The Network of Robotic Echelle Spectrographs

Signal-to-Noise

The targets for commissioning observations are typically isolated stars with V magnitudes between 3 and 8. Fewer observations are made of targets in the range 8 < V < 12. Based on the spectra from those targets, the signal-to-noise per resolution element is estimated from the order of the spectrum that includes the Mg b lines (5167 - 5184 Å). The results are scaled to an exposure time of 60s, and then a model is fit to the points with the highest quality spectra (i.e. successful acquisition and clear sky conditions).

The models for the LSC-NRES, CPT-NRES, and ELP-NRES units are shown in the graph below. Although the scheduler regards the NRES units as identical, the commissioning observations have demonstrated that their performance somewhat differs. Those differences are predominantly driven by the state of the feeding telescope, not the spectrograph itself. We recommend to set the exposure time based on the performance of the lowest throughput NRES system. 

Overheads

Known Limitations:

1. Target Acquisition: Successful target acquisition typically takes 3 minutes or less. However, in some cases, acquisition can take up to 10 minutes. While the NRES team is working to improve the acquisition efficiency, the overhead time is set to 10 minutes to increase the likelihood that observing requests complete. Also, because the target acquisition code sometimes fails to center the star on the fiber, some exposures have lower than expected signal-to-noise. The acquisition strategy is continually being improved.
   
   
2. System throughput: Initial data indicate that all three deployed spectrographs have a comparable system throughput. Differences in mirror reflectivity and collimation status can partially account for the differences between the NRES units.
   
   
3. Exposure Meters: The exposure meters in all NRES units are misaligned and not functional. The cause of the misalignment and possible mitigation efforts are under investigation. 

Shared Risk