NDLI: A New Look at Johnson/Cousins Photometric Filters

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A New Look at Johnson/Cousins Photometric Filters

Content Provider	Semantic Scholar
Author	Goldman, Don S. Schuler, Chester A.
Copyright Year	2006
Abstract	Some of the Schott glasses originally prescribed by Bessell in creating Johnson/Cousins UBVRcIc photometric filters for use in CCD cameras are no longer available. We have selected new glasses, and added interference coatings, to create a new set of filters that accurately match the original Johnson/Cousins bandpasses. This paper will describe the new prescriptions and give preliminary results from on-telescope tests. Introduction Photometric filters were first used to better match photomultiplier tube output to photographic magnitudes. The first UVB (U= ultraviolet; V=visible=green; B= blue) filters were used in this way. The advent of CCD (charge-coupled device) cameras led to further filter development, because CCDs were sensitive in the red and near-infrared, and picked up light “leaks” from the filters that the early photomultiplier detectors could not detect. These “leaks” arose from the glass filters also transmitting light in the nearinfrared. For example, deep-blue Schott B25 glass used in the B filter also transmits nearinfrared light above 700 nm. Cyan Schott glass BG39 is used in the B filter to eliminate the near-infrared “leakage” from BG25. These photometric filters were developed with colored glasses produced by Schott Glass Industries of Duryea, Pennsylvania. For example, the photometric V filter is made from two Schott glasses; 2 mm of cyan BG39 and 2 mm of yellow GG495, as shown below. BG39 defines the trailing edge, or high wavelength side of the V band pass. It cuts out all near-infrared light. GG495 defines the leading or low-wavelength edge as a cut-on filter, eliminating UV and blue light. These glasses are bonded together with UV-cured, UV-transmitting epoxy. The resulting V spectral curve is shown in green.
File Format	PDF HTM / HTML
Alternate Webpage(s)	http://www.ulo.ucl.ac.uk/docs/AAVSOPaper2.pdf
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article

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