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Cryostat Window, Filter and Dichroic Specification and MeasurementsIntroductionThis document details the specifications for the SCUBA-2 cryostat window, filters and dichroic. It also includes measured plots for the final filters. The general philosophy has been to adopt a scheme in which the instrument is over-filtered, i.e. extra provision is made for filters at critical apertures, although some of these may later prove unnecessary. There has been an optimization procedure for the entire filter chain to maximize transmission (e.g. relative fringing between filters) and minimize out-of-band radiation.Estimated Filter profilesThe profiles of the SCUBA-2 filters have been measured individually at Cardiff. The text files linked below contains the total profile obtained by stacking all the measurered filter profiles togheter, including the cryostat window and the dichroic. Some of the IR edge blocking filters have been estimated from higher frequency data. These filters are not expected to contibute to the profile since they are flat over the bandpass filters but it will affect the total transmission. No attempts have been done to include losses in mirrors or due to scattering. The filter measurements has been done at room temperature - not the operating temperature. Further, any fringing due to reflections between filters in the cryostat is not taken into account. The actual profile in the instrument will be measured when the SCUBA-2 FTS is operational.Estimated 850 filter profile from 2009 to present Estimated 450 filter profile from 2009 to present SCUBA-2 originally had a wider 450 bandpass filter. This was replaced with a narrow filter in 2009 due to array saturation. It is possible the original filters will be reinstalled in the future. Estimated original 450 filter profile 
SpecificationsThe filter and dichroic specifications are detailed in the following tables.Low- and high-pass edge and thermal filters: Table 1
Dichroic and Bandpass: Table 2
Window: Table 3
Filter descriptions, sizes, clear apertures and measurementsWindow ProtectorDimensions: clear aperture diameter 235mm, outer diameter 245mm, 4um thick.Cryostat windowUHMW polyethylene with anti-reflection coating to maximize transmission at 450 and 850um. Dimensions: clear aperture diameter is 200mm, outer diameter 255mm, and 8.63mm thick. Transmission data for the UHMW polyethylene window are given in Figure 1.Figure 1: Transmission of the AR coated UHMW polyethylene window material for 8.63mm thickness.
C_B0: 300 K filter behind cryostat window5um thermal filter Dimensions: clear aperture diameter 200mm, outer diameter 210mm, 4um thick.C_B1: Radiation shield (outer) filter (60K)10um thermal filter Dimensions: Clear aperture diameter 185mm, outer diameter 195mm, 4um thick.C_B2b: Radiation shield (middle) filter (60K)20um thermal filter Dimensions: clear aperture diameter 185mm, outer diameter 195mm, 0.4um thick.C_B2a: Radiation shield (inner) filter (60K)37cm-1 LPE Dimensions: clear aperture diameter 185mm, outer diameter 195mm, 0.65mm thick.C_B3a: Helium shield (outer) filter (4K)30um thermal filter Dimensions: clear aperture diameter 185mm, outer diameter 195mm, 0.4um thick.C_B3b: Helium shield (inner) blocking filter (4K)33cm-1 low-pass edge Dimensions: clear aperture diameter 185mm, outer diameter 195mm, ~0.6mm thick.Figure 2: Thermal filters. C_B0, C_B1, C_B2b and C_B3a. The effect of poor signal to noise is visible in the low frequency data - the real transmission from 0 to 10 cm-1 is over 0.99.
C_B4a: Cold stop blocking (outer) filter (1K)29cm-1 low-pass edge Dimensions: clear aperture diameter 102mm, outer diameter 117mm, ~0.6mm thick. This is the filter that was added in December 2007 and is the outer filter at 1K, i.e. closest to the cold shutter.C_B4b: Cold stop blocking (inner) filter (1K)32cm-1 low-pass edge Dimensions: clear aperture diameter 102mm, outer diameter 117mm, ~0.6mm thick.Figure 3: Instrument blocking filters. CB2a (W1054), CB3 (W1032) and CB4 (W1045).
Dichroic (1K)16.5cm-1 low-pass edge. This filter is designed to transmit (>90%) the 850um band, whilst reflecting (>90%) the 450um band. The prototype tests showed the dichroic to be effective in both reflection and transmission at 30 degrees incidence for both s- and p-polarizations. Dimensions and angle of incidence: clear aperture diameter 145mm with an angle of incidence of 35 degrees. Filter thickness ~1mmFigure 4: Measured transmission and reflection data for W1091 dichroic.
B5: Arrays edge filters (1K)Both arrays will have an additional blocking filter to eliminate any higher-frequency leaks. 450_B5 16cm-1 high-pass edge and 26cm-1 low-pass edge. See figure 5. 850_B5 14cm-1 low-pass edge See figure 6. Dimensions: clear aperture diameter 110mm. Note the 450 edge has changed to two filters, both a low-pass and a high-pass, for the new narrower bandpass.BP: Bandpass filters (1K)The passbands and cut-offs for these filters are derived from the atmospheric model for 1mm of PWV. They are of the hot-pressed technology. 450_BP 450um BP See figure 5. Note: The 450 bandpass was changed to a narrower version following the first on-sky tests. The plot showes the installed narrowed 450um bandpass. The 26cm-1 low-pass edge filter is not shown in the figure. 850_BP 850um BP See figure 6. Dimensions: clear aperture diameter 110mm.Figure 5: Array filters for 450um band. 450 BP (W1417) and 450 B5 16cm-1 HPE (W1426). The LPE (W1058) is not shown, and will be added.
Figure 6: Array filters for 850um band. 850 BP (W1104) and 850 B5 14cm-1 LPE (W1060).
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