filter designator

Telescope filters are often indicated by the letter code indicating the filter's passband; this does not precisely specify the filter because passband-characteristics of the letter depend upon the particular photometric system, and in precise detail, the filters are affected by manufacturing inconsistency. For filters other than wide-band, and for some telescopes' filters, more-specific terms are used. Some filters are for observing specific spectral lines and a spectral line designator may be incorporated in the term used for the filter. Another convention is the inclusion of the string LPR to indicate light pollution reduction, generally a telescope's filter that removes known light pollution in its location, such as from street lamps designed to produce light that can be filtered out in such a way.

For HST and JWST, a particular convention has been used for specifying filters, examples being "F606W" and "F1140C". It begins with the letter "F" (for "filter") followed by a rough wavelength (for HST, in nanometers), followed by a letter generally indicating the width of the band: N for narrow-band, M for mid-width band, W for wide-band, X for extremely wide, or LP for long pass (which, rather than allowing only a band, imposes just a single cut-off, allowing all longer wavelengths). For example, "F606W" is HST's wide-band filter in the region of 606 nanometers. The same type of designation system has been adopted for the JWST (but with numbers in units of 10 nanometers).

HST uses the letter Q for quad as well, placed immediately after the initial "F", which indicates a filter that covers only a fraction of the field of view (FOV): in order to provide more types of filters, some of HST's single physical filters actually filter with multiple passbands, such that each of the four quarters of the FOV is filtered with a different passband. The letter Q suggests a quarter of the FOV, but the same letter is used for filters with other fractions, e.g., 1/6. The letter R after the F means ramp, which indicates a filter for a passband emphasizes either the shorter or longer wavelengths of its wavelength range. JWST, in addition to the suffixes "W", etc., also uses the suffix C, standing for coronagraph.