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When TAURUS is used in imaging mode, the field of view is limited
by the following factors:
- The field cutoff of the collimator, equal to 9 arcmin.
- Filters placed in the focal plane may further limit the
field of view. The scale at the input focal plane is
4.51 arcsec mm-1 for TAURUS-2. Thus, for example,
a filter with a diameter of 75 mm would limit the field
to 5.6 arcmin. This constraint can be avoided by using filters in the
pupil plane. However, using filters in the collimated beam leads
to an increased risk of ghost images, and in the case of the smaller
filters may cause vignetting.
- The size of the detector. The scale at the detector focal plane is
23.25 arcsec mm-1 for TAURUS-2 with the f/2.1 camera and
12.40 arcsec mm-1 for TAURUS-2 with the f/4 camera. Thus, for
example, the EEV3 CCD, whose size is 27.9x25.7mm, will
provide a field of 5.8x5.3 arcmin when used with the f/4
camera.
- Optical quality. The f/4 camera is designed to give high spatial
resolution at the expense of field. The quality of the images
therefore degrades somewhat with field radius (see section ?).
This may place a limit on the fieldsize useable for a particular
project.
When TAURUS-2 is used as a Fabry-Perot interferometer, the field may
be further limited by the following factors:
- The field of view of TAURUS-2 is not limited in the conventional way
by the Jacquinot criterion
(Journal of the Optical Society of America, vol 44, p761, 1954).
At high resolutions, the field will however be limited by the
width of the interference fringes. The wavelength change across
a detector pixel increase with both the distance from the field centre
and with the resolution, so there exists an off-axis angle
beyond which the wavelength change across a single pixel is greater
than the spectral resolution. For a pixel size of p micron and a
resolving power R, the off-axis angle in arcmin at which this
results in a
a degradation of resolution by a factor of sqrt(2) is given by:
k = 12.5 for TAURUS-2 with the f/2.1 camera
*
k = 23.5 for TAURUS-2 with the f/4 camera
- The total size of a datacube must not exceed the amount of
memory available in the Detector Memory System (48 Mbyte as of 17/5/91).