The exposure meter for HIRES consists of a pickoff mirror on the end of spinning arm, a photomultiplier tube (PMT), and the controlling hardware and software for the device. The PMT is a Hamamatsu HC135-01 sensor module having a spectral range of 300-650 nm and a peak counting efficiency of 22% at 400 nm. The exposure meter is generally used to optimize observations of bright sources (< 10 mag) when specific signal to noise levels are desired for a given program before moving on to the next target. Its use is not recommended for faint sources when long integrations are required as the pickoff mirror will induce a 1% light loss as it passes through the optical beam. If you plan to use the exposure meter, it is recommended to select "Start HIRES control software with HEXPOCON" when bringing up the HIRES control software. Alternatively, one could start the exposure meter software from the HIRES control menu - subcomponents pulldown. The GUI consists of a 2-panel window, the left panel displaying the light/signal/dark counts and the right panel the target/signal display. To start the exposure meter simply click the "Start" button in the upper left corner. This will begin spinning the propeller arm with the pickoff mirror and warming up the PMT. Please allow at least 10 minutes for the PMT to warm up and the exposure meter arm to achieve proper rotational velocity. Note that any significant quantity of light entering HIRES (such as the ThAr and quartz lamps) will automatically shutdown the exposure meter to safeguard the PMT. The "Reset" button should be pushed to clear any errors should this occur.

Once warmed up, the exposure meter can be armed by clicking the "Arm" button in the upper center of the GUI. Arming the meter will effectively relinquish control of closing the shutter from the HIRES exposure control GUI to the exposure meter. When the desired count level is attained by the exposure meter, the shutter will close and the image will read out. If the integration time entered in the HIRES exposure dashboard is reached first, however, the exposure will terminate and read out. Therefore ensure that a long enough integration time is entered in order for the desired SN level to be attained.

During the exposure, a horizontal yellow line will appear in the right panel of the GUI that represents the desired level of counts (set by the user in the Target Signal block). A second blue line steadily moving upwards toward the desired count level represents the current detected signal. A PMT count to spectrum SN level relationship is not available at this time. Routine users of the HIRES exposure meter have devised their own metrics for their respective programs. To increase or decrease the number of points displayed in the left-hand plot, simply click on the buttons labeled "MORE PTS" or "LESS PTS." The exposure chronological midpoint and flux-weighted midpoint are preserved in the fits header (keywords: EXM0MIDP and EXM0FWMP).

To disarm the exposure meter, simply click the "Disarm" button and exposure control will be relinquished to the HIRES expsoure control GUI. To stop the HIRES exposure meter, simply click the "Stop" button. The pickoff mirror will stow out of the optical beam. Running the end of night shutdown will also turn off the PMT and stow the propellel arm.

An SPIE paper describing the exposure meter is available here (Kibrick, R.I., et al., 2006, Proc. SPIE, 6274, 58).