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In The Pod Bay, we were talking about some pictures that Cassini took of Enceladus recently. We were discussing the calibration pictures and the filters that the spacecraft uses. A NASA FAQ page for Cassini shows a table of the filters it uses, and I've been trying to understand some of them.

Right now I'm trying to understand the Continuum Band filters. There is a sentence on the FAQ page that says "For the Continuum Bands, Band 1 and 2 are in the red part of the spectrum while Band 3 is in the infrared." What is the difference between these filters and the other filters that are just labeled for "methane", "green", "infrared", and so on? For instance, what is the difference between Continuum Band 3 (Infrared) and one of the filters simply labeled "Infrared"?

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They are very narrow passband filters with FWHM pass-through of usually only a few nanometers and a sharp cutoff. Say, a red continuum filter might be centered on ~ 620–740 nm with a FWHM of, say, 5–20 nm. They can be used standalone to only pass through emissivity in that narrow spectral band, or to use produced images as a mask for other exposures and then subtract the continuum band from it, for example to remove background stars from the final image.

Other Cassini filters are either wide band, clear (no filtering), polarizing filters, or a Hydrogen Alpha which is also a single-band filter passing through a narrow band centered at 656.28 nm frequency range, and a Methane Band filter, which is likely a multi-band filter with visible spectrum band at 880-900 nm and a near-IR band centered at ~ 1.3 μm.

Cassini uses two filter wheels per camera (narrow angle camera has 12 filters, wide angle camera has 9 filters per wheel) and can combine up to two such filters for each single frame.


OK, I found a Cassini Imaging Science Subsystem (ISS) reference so we can see how well I did above with my guesstimates. Fairly good I think. Most of it is spot on, some fair (CB continuum band filters aren't really used standalone), and the things that are off are:

  • There's a two-lobed narrow-band filter, the CB1. CB1-CB3 serve as continuum filters to MT methane filters from 619 nm (weak) to 898 nm (strong) methane absorption bands.

  • Methane filters don't go up to 1.3 μm into the near-IR frequency range, but up to 1 μm due to quantum efficiency for the ISS of > 1% limiting it to that EM frequency floor. But Cassini also has VIMS, Visual and Infrared Mapping Spectrometer, that, while it has smaller spatial resolution than ISS, has better spectral coverage, so there's that.

From words to figures, the following is all from Cassini Imaging Science: Instrument Characteristics And Anticipated Scientific Investigations At Saturn, Carlyn C. Porco et al., Space Science Reviews 115:363–497, 2004 (PDF), please refer to the document for more detailed information:

Cassini ISS filter characteristics

Filter  λcen,NAC  λeff,NAC  λcen,WAC  λeff,WAC  Science justification
------------------------------------------------------------------------------------------
UV1     258W      264       –         –         Aerosols
UV2     298W      306       –         –         Aerosols, broad-band color
UV3     338W      343       –         –         Aerosols, broad-band color, polarization
VIO     –         –         420SP     420       Broad-band color
BL2     440M      441       –         –         Medium-band color, polarization
BL1     451W      455       460W      463       Broad-band color
GRN     568W      569       567W      568       Broad-band color
MT1     619N      619       –         –         Methane band, vertical sounding
CB1     619N      619       –         –         Two-lobed continuum for MT1
CB1a    635       635       –         –
CB1b    603       603       –         –
RED     650W      649       648W      647       Broad-band color
HAL     656N      656       656N      656       H-alpha/lightning
MT2     727N      727       728N      728       Methane band, vertical sounding
CB2     750N      750       752N      752       Continuum for MT2
IR1     752W      750       742W      740       Broad-band color
IR2     862W      861       853W      852       Broad-band color; ring absorption band
MT3     889N      889       890N      890       Methane band, vertical sounding
CB3     938N      938       939N      939       Continuum for MT3; see thru Titan haze
IR3     930W      928       918W      917       Broad-band color
IR4     1002LP    1001      1001LP    1000      Broad-band color
IR5      –        –         1028LP    1027      Broad-band color
CL1     611       651       635       634       Wide open, combine with wheel 2 filters
CL2     611       651       635       634       Wide open, combine with wheel 1 filters
P0      617       633       –         –         Visible polarization, 0°
P60     617       633       –         –         Visible polarization, 60°
P120    617       633       –         –         Visible polarization, 120°
IRP0    746       738       705       705       IR polarization; see through Titan haze
IRP90   –         –         705       705       IR polarization; see through Titan haze

TABLE VIII: All wavelengths in nm. Central wavelengths (‘cen’) are computed using the full system transmission function. These numbers are taken to be the numerical name assigned to the filter. Effective wavelengths (‘eff’) are computed using the full system transmission function convolved with a solar spectrum. Bandpass types: SP: short wavelength cutoff; W: wide; N: narrow; LP: long wavelength cutoff.


Cassini ISS WAC & NAC Broad-band and Medium-band Filters

Cassini ISS WAC & NAC Broad-band and Medium-band Filters

Figure 20: System transmission functions for NAC and WAC broad- and medium-band filters. Filters for the NAC are as follows: UV1, UV2, UV3 (purple: solid, dotted, and dashed); BL1, BL2 (blue: solid and dashed); GRN (green); RED, (red); IR1, IR2, IR3, and IR4 (orange: solid, dot-dash, dashed, and dotted). For the WAC: VIO (purple), BL1 (blue), GRN (green), RED (red), IR1, IR2, IR3, IR4 and IR5 (orange: solid, dashed, dotted, and the latter two not shown). The CL1/CL2 filter combination is given by the solid black line in both plots.


Cassini ISS WAC & NAC Narrow-band Filters

Cassini ISS WAC & NAC Narrow-band Filters

Figure 21: System transmission functions for the NAC and WAC narrow-band filters. NAC filters are as follows: MT1, MT2, MT3 (purple: solid, dotted, and dashed); CB1, CB2, CB3 (green: solid, dotted, and dashed); and HAL (red). WAC filters are HAL (red), MT2, MT3 (purple: solid and dashed); CB2 and CB3 (green: solid and dashed). The geometric albedo of Titan is also given (solid black) to illustrate the placement of the methane band and continuum filters relative to the methane features in the spectrum of Titan.


Cassini ISS WAC & NAC Polarizing Filters

Cassini ISS WAC & NAC Polarizing Filters

Figure 22: System transmission functions for the NAC visible and infrared, and the WAC infrared, polarizers.

Some additional explanations can also be found in Cassini Imaging of Jupiter's Atmosphere, Satellites and Rings - Supplemental Online Material, C. Porco et al. (PDF).

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