INGRID filter and pupil stop sizes

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Calculation of the narrowband filter transmission curves at 5 deg incidence and 77K

The transmission curves of the narrowband filters at 77K and at an angle of incidence of the light of 5deg have been calculated as follows:

Using the change of central wavelength (CWL) per kelvin given by the manufacturer (column 7 of the table), we have estimated the blue-shift due to a decrease in temperature from 293K to 77K.
The formula
cwl(theta)=cwl(0deg) * sqrt(1-(sin(thetha)/n)^2)
relates the CWL at 0deg incidence, cwl(0deg), with the CWL at incidence theta for an interference filter with a given effective refractive index n. We have used the formula above over the previously calculated transmission curve at 0 deg incidence and 77K, to get the final transmission curve at 5 deg incidence and 77K. An effective refractive index equal to 2 has been assumed for all the filters (Ghinassi et al. 2002, A&A 386, 1157).

An increase in transmission of about ~6-8% is expected for the filters at 77K, which has not been taken into account in the calculation.

Last Updated: 27 June 2002
Almudena Zurita azurita@ing.iac.es