The more relevant instrumental characteristics are reported in Table 2 (click here), and only those up to now not examined will be discussed in this section.
Wavelength range:
while for the UBF the useful wavelength range is 4200 
7000 Å,
and for the interferometer 4000 7000 Å , the IPM can
be used only on the range 4600 6800 Å. This limitation
is imposed by the achromatic ranges of the wave-plates and of the
polarizing beemsplitter cubes.
Calibrated ranges: due to its very complex construction, the UBF shows an optical behaviour which significatively differs from the theoretical one. The calibration procedure, therefore, demands a lot of time to be performed and the use of a spectrograph with a medium-high spectral resolution. For this reason the filter has been calibrated only on six wavelength ranges, 40 Å wide, centered on lines of astrophysical interest. The range at 6438 Å has been calibrated to allow the measurement of the spectral dishomogeneities of the focal plane, but obviously it can be also used for solar observations. In principle the calibration of the UBF can be extended to other ranges, but since in this case the filter should be brought back to Arcetri, such a possibility will be re-examinated only in the future.
Field of view:
in telecentric mounting the maximum
obtainable angular field 
linearly depends on the
allowed incidence angle 
(see Paper I):
where 
and 
respectively are the
interferometer and the telescope diameters.
Assuming 
(see Sect. 2.2),
= 50 mm and = 900 mm,
a maximum angular field 
is obtained.
In practice, as not all the clear aperture of the FPI can be
used, the useful angular field is 
, which, as
described in Sect. 3.2, circumscribes a square CCD:
the resulting useful field is therefore 
.
Wavelength setting time:
as shown in Table 1 (click here), the wavelength positioning of the
interferometer is very fast (
1.5 ms).
The instrumental wavelength setting time is therefore imposed by the UBF
and it is not constant, depending on the angular rotations of
the crystal groups, necessary to set the passband to
the demanded wavelength. If then a different wavelength range
is requested, also the two filter wheels,
preceding the CCD cameras, must be positioned.
The maximum setting time amounts therefore to about 1 s; however,
in the case of small wavelength steps, as for example in scanning a
photospheric line, the setting time typically is 0.1 s.