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A novel use is made of the small amount of (1.5%) of optical light reflected from the cross-dispersing prism, by focusing it with a Fresnel lens onto a photomultiplier, in front of which is located a filter wheel with ten filter positions. There are five broadband color filters (Johnson UBVR and I), four neutral density filters (with ND values of 0.5, 1.0, 2.0, and 3.0 dex) and one clear aperture. The exposure meter can be used whenever the main bHROS shutter is open, including during science exposures.
One important function of the exposure meter is to enable peak-up on the light from a target source, as part of the acquisition process. To minimize the the effects of atmospheric dispersion, one of the UBVRI filters can be selected to ensure that the peak-up is carried out using light from the same region of the spectrum as recorded by the echellogram. However, for very bright or faint sources, the neutral density filters or clear aperture must be used.
There is an analog display of the count-rate from the exposure meter which can alert the observer to changing sky conditions, such as worsening seeing or clouds.
Last update September 2, 2005; Steven Margheim