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On-Line Filter Curves (1Jun92)
(from CTIO, NOAO Newsletter No. 30, 1 June 1992)

CTIO has a collection of 150 optical filters of various types. Each
filter as it is received is scanned on the CTIO scanning photometer,
and its transmission curve recorded on floppy disk. These records are
kept on-line on the Sun network and are available to users over the
network, or via FTP from other institutions. The catalog is called
catalog.filters.Z and is found in /us12/ftp/pub/. The .Z indicates that
the data are compressed. An explanation is found in README.FILTERS in
the same directory.

The transmission curves are sampled every 2 A for the narrow band
filters, and every 5 A for the broad band filters. The physical sizes of
the filters include the thickness. This number could be used to
estimate the focus setting offsets for the various filters. The
measurements were made in an f/13.5 beam, at a temperature (normally)
of 22 C. When using the interference filters at other f-ratios and
temperatures, allowance should be made for the center wavelength and
bandwidth shift.

The filter curves in the FTP public directory are reproduced exactly as
written in the filter measuring system by the Instrument Group. The
files comprise a header giving the filter data and any relevant
comments and a tabulation of lambda and percent transmission. No
attempt has been made to judge the optical quality of the filters, but
it is known that some are suspect, especially in a fast beam. Users are
advised to check the suitability of filters for their specific needs.

The disk version of the catalog of filters is written in Unix tar
format, and then compressed using the standard Unix compress utility.
The catalog occupies 1.1 Mbyte in tar format and 2.1 Kbyte in
compressed format. The commands used were:

   tar cvf catalog.filters *.flt filters.tex
   compress catalog.filters

In order to rebuild the file structure of the catalog just type:

   uncompress catalog.filters
   tar xvf catalog.filters

An expanded version of the text of this file is included as
filters.tex, and may be printed using the TeX program and a suitable
dvi routine.

William Weller

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