The ACIT Imaging System
The ACIT Imaging System
An ultra thin, eight position filter wheel with standard BVRI CCD photometric filters has been added to the ACIT system. They are made by Omega Optical and combined according to "Glass filters for coated CCDs" proposed by Bessell [Bessell, M.S. (1990), PASP, 102, 1181] which match the standard BVRI passbands. Filter wheel is integrated into the system and it is possible to image one or more sequences each night with any combination of filters.

Recently, the photometric setup has been used for precise color photometry of eclipsing variable stars. It has relatively large field, covering nearly one square degree which enable us to image the variable and several comparison stars at the same time. Its main advantage is that it is possible to do accurate photometry even in mediocre sky conditions, eg. with some cirruss clouds present. This is the case in our test observation of V505 Per, obtained on November 13, 1999. The V505 Per instrumental V band light curve shows random variations due to bad sky conditions but the final light curve is quite consistent, except near the binary minima, when the sky was completely cloudy.

Data processing
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 single field 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: Kv=0.466, Kb=0.603
First order extinction, obtained on UV Leo sequence on March 2, 2000: Kv=0.258, Kb=0.333

Transformation of our CCD and filter combination to standard photometric system was obtained by using standard stars in M67 open cluster. Standard magnitudes were taken from WEBDA open cluster database. Star coordinates 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 Rc and Ic filters.

The relationship between the standard and ACIT observed values is:
V = v0 - 0.004(B-V) + zv
V = v0 + 0.03(V-R)c + zOv
(B-V) = 1.520(b-v)0 + zbv
(V-R)c = 1.031(v-r)0 + zvr
(V-I)c = 0.966(v-i)0 + zvi
(R-I)c = 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 UV Leo eclipsing variable, obtained in V and B 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.

Program stars
Eclipsing Binaries

Current status

Other objects
V886 Her

Eclipsing Variable Minima Calculator

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