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INSATLLATION
SCHEMATICS
DETECTORS
FILTERS
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filters part 2 previous, filters part 1next , filters part3

Filters are made of transparent and thin metallic materials that are carefully designed to meet the specifications of astronomers and scientists. In addition to filters, there are also grisms, which are optical devices that work somewhat like a prism. It has the effect of spreading the spectrum of light across an image plane. So it also controls what wavelengths hit a detector.
Specifications of Filters/Grisms
Each filter or grism goes through extensive processing and testing until the desired specifications are met. Among the most important ones are:

1) central wavelength (lambdasymbol lamda): The central wavelength of light which the filter lets through to the detector.

2) bandwidth (fwhm): The ranges of light wavelength the filter lets through. "fwhm" stands for Full Width Half Max. This means bandwidth is the range covered between the two points where the transmission % is half of its maximum (see image below).

3) transmission %: The percentage of light that makes it through the filter out of the possible light that could have made it through at a specific wavelength. This is more of a performance measure than a specification as scientists would always like transmission to be 100%.


Specifications and Performance Measurements
Two filters are represented in the graph below. Filters are designed with variation so that scientists can study specific wavelengths of light a stellar object might be producing.

spec. and perf. of 2 filters

Actual performance measurements of filters are not quite as clean-cut as what is shown above. Below is a more accurate representation (black wavy line).

actual performatnce
 

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