NRES performance

The Network of Robotic Echelle Spectrographs

Please see the Observatory Status page for the current status of each NRES unit.

Signal-to-Noise

The plot below shows the expected signal-to-noise ratio obtainable from NRES under good conditions; the actual SNR obtained may be lower due to clouds, acquisition inaccuracies, etc. 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 from a set of test observations 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). 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 setting the exposure time based on the performance of the lowest throughput NRES system, which is shown by the model below.

A signal-to-noise ratio of at least 25 is recommended in order to obtain precise RVs, and at least 40 for stellar parameter determination. These SNRs are difficult to obtain for targets with V>10.

Overheads

Beyond the exposure time, NRES observations are subject to typical overheads. The overheads we've included in our database are the following:

The total, 667s, will be added to the exposure time of the request. You must make sure that your time window can accommodate the sum of exposure time plus overheads.

• slew & settle time: 180s
• time to switch between calibration mode and "sky observing" mode: 20s
• acquisition camera exposure time: 10s
• acquisition image processing time: 400s
• post-exposure readout time: 57s

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. Additionally, the accuracy of the acquisition can vary, and some exposures therefore have lower than expected signal-to-noise. Two acquisition strategies are available, centering either the brightest object in the field or using a WCS-based solution to the background stars. A detailed description of the acquisition modes can be found here.
   
   
2. System throughput: Initial data indicate that all four 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: Only the exposure meter at ELP is currently functional. Those in the other NRES units are misaligned and not functional. Currently the exposure meters provide flux versus time data as an additional extension in the output FITS file; there is no capability to cut off an exposure once a target SNR has been reached.
   
4. RV accuracy: The NRES Commissioning Pipeline typically achieved an RV precision of order 50-100 m/s, but the newly-deployed BANZAI-NRES pipeline has demonstrated RV precision as good as 10 m/s on bright (V~6) standard stars. The design goal of 3 m/s RV precision appears achievable with the spectrograph hardware, but will require further improvements to software and operational procedures.
5. Spectrograph focus: the NRES unit at TLV is in slightly poorer focus than the other units, and consequently achieves a slightly lower spectral resolution, by factor of 1 to 1.5 depending upon position on the chip.

We recognize that the instrument performance is not yet at the level of its ultimate capability; we expect to continue to deliver significant improvements. We welcome your assistance in that effort. We encourage you to send us your criticism and your suggestions for improvements: Send email to science-support@lco.global and put "NRES" in the subject line.