On the corner between Sides 1 and 2 of the spectrograph is a square box containing a dial indicator of the grating tilt [F]. To change the tilt, first turn fully CCW the large, gold, grating rotation clamp knob [E] on Side 1 of the spectrograph to the right and up from the indicator box. This unlocks the grating. On the side of the indicator box is a knob with crank handle (folded within) which changes the tilt. The indicator displays units of tilt angle, with the fine wheel divided into divisions of 0.01 units. Always approach from lower values to higher ones to ensure repeatability. Finally, lock in the new grating position by turning the gold clamp knob.
Note: The tilt adjustment clamp is fully adequate to freeze the grating rotation against normal torques encountered while observing, but can be easily overwhelmed by the high gear ratio of the manual drive. Always release the clamp before rotating to a new position. Failure to do so will result in premature wearing of the clamp and eventual slippage!
When zeroed such that a tilt reading of 0.00 places the 0th-order slit image in the middle of the detector, tilt reading (in degrees) is related to the central wavelength of a grating by the following equation:
where is the central wavelength (in Å), m is the order number, and r is the ruled grooves/mm of the grating. Appropriate tilt settings can also be estimated from the plot in Figure 2.4. Appendix B provides an atlas of the emission lines seen in the comparison lamps (see Chapter 4).
Figure 2.4: Grating tilt.
Note: At high grating tilts ( 20), the beam projects to an eccentric ellipse which overfills the grating. The amount of light lost by this vignetting is related to tilt angle as depicted in Figure 2.5.
Figure 2.5: Grating vignetting.