A CVF is a multi-layer thin film interference filter mounted on the filter wheel. Its substrate is a sector of a ring. The thickness of each layer of thin film is a function of the polar angle on the sector. An elementary area of the ring behaves as a narrow band pass filter. Both its central wavelength and its band pass are functions of the polar angle on the sector, thus of the position of the filter wheel (WARNING: the first and last two steps of each sectors suffer significant vignetting).
The CVF's allow spectro-imagery, or line mapping. Table
gives an overview of the ranges of variation of the
parameters of the different segments of CVF.
Table: Limits of the CVF parameters
Note that, for the LWCVFs, the resolution is degraded towards shorter wavelengths because of the size of the pupil image which spreads over more and more spectral elements of the CVF.
Figure gives the spectral resolution R =
/
.
Figure gives the FWHM and /step as
a function of .
Figure and give the spectral response
(CVF transmission 
detector response) as a function of
.
Tables to give for each CVF
part, as a function of the step number: , /step, FWHM, spectral transmission and sensitivity in
ADU/sec/mJy/pixel.
The step motor drive of the filter wheels is such that the minimum
interval between two consecutive positions is 1 degree, or 1 step
of the CVF, so that only the predefined set of central wavelengths
given in Tables to can be
obtained
CVF's can be used in a fixed mode or in a scanning mode:
_begin, _end, and a
sampling step (an integer number 
of steps of the
filter wheel between two positions). The system determines the
``begin" and ``end" positions of the wheels closest to the requested
wavelength range limits, and scans between these positions at
intervals.
Figure: Spectral Resolution of the CVFs
Figure: FWHM (upper curves) and 
/step (lower
curves) for the 3 CVFs
Figure: Spectral Transmission of the SW CVF
Figure: Spectral Transmission of the LW CVFs
