Inhomogeneity in Spectral Transmission of the Coated UBVRI Filters of BFOSC and its Influence on Photometry

Details Inhomogeneity in Spectral Transmission of the Coated UBVRI Filters of BFOSC and its Influence on Photometry Inhomogeneity in Spectral Transmission of the Coated UBVRI Filters of BFOSC and its Influence on Photometry

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002chjaa...2..563y&link_type=abstract

Chinese Journal of Astronomy & Astrophysics, Vol. 2, p. 563-575

Astronomy and Astrophysics

Astrophysics

Methods: Data Analysis, Techniques: Photometric

Scientific paper

In order to realize the Johnson UBV and Kron-Cousins RI photometric systems in BFOSC, glass filters were purchased according to the Bessell prescription. All the filters are anti-reflection coated on both sides to increase the transmission. By comparing two dome flat-field exposures taken through any one of these filters set in two orientations 90 deg apart, inhomogeneities in the filter transmission is clearly demonstrated. Using a PDS microphotometer, we have confirmed that the form of their spectral transmission curve varied from point to point on the filters. This kind of inhomogeneity cannot be eliminated by dividing by flat-fielding exposure and must be ascribed to inhomogeneities in the coating. This is not some accidental defect in coating, it is in the existing coating technique because all the filters (including the narrow-band interference ones) for the BFOSC and a Hβ filter made by another manufacturer showed a similar pattern. We found that for the studies of variable stars with amplitudes less than 0.05 mag. or studies aiming at establishing accurate C-M diagrams of star clusters, the usual uncoated Schott glass filters of Bessell's prescription are to be preferred.

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