7.1.3 Filter Suite

Imaging with FORCAST can be performed in either a single channel or in both the SWC and LWC channels simultaneously (dual channel mode). In single channel mode any one of the available filters may be used. In dual channel mode, a dichroic is used to split the incoming beam, directing it to both the SWC and LWC.

The dichroic reduces the overall throughput. Table 7-1 shows the throughput with the dichroic in (Dual Channel mode) relative to that for the Single Channel mode. The degradation of the system throughput by the dichroic can have a significant effect on the instrument sensitivities as discussed in more detail in Section 7.1.4. In addition, since there are more short wavelength filters available than slots in the SWC filter wheels, some short wave filters will be housed in the LWC. The final filter wheel configuration for each observing cycle will be driven by proposal pressure. Depending on the final filter configuration, it is possible that not all LWC filters will be able to be used in dual channel mode due to the cutoff in the dichroic transmission at short wavelengths. Additional details are provided below and will be updated once the results of each cycle's Call-for-Proposals are known.

Table 7-1: FORCAST Dichroic Throughput

Bandpass Throughput
5-10 μm 60%
11-25μm 85%
25-40 μm 40%

The filters in the SWC are standard Optical Coating Laboratory, Inc. (OCLI) thin-film interference filters. These filters are stacked with blocking filters to prevent light leaks. The only exception is the 25.4 μm (FOR_F253) University of Reading filter, which is a custom double half-wave (three mesh) scattering filter stacked with a diamond scattering blocking filter to provide blue-light rejection. The 33.6 (FOR_F336), 34.7 (FOR_F347), and 37.1 μm (FOR_F371) filters in the LWC are LakeShore custom double half-wave (three mesh) scattering filters. The 31.5 μm filter is a Fabry-Perot Interferometer filter.

The central wavelengths, bandwidths, and the expected FWHM IQ in each of the filters are given in Table 7-2 below. The first column under "Imaging FWHM" presents the best measured FWHM IQ with a telescope jitter of ~1.25'' rms. These values are representative of the IQ observed in Cycle 2. The second column is the average measured FWHM IQ from Basic Science with a ~2.1'' rms jitter and indicates the "worst case" IQ. Not all of the filters have measured IQ values, but we expect that they will be comparable to those with measured values that are of similar λeff. Figure 7-3 shows the filter transmission profiles (normalized to their peak transmission) over-plotted on an ATRAN model of the atmospheric transmission. The filter transmission curves (text tables) are available as a zip file or individually from Table 7-2.

Due to the limited number of slots available in the filter wheels, not all of the filters listed in Table 7-2 are available at any one time. For Cycle 4, there will be a "default" filter set (indicated by bold type in Table 7-2). GIs may request that other filters be used for their observations, but they must make a convincing scientific argument to justify swapping out the existing filters for those they desire. We anticipate that a single filter swap may be permitted during Cycle 4 provided there is adequate scientific justification. More details are available in the Cycle 4 Call for Proposals.

Table 7-2: FORCAST Filter Characteristics

Channel Filter λeff (μm) Δλ (μm) Imaging FWHM ('') Profiles
SWC FOR_F054 5.4 0.16 b txt file
FOR_F056 5.6 0.08  
FOR_F064 6.4a 0.14 3.0 3.5 txt file
FOR_F066 6.6 0.24 2.9 3.5 txt file
FOR_F077 7.7 0.47 2.7 3.5 txt file
FOR_F111 11.1 0.95 2.7 3.6 txt file
FOR_F197 19.7 5.5 2.9 3.8 txt file
FOR_F253 25.3 1.86  
LWC FOR_F086 8.6 0.21 txt file
FOR_F113 11.3 0.24 txt file
FOR_F118 11.8 0.74  
FOR_F242 24.2 2.9 3.3 4.0 txt file
FOR_F315 31.5 5.7 3.4 4.3 txt file
FOR_F336 33.6 1.9 4.5 txt file
FOR_F348 34.8 3.8 3.6 4.5 txt file
FOR_F371 37.1 3.3 3.5 4.7 txt file

a Entires in bold font are expected to be part of the "default" filter set for Cycle 4.

b IQ values for some filters have not been measured at this time, but it is expected that they will be similar to those of similar λeff with measured values.

 

FORCAST filer transmission profiles

Figure 7-3: FORCAST filter transmission profiles along with an ATRAN model of the atmospheric transmission across the FORCAST band (assuming a zenith angle of 45 degrees and 7 μm of precipitable water vapor). For clarity the filter profiles have been normalized to their peak transmission. SWC filters alternate between green and blue, while LWC filters alternate between red and yellow.

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