8.1.4 Filter Suite

The wavelength range of the FPI+ is 360 nm to 1100 nm. Six spectral filters are available within this range. These are five Sloan Digital Sky Survey filters u’g’r’i’z’ and a Schott RG1000 near-IR cut-on filter. Additionally, three neutral density (ND) filters can be used to attenuate bright stars. The ND filters are required for the tracking function of the FPI+ and the optical densities are chosen in such a way that stars within the brightness range of 0 < V mag < 16 can be imaged with an exposure time of 1 second. The “daylight” filter is also a requirement for telescope tracking to be able to acquire bright guide stars in twilight.

Table 8-1: FPI+ filter wheel configuration for the two filter wheel carousels

Carousel 1 Carousel 2
OPEN OPEN
Sloan u' ND 1
Sloan g' ND 2
Sloan r' ND 3
Sloan I' Daylight
Sloan z' Blocked

Table 8-1 shows the configuration of the FPI+ double filter wheel. Filters from carousel one and two can be combined freely with a few exceptions. The daylight tracking filter from carousel two can only be used with the OPEN position of carousel one to avoid non-overlapping wavelength bands. The “blocked” position cannot be selected for observations, but instead is used for taking calibration data (bias frames, dark frames).

Filter Throughput

Figure 8-1 shows a plot of the FPI+ total system throughput, which includes a model for atmospheric extinction, the calculated SOFIA telescope throughput, and the instrument quantum efficiency. The filter spectral response has been measured and is added to the plot. At the wavelengths where the Sloan u’ filter is transparent, other elements in the FPI+ light path (dichroic tertiary mirror, protected silver coatings, ZEISS lens) are nearly opaque. This results in a very low throughput (~0.5%) for the selection of the FPI+ with the Sloan u’ filter.

 

Total system throughput for filters

Figure 8-1: Total system throughput for Sloan filters, the Schott RG1000 (daylight) filter and the "OPEN" FPI+ configuration.

Figure 8-2 is a plot of the neutral density filter transmittance vs. wavelength for the three installed ND filters. Over the entire wavelength range of the FPI+, the ND filters have the average optical density listed in Table 8-1. However, there is a wavelength dependence of the optical density of all ND filters that has to be considered when using the ND filters in conjunction with the Sloan filters. All filters are par focal despite their different thicknesses, because they are installed in the parallel beam in front of the Zeiss lens.

 

Transmittance curves of the FPI+ neutral density filters

Figure 8-2: Transmittance curves of the FPI+ neutral density filters.

Table 8-2: FPI+ Neutral density filter properties

Filter name Glass Type Thickness Average Optical Density
ND1 Schott NG9 4.0mm 4
ND2 Schott NG3 3.5mm 2.6
ND3 Schott NG4 2.8mm 1.3