Orienting the object in the field

 Another consideration may be the orientation of the object relative to the optical axis of the Fabry-Perot. Note that this issue is essentially irrelevant at the 60-inch telescope, where there are only two possible orientations for the instrument, relative to the telescope field of view. The etalon wedge places the optical axis at the 60-inch telescope on the east (or possibly the west) edge of the field of view. (Note also that the rotation angle of the CCD chip, as adjusted in Section 3.10 does not affect the relationship between the field of view and the Fabry-Perot optical axis.) Finally, if you are observing a completely sampled velocity cube, the orientation of the object in the field of view is almost irrelevant. You may still want to consider the issue, however, in case weather-related or other problems prohibit you from actually obtaining a completely sampled data cube.

Given these constraints, the existence of a preferred orientation for the object in the field of view depends upon whether the object has a preferred axis of symmetry, in terms of either its spatial or velocity structure. For example, an edge-on disk galaxy might be oriented perpendicular to a line from the optical axis, allowing most of the galaxy's disk to be scanned through the relevant velocity space simultaneously, with only a few etalon spacings.