Positioning the object in the field

 The first consideration when observing a science target is the position of the object in the field of view. The optimal position depends upon the angular size of the object and the instrument field of view. In the usual configuration (306 mm collimator; 105 mm camera lens), the field of view of the Fabry-Perot system is ~16' at the 60-inch telescope; ~5' at the 200-inch telescope (see Table 2 above). There are two primary constraints concerning the positioning of the object in the field of view:

1.

The image quality degrades somewhat toward the corners of the field, due to the instrument and telescope optics.

2.

The velocity resolution and accuracy degrade with distance from the optical axis, due to the nature of the Fabry-Perot. For example, the resolution of the standard Palomar ET-50 etalon is ~23 km/s on-axis at H$\alpha$, but only ~50 km/s in the corners farthest from the optical axis. This restriction is due to the decreasing spatial separation between Fabry-Perot orders, and is primarily an issue only if you choose not to obtain a completely sampled velocity data cube.

Therefore, in order to optimize the spatial and velocity resolution, you will probably want to position the target somewhere between the optical axis and the center of the field, depending on its angular size relative to the field of view.