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Circular Variable Filters (CVF)

 

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 gif 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 gif gives the spectral resolution R = /.

Figure gif gives the FWHM and /step as a function of .

Figure gif and gif give the spectral response (CVF transmission detector response) as a function of .

Tables gif to gif 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 gif to gif can be obtained

CVF's can be used in a fixed mode or in a scanning mode:

  
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



next up previous contents
Next: Polarizers and polarization Up: GENERAL PERFORMANCE OVERVIEW Previous: Filters



ISOCAM Observer's Manual - V1.0
Tue Oct 31 12:06:23 MET 1995