Data processing is done with IRAF using Daophot psf photometry. For each image, we obtain a list
with instrumental magnitudes
for all stars in the CCD field. The magnitudes and colors are all reduced to no-atmosphere with extinction coefficients
determined each night.
The 1st and 2nd
order extinction is determined either by observing comparison stars in the
or in many fields over the large range of air mass. It depends on sky
transparency and show significant
night to night variations.
The derivation of these coefficients, obtained on two nights with good and bad transparency is illustrated below.
First order extinction, obtained on UV Leo sequence on February 27, 2000:
First order extinction, obtained on UV Leo sequence on March 2, 2000:
Transformation of our CCD and filter combination to standard photometric system was obtained by using
standard stars in
M67 open cluster.
were taken from
WEBDA open cluster database.
were processed to J2000 and matched with our measurements.We took a deep BVRI exposures for color calibration.
After the extinction correction, we plot
V-v against B-V (for V) and
(B-V) - (b-v) against B-V
(for B). Similar plots were prepared for
The relationship between the standard and ACIT observed values is:
v0 - 0.004(B-V) + zv
v0 + 0.03(V-R)c + zOv
1.520(b-v)0 + zbv
1.031(v-r)0 + zvr
0.966(v-i)0 + zvi
0.909(r-i)0 + zri
Differential extinction and color correction is then applied and standard BVRI
(Johnson-Cousins) magnitudes calculated. For illustration, the extinction and
color corrected light curve of
eclipsing variable, obtained in
filters is added. Also reduced
single night data
of UV Leo, containing standard V magnitude vs. MJD of variable and seven comparison stars shows that the system is well calibrated.