Narrow Band Filter Transmission Curves
All the filters were scanned at room temperature (~293K) and
an angle θ=0° with respect to the incident light. However, the
central wavelength of the narrow band filters depends on (i) the angle
of incidence and (ii) the temperature, which in the case of INGRID are
θ~5° and T~77K respectively. Transmission curves for these values
have been estimated from the measured data as follows:
- The blueshift resulting from the decrease in temperature (from
293K to 77K) was estimated using the change
of CWL (central
wavelength) per Kelvin given by
the manufacturer (see column 9 of the narrow
band
filter table).
- The effect of the 5° tilt on the central wavelength
was then
taken into account using the following equation, which relates the CWL
at angle θ to that at 0° incidence, for an interference filter of
effective refraction index n.
A value of n=2 was assumed
for all the filters (see Ghinassi
et al. 2002, A&A, 386, 1157).
An increase in transmission of about ~6-8% is expected for the filters
at T=77K, which has not been taken into account in the calculation.
