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.  Time  S/N @ Wavelength 
LRS_g1_2.0_GG385  stellar/QSO  20  1800  1520 @ 6500 Å 
LRS_g1_2.0_GG385  stellar/QSO  21  1800  38 @ 6500 Å 
LRS_g1_2.0_GG385  distant galaxy  21  1800  914 @ 6500 Å 
LRS_g2_2.0_GG385  distant galaxy  21  1800  49 @ 6500 Å 
LRS_g3_1.0_OG515  stellar  20  1800  1417 @ 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.
Throughput
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 preslit 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
