FHiRE Electronics

The electronic components of FHiRE can be broadly catagorized as: instrument control and detector array control. Instrument control referes to the electronics needed to control the mechanical devices within the instrument. The FHiRE spectrograph itself is designed to contain no moving parts in order to maximize wavelength stability. As a result, the only moving parts are located within the guider and acquisition module (GAM) that is mounted on the telescope. The detector array will be controlled with a control system known as MONSOON. The MONSOON system was developed by the National Optical Astronomy Observatories (NOAO) and is now in use at several observatories around the world. The detector array is a 2048 x 4096 CCD with 12 micron pixels developed by SAIC and is housed within an Infrared Labs ND-8 liquid Nitrogen (LN2) dewar. The array is configured so that the spectral dispersion is along the short dimension with the spectral orders arranged along the long dimension. The MONSOON controller provides the bias and high frequency clocking voltages that are used to read and reset the array. The controller also provides for temperature sampling of the detector and for shutter control. A Point Grey Blackfly CMOS camera will be used on the GAM for target acquisition and to provide for telescope tracking (guiding). A wedge beam splitter within the GAM reflects about 4% of the light from the target star to the camera so that its image can be used to: 1) position the telescope so that the remaining 92% of the star's light enters an optical fiber leading to FHiRE, and 2) compute and monitor the centroid of the star in order to provide correction for the drift in the telescope's tracking. The GAM contains a Raspberry Pi microcomputer which controls the camera and the Thorium-Argon and Fabry-Perot lamps. The R-Pi acquires the camera images of the target and provides positioning signals to WIRO's telescope control system (TCS). A centroiding algorithm monitors the target position and sends error signals to the TCS in order for it to use this "guide signal" to make adjustments in the telescope pointing and thus keep the light from the target going down the optical fiber to FHiRE. An instrument control interface (ICI) written in Python/TCL and running on the R-Pi is used to provide the user with a convenient control of the GAM, the guider camera and the MONSOON controller. The interface uses the camera signal to monitor the flux from the star in order to compute an intensity-weighted time for mid-exposure, a quantity that is vital for precision radial velocity work in the presence of clouds. This signal is also used to estimate the real-time signal to noise ratio as the exposure progresses. The GAM also contains small shutters that are used to restrict the light from the TH-Ar and Fabry-Perot lamps in order to avoid saturation of the detector. Finally, the GAM contains a flat-field lamp that can controlled by the user. The ICI also queries the TCS for telescope information and constructs a data header for each FHiRE exposure containing the instrument and telescope information for FHiRE's data reduction pipeline. These data files and ancillary "meta-data" from the GAM are written to a data archive located at WIRO.