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
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
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
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).
