AAVSO: BVRI CCD Photometry Observing Program

The AAVSO has a program to monitor selected stars in the B, V, R, and I filters using CCD observations. We began the program with a limited number of stars for two reasons. First, we had developed finder charts with comparison stars in the four filters for only 8 stars. Second, we wished to see how well data can be combined from different observers with different telescopes, cameras, and software. Please send BVRI data only for the stars for which we have BVRI charts: S Per, U Ori, VX Gem, DH Dra, VX UMa, W Leo, RU Vir, and RR Boo.

Each night you observe, you should take bias images, dark images, and flat fields with each of the filters you intend to use. All images should be processed before you do the photometry (bias and dark subtracted and divided by the flat). We suggest that you take a minimum of three images in each filter for each star you observe. In order to transform the images to a standard system, you need to observe with at least two different filters in each session.

Choose three comparisons for each variable you observe and find the magnitude of the variable by comparing it to each of the three comparison stars. The data you enter on the report form should be corrected using your transformation instructions.

First you will get differential magnitudes between the variable and each of the three comparisons. Then, obtain the differential colors (B-V, V-R, R-I). Then, using the transformation equations, transform the colors. Calculate the color- corrected values of the differential magnitudes. Finally, add the calibrated value (from the AAVSO CCD chart) for each of your three comparisons to obtain the correct magnitude of your variable for each filter you have observed. An example calculation for observations with the V, R, and I filters is given on the next page.

If you have observed only with the R and I filters, you will need only the R-I and R transformation equations. If you observe only with V and R, use the transformation equation for T_v and T_vr. If you observe with the B filter, then you must also transform B-V.

Sample Calculation of Transformed Magnitudes

		Instrumental Magnitudes			Standard Magnitudes
		v	r	i	V	R	I
RR Boo		21.284	18.508	16.974
Comp 2		19.118	17.599	17.519	9.661	9.046	8.466
Comp 4		22.516	20.999	20.969	13.073	12.522	11.980
Comp 6		23.151	21.625	21.649	13.756	13.154	12.625
RR Boo	- Comp 2	2.166	0.909	-0.545
RR Boo	- Comp 4	-1.232	-2.491	-3.995
RR Boo	- Comp 6	-1.867	-3.117	-4.675

		Differential Colors
		v-r	r-i
RR Boo	- Comp 2	1.257	1.454
RR Boo	- Comp 4	1.259	1.504
RR Boo	- Comp 6	1.250	1.558

Assume your transformation coefficients are as follows:

T_vr = 0.95; T_ri = 1.00; T_r = 0.09 and: V-R = T_vr * (v-r); R-I = T_ri * (r-i); and R = r + T_r* (R-I)

to transform the instrumental differential colors to standard differential colors:

		Transformed Differential Colors
		V-R	R-I	R
RR Boo	- Comp 2	1.194	1.454	1.040
RR Boo	- Comp 4	1.196	1.504	-2.356
RR Boo	- Comp 6	1.188	1.558	-2.977

Using V = (V-R) + R and I = R - (R-I) to obtain the transformed differential magnitudes:

	Transformed Differential Magnitudes
	V	R	I
RR Boo - Comp 2	2.234	1.040	-0.414
RR Boo - Comp 4	-1.160	-2.356	-3.860
RR Boo - Comp 6	-1.789	-2.977	-4.534

Use the standard magnitudes of the comparison stars to obtain the magnitudes of the variable star:

	Magnitudes of RR Boo
	V	R	I
Based on Comp 2	11.895	10.086	8.052
Based on Comp 4	11.913	10.166	8.120
Based on Comp 6	11.967	10.177	8.090

search engine | site map | links | contact us