An important point when planning observations with TAURUS is that since the interferometer operates at a high order of interference (typically a few hundred), it is necessary to isolate the wavelength region of interest using an order sorting filter. For observations of objects at high redshift, or of unusual emission lines, it is important to check that a filter with a suitable central wavelength is available. Also, the bandwidth of the filter should match the free spectral range of the etalon being used. If the bandwidth of the filter is less than the free spectral range of the etalon, this will restrict the wavelength range covered, whereas if the bandwidth of the filter is much larger than the free spectral range of the etalon, there is a risk of order confusion.
The filters can either be placed in the focal plane or the pupil plane, and there are a number of points to be considered before deciding which is most sensible. An advantage of using filters in the focal plane is that it is possible to shift the central wavelength to the blue ( not to the red) by tilting the filter (Lissberger & Wilcock Journal of the Optical Society of America, vol 49, p126, 1959). At a tilt angle radians, the effective wavelength depends on the nominal wavelength as:
A disadvantage of using filters in the focal plane is that they may limit the available field size. This problem can be avoided by using filters in the pupil plane. As long as the diameter of the filter is larger than that of the collimated beam (about 68 mm) this will not cause vignetting. However, the risk of creating ghost images is greater.
It is important to note that the 50 mm square filters normally used for CCD imaging on the INT and JKT are not well suited for observations with TAURUS-2. When used in the focal plane filter wheel, they will restrict the field size to less than 3.76 arcmin. When used in the pupil plane filter wheel, they will cause vignetting. The larger filters which have been purchased specially for use with TAURUS-2 are listed in Appendix .