0.005	-0.079	0.035	fixed extinction
2001-Jan-14	NGC2419, M92	24.80 ± 0.01	-0.082 ± 0.005	-0.008 ± 0.007	0.032	free fit
2001-Feb-08	NGC2419	25.14 ± 0.03	-0.056 ± 0.006	not derived	0.026	free fit
2001-Feb-08	NGC2419	24.888 ± 0.009	-0.059 ± 0.007	-0.079	0.032	fixed extinction
2001-Mar-08	NGC2419	24.89 ± 0.01	-0.057 ± 0.007	-0.116 ± 0.005	0.038	free fit, 3-4" seeing
2001-Apr-07	M92	24.943 ± 0.007	-0.046 ± 0.008	-0.079	0.029	fixed extinction
2001-Oct-04	NGC7790	24.73 ± 0.01	-0.043 ± 0.003	-0.060 ± 0.009	0.030	free fit