# 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 in comparison 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 center of track throughputs.
We have no imaging performance benchmarks yet because the depth depends so much on image quality and
sky brightness.

## Rough Estimates

These are sky noise dominated observations, so remember to scale by the sqrt(time).

ALL S/N are per assuming 2x2 binning and are per resolution element.

Configuration | Type of Object | Mag. | Time | S/N @ Wavelength |

LRS_g1_2.0_GG385 | stellar/QSO | 20 | 1800 | 15-20 @ 6500 Å |

LRS_g1_2.0_GG385 | stellar/QSO | 21 | 1800 | 3-8 @ 6500 Å |

LRS_g1_2.0_GG385 | distant galaxy | 21 | 1800 | 9-14 @ 6500 Å |

LRS_g2_2.0_GG385 | distant galaxy | 21 | 1800 | 4-9 @ 6500 Å |

LRS_e2_1.0_E2F | stellar | 20 | 1800 | 8-12 @ 5000 Å |

If you find these values to be incorrect over a significant data set PLEASE
send us your better estimation and what program number this data comes from.

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

## Throughput

##

In the following plots we give the throughput for each grating assuming
a 9.2m telescope with a 3.713m obstruction, observed 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**

**LRS g2**

**LRS g3 now defunct**

**LRS e2**

**All LRS grisms **

## Using this Data

##

To properly use this data the investigator should calculate the number
of photons incident on a 9.2m aperature 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 1.2). This gives
the number of photons you should expect without slit losses per wavelength
unit. 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 (ie photometric which occurs only ~25% of the time).
To correct to typical spectroscopic conditions remove ~20% of the photons.
The sqrt of this number gives you
the S/N in the absense of sky noise (the largest noise source). Include
a sky noise term by scaling the preslit photons by the relative magnitudes
of the target and the sky (never fainter than 21.0) and
multiplying by the slit width in arcseconds.
Add the noise sources in quaderature and divide into your
source flux. See simple! Or just use the rough estimate at the top of
the page.