The 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 bandpass filter. Both its central wavelength and its bandpass are functions of the polar angle on the sector, and thus of the position of the filter wheel. The flux and wavelength calibration are described in Sections 4.8 and 4.8.1. The first and last two steps of each sector suffer significant vignetting.

**Table 2.5:** *Wavelength range and spectral resolution of the different CVF segments.*
CVF segment	$\lambda_{begin} \rightarrow \lambda_{end}$	$\Delta \lambda$	FWHM	$\lambda / \delta \lambda$
	[ $\mu m$ ]	[ $\mu m$ /step]	[ $\mu m$ ]
CVF SW	$2.273 \rightarrow 5.122$	$0.0252 \rightarrow 0.0252$	$0.0954 \rightarrow 0.125$	41
CVF1 LW	$4.956 \rightarrow 9.583$	$0.062 \rightarrow 0.0483$	$0.134 \rightarrow 0.220$	$37 \rightarrow 43$
CVF2 LW	$8.782 \rightarrow 17.34$	$0.110 \rightarrow 0.090$	$0.258 \rightarrow 0.336$	$34 \rightarrow 52$

2.6 Circular Variable Filters (CVF)