11.1.3 Filter Suite

Wavelength range: 0.3 - 1.1 µm. HIPO currently includes standard Johnson UBVRI  and Sloan Digital Sky Survey u' g' r' i' z' filters. Occultation observations will normally be unfiltered for events involving faint stars or will use specialized filters such as the narrow-band methane filter (λc ~ 0.89 µm) for events with bright stars. Additional custom filters will be added as needed for specific events.

HIPO uses a dichroic reflector to separate its blue and red channels. Two dichroics are currently available with transition wavelengths of 0.575 and 0.675 µm respectively. Other dichroics will be added as necessary for specific events. The 0.575 µm dichroic transition wavelength is close to the boundary between the Sloan g' and r' filters and the 0.675 µm transition is close to the boundary between the Sloan r' and i' filters.

HIPO has been upgraded recently with the addition of deep depletion CCDs having a multi-layer antireflection coating. This provides higher quantum efficiency across the board with much higher quantum efficiency at longer wavelengths. It also nearly eliminates fringing at longer wavelengths. The figures below reflect this improvement.

Below are two plots (Figures 11-2 and 11-3) of the HIPO total system throughput including atmospheric extinction, SOFIA telescope throughput, and instrument throughput for each of the available bandpasses. The first figure shows the total system throughput for the Johnson and methane filters with the 0.575 µm dichroic. The second figure shows the Sloan filters with the 0.675 µm dichroic. These figures assume that the FLITECAM dichroic beamsplitter is not installed. The very low total system throughput for HIPO is due to the dichroic tertiary mirror in the telescope. The long-planned aluminized tertiary will increase our throughput by a factor of nearly 3, on average, when it becomes available. The corresponding throughputs are presented in Figures 11-4 and 11-5.

HIPO total system throughput 0.575 µm dichroic

Figure 11-2.

HIPO total system throughput 0.675 µm dichroic

Figure 11-3.

HIPO throughput 0.575 µm dichroic

Figure 11-4

HIPO throughput 0.675 µm dichroic

Figure 11-5

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