Orient the CCD

 Now is a convenient time to verify the orientation of the chip. Because of the presence of the etalon wedge and the internal optical path of the Fabry-Perot, the only rotational degree of freedom is the position angle of the CCD. Although certain observational constraints may also exist, such as the position angle of a target galaxy, it is often convenient to orient the chip such that the optical axis appears on the left side of the image display, for data reduction purposes.

  

Figure 16: The default orientation of the field of view at each telescope. Since only the CCD may be rotated at the 60-inch telescope, the ring pattern there is fixed relative to the directions on the sky. At the 200-inch telescope, however, the Cassegrain ring may be rotated to orient the Fabry-Perot optical axis in any direction desired.

At the 60-inch telescope, assuming the Fabry-Perot has been attached to the telescope with the cable boot box oriented toward the north, the tilt of the etalon places the optical axis on the eastern edge of the field. The preferred orientation will then appear on the display with east to the left and north down. In contrast, on the 200-inch telescope, the instrument is rotated by 45°, placing the optical axis on the northwest edge of the field of view. If we again orient the chip so that the optical axis appears on the left side of the image display, north is then oriented toward the upper left. (This is not strictly true, as the Cassegrain ring on the 200-inch telescope may be rotated to orient the field in any desired direction.) See Figure 16 for a graphical explanation.

The dewar plate on the Fabry-Perot allows the CCD to be attached at any rotation angle. By loosening the clamps on this plate, the dewar can be rotated without removing it from the instrument. Be careful to loosen the dewar clamps just enough that the dewar will rotate. Someone should be supporting the dewar at all times, as the clamps may easily slip outward. Use the label on the side of the dewar, as well as the apparent chip orientation from the test exposures, to align the dewar appropriately. There is also an OBSERVING command, CHIPORIENT, which you may find useful. You can use the fact that the square CCD chip is aligned with the octagonal dewar bases of both CCDs #15 and #16. Before re-tightening the dewar clamp bolts, be sure that the dewar is seated within the slightly indented central area of the dewar plate.