This form can be used to run ATRAN in order to compute a synthetic
(i.e., theoretical) spectrum of atmospheric transmission (transmission as a function of wavelength).
The output data file generated
by ATRAN (atran.plt...dat), as well as a SM script to produce a plot (atran.plt...sm),
can be saved to your machine from your browser window once the program
has been run.

Give the altitude of the observatory in feet. (The maximum altitude allowable by ATRAN is 95470 feet.)
ATRAN will use a model of the atmosphere to
compute the predicted water vapor overburden at the zenith. If you wish to change the
zenith water vapor content to a specific value, give the new value in microns in the
appropriate field; otherwise leave the value at 0 and ATRAN will use the predicted value
derived from the model. Specify the zenith angle (the angle between the zenith and the line of sight) for the observations and the
wavelength range of interest, in terms of the minimum and maximum wavelengths given in
microns. The minimum wavelength possible is 0.85 microns. (Note that the larger the wavelength
range chosen, the longer it takes to perform
the calculations.) The model spectrum will be smoothed with a Gaussian with a constant FWHM value, where the FWHM is determined
from the central wavelength of the spectrum divided by the resolution value R if one is specified. (A value of 0 for R
means that no smoothing will be done.) Although
this version of ATRAN allows the user to specify the latitude of the observatory,
experience has shown that the day/night and seasonal variations in ozone are larger than
the latitude dependence in the total ozone column. Therefore, we recommend users adopt the
latitude of 39 degrees, in which case ATRAN will use a `standard' ozone value to compute the
atmospheric transmission model.

**Note:** Effective 01 July 2020, the parts per million of greenhouse gases used by ATRAN have been modified to reflect current values (CO2 changes from
330 to 415 ppm and NH4 changes from 1.6 to 1.888 ppm). These changes will result in deeper absorption lines.

All wavelengths are in vacuum.

**Note:** In order to prevent form time-outs and the generation of huge output files, we have
restricted the maximum size of the wavelength range to be 60% of the central wavelength. We have also restricted the altitude to be less than 60000 feet.

ATRAN was developed and kindly
provided to the SOFIA program by Steve Lord.
The reference for ATRAN is Lord, S. D., 1992, NASA Technical Memorandum 103957.
The current version uses the 2000/2001 version of the HITRAN database.

Users of this web form are requested to reference the Lord technical memorandum and this web site in any publications. If you have problems with this form, please contact the SOFIA Help-Desk (sofia_help.usra.edu). If you have questions about ATRAN itself, or would like a copy of the documentation on ATRAN, please contact Steve Lord (lord_91106@yahoo.com).

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