LRS Throughput

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 or pupil compared to the middle of a trajectory. As such two medium length visits centered on middle of a track are sometimes more valuable than a single long visit. All throughput estimates are based upon the center of track pupil.

Rough Estimates

These are sky noise dominated observations, so remember to scale by the sqrt(time).
All S/N values assume 2x2 binning and are per resolution element.

Configuration Type of Object Mag. TimeS/N @ Wavelength
LRS_g1_2.0_GG385stellar/QSO 20 1800 15-20 @ 6500 Å
LRS_g1_2.0_GG385stellar/QSO 21 1800 3-8 @ 6500 Å
LRS_g1_2.0_GG385distant galaxy2118009-14 @ 6500 Å
LRS_g2_2.0_GG385distant galaxy2118004-9 @ 6500 Å
LRS_g3_1.0_OG515stellar20180014-17 @ 8000 Å

If you can obtain superior S/N estimates from your own data set PLEASE send us your improved values and the program number of the source data.

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


In the following plots we give the throughput for each grating assuming a 9.2m primary with a 3.713m obstruction, observing right at the center of the track (see fill factor information for more on this) and corrected to above the atmosphere using the KPNO extinction coefficients. Click on the plot to download the text file used to generate the plot.

LRS g1

All LRS grisms

Using these Data

To use these data properly the investigator should calculate the number of photons incident on a 9.2m aperture with a 3.713m obstruction (55.6 square meters of collecting area) from their source. Multiply by the system throughput at the wavelength of interest for the configuration of interest. Correct for extinction (a typical airmass for the HET is 1.22). This gives the number of photons per unit wavelength you should expect without slit losses. Multiply by the resolution element width to get the photons per RE. Correct for the slit losses by assuming a typical seeing (say 2.0") and your slit configuration of interest. This is still assuming perfect sky transmission (i.e. photometric which occurs only ~25% of the time). To correct to typical spectroscopic conditions remove ~20% of the photons. The square root of this number gives you the S/N in the absence of sky noise (the largest noise source). Include a sky noise term by scaling the pre-slit photons by the relative magnitudes of the target and the sky (assumed 21.0 mag at faintest) and multiplying by the slit width in arcsec. Add the noise sources in quadrature and divide into your source flux. See simple! Or just use the rough estimate at the top of the page.

Last updated: Sun, 08 Jan 2012 02:06:52 -0600 caldwell


The Telescope

Technical Overview

Object Observability




HRS Instrument Summary

HRS - Details

HRS Configurations

HRS Throughput

HRS Wavelength Calibration


MRS Instrument Summary

MRS Configurations

MRS Throughput

MRS Wavelength Calibration


LRS Instrument Summary

LRS Configurations

Multi-object Unit

LRS Throughput

LRS Spectral Resolution

LRS Wavelength Calibration

Visitor Filter Information

LRS Position Angles

Blind Offsetting

Guide Stars Limits

LRS Update from Gary Hill

Program Preparation

Web Management System

Other HET Documentation