LRS2 throughput and signal-to-noise estimates

It is important for the PI to realize that due to the design of the HET the effective collecting area changes over a trajectory. Near the end of a trajectory the HET has half the collecting area that it has near the middle. As such two medium length visits centered on middle of a track could be more valuable than a single long visit.

To see the effect of changing exposure times in a moving aperture try the HET Filling Factor Calculator.

UPDATE As of January 2020, Greg Zeimann has provided a new exposure time calculator for LRS2 using real data and his data reduction pipeline (which is used on all science data). The old 5-sigma line/continuum flux limit graphs are still maintained at the bottom of this page.

The LRS2 exposure time calculator is meant as a toggle tool for many generic science goals. The HTML can be opened with nearly any browser (FireFox has been tested the most) and will activate once any of the toggles have been adjusted. The channels UV and Orange reside in LRS2-B while Red and Farred reside in LRS2-R. The calculator matches well with the signal to noise delivered by the automatic pipeline and the spectrum*.fits products. Feel free to email with any questions.

DOWNLOAD exposure time calculator

What follows is a brief description of a few aspects of the tool:

  • Source Model - We assume a point source set by the seeing parameter and a flat F_nu spectrum set by the AB magnitude.

  • Extraction Method - We assume a symmetric Gaussian PSF model set by the seeing (exactly as the source model) and perform an optimal extraction following the same methodology as the automatic LRS2 pipeline.

  • Sky Brightness - Three sky templates were built from measured skies ranging from 20.0 < m_5000A < 21.0 [bright], 21.0 < m_5000A < 22.0 [grey], and 22.0 < m_5000A < 23.0 [dark]. Thus a bright sky is roughly 20.5 mag/arcsec^2 at 5000A, grey sky is 21.5 mag/arcsec^2, and dark sky is 22.5 mag/arcsec^2

  • Average Pupil Fill Factor - The HET pupil illumination varies with declination, track start time, and exposure time. A fill factor of 1.0 is nearly impossible for all but short exposures at center track. The default value of 0.9 will suit most programs. If your exposures are quite long, >30 mins, a value of 0.6-0.8 may be more appropriate.

  • Transparency - This factor accounts for sky conditions of your observation. Spectroscopic conditions are between 0.5-0.9, and photometric conditions are >0.9.

(note: what follows is older information; please use exposure time calculator above, if possible)

Below we give the estimated on-sky performance for LRS2-B and LRS2-R in 10E-16 ergs (A) and in AB-mag (B) under a dark sky with 2x30 minute exposures with 1.2" FWHM image quality, ie. best possible conditions.

Last updated: Fri, 24 Jul 2020 16:06:36 -0500 stevenj


The Telescope

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