The amount of telluric water vapor along the line of sight of the Kuiper Airborne Observatory telescope as obtained concommitantly on 23 flights is compared with the NASA-Ames Michelson interferometer and with the NOAA-Boulder radiometer. A strong correlation between the two determinations exists, and a method for computing the atmospheric transmission for a given radiometer reading is established

Erickson, E. F.

Kuhn, P. M.

Simpson, J. P.

Stearns, L. P.

English

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RUGA
National Aeronautics and
Space Administration
NASA TochnkW Memorsndum 78M
Determination of Use Telluric Water
Vapor Absorption Correction for
Astronomical Data Obtained From
the Kulper Airborne Observatory
E. F. Erickson, J. P. Simpson,
P. M. Kuhn, and L. P. Steams
(NASA-TM-78582) DETERMINATION OF THE
	
M79-20941
TELLURIC HATER VAPOR ABSORPTION CORRECTION
FOR ASTRONOMICAL DATA OBTAINED FROM THE
KUIPER AIRBORNE OBSERVATORY (NASA) 13 p HC	 Unclas
A02/MF A01	 CSCL,03A G3/89 16652
April 1979
F-
s
E	 NASA Technical Memorandum 785M
1
Determination of the Telluric Water
Vapor Absorption Correction for
Astronomical Data Obtained From
the Kulper Airborne Observatory
E. F. Erickson
J. P. Simpson, Ames Research Center, Moffett Field, California
P. M. Kuhn
L. P. Stearns, NOAA-Atmospheric Physics and Chemistry Laboratory, Boulder, Colorado
RVGA
National Aeronautics and
Space Administration
Ames Research Center
Moffett Field, California 94035
DETERMINATION OF THE TELLURIC WATER VAPOR ABSORPTION CORRECTION FOR
ASTRONOMICAL DATA OBTAINED FROM THE KUIPER AIRBORNE OBSERVATORY
E.F. Erickson and J.P. Simpson
Astrophysical Experiments Branch
NASA-Ames Research Center
Moffett Field, CA 94035
and
P.M. Kuhn and L.P. Stearns
NOAA-Atmospheric Physics and Chemistry Laboratory
Boulder, CO 80302
Summary
This report compares the amount of telluric water vapor along the line of
sight of the Kuiper Airborne Observatory telescope as obtained concommitantly
on 23 flights with the NASA-Ames Michelson interferometer and with the NOAA-
Boulder radiometer. A strong correlation between the two determinations exists,
and a method for computing the atmospheric transmission for a given radiometer
reading is established.
..14
lIntroduction
A problem commonly faced by scientists making astronomical observations from
within the Earth's atmosphere is the correction of the measured intensity for telluric
absorption. From the operating altitude of the Kuiper Airborne Observatory (KAO),
the problem can still be severe, depending on the wavelength range and spectral
resolution of the observation. Over most of the infrared portion (1-1000 um) of
the spectrum the major absorbing constituent is water. Two NOAA-Boulder water
vapor radiometers (Kuhn et al., 1975; Kuhn et al., 1976) are operated routinely
onboard the observatory to provide experimenters a record of the amount of water
vapor (1) along the line of sight (LOS) of the telescope, and (2) on a vertical.
line or zenith (z) above the aircraft.
Water Radiometer Measurements
The NOAA radiometers measure the absolute energy in a spectral band extending
from approximately 19-37 um. The response of the radiometer in this band is a pro-
duct of the window and filter transmissions, the detector response function, and
the amplifier gain. The shape of the radiometer response function versus wave-
length is determined from the response curves of the separate components. The
absolute responsivity (V/W) is calibrated in the laboratory using a cooled and
purged black body source. In flight, the radiometer sees the sky chopped against
a 2730K reference source. The radiant power P from the sky is determined from the
radiometer output. A separate radiometer, operating from 14-16 u on the edge of
the CO2 band determines the air temperature T. The amount of precipitable water
vapor W is then determined from predictions of P by a multilayer atmospheric
model which have been previously calculated using the radiometer response function,
for various values of T and W. The water vapor line strengths for the radiometer
2
emission are based on the current Air Force Geophysical Laboratory line strength
compilation adapted to a specific transfer computer model. Pressure broadening
and temperature corrections to the absorption coefficients are introduced for
each layer. Program DEGRADE and NOAA programs KBAR and LINES (NOAA-APCL-R31-1977)
are combined to obtain the broad band absorption coefficients employed in the
model. A preliminary value of W is available on-line during the observations,
and a final record of W versus time during a flight is available within a few days
of the flight after processing on the NOAA computer at Boulder. The record of W
(both LOS and z) as a function of time is available to each telescope user for
the period of his observations.
Far Infrared Spectral Observations
An independent estimate of the LOS value of W has been obtained a number of
times from measurements of the atmospheric transmission made with the NASA-Ames
Michelson interferometer. This instrument flies typically a few times per year
on the KAO to observe astronomical sources. The wavelength region covered on
anyone flight is broad, with extreme limits of •ti20p and 200u. The exact wave-
length range is determined by filters and beamsplitters (Erickson et al., 1977).
Over this wavelength range the atmosphere is, on the average, only about 70%
transmitting due to absorption by r'ie atmospheric water vapor. The water vapor
absorption consists of optically thick broad lines spaced at irregular intervals
throughout the spectrum, with relatively clear wavelengths inbetween. Since the
Michelson interferometer sees each of these lines (or groups of lines at low
resolution), it is possible to determine the amount of atmospheric absorption
in each individual spectrum.
Transmission spectra for the Earth's atmosphere can be computed for any
amount of water vapor, using program "DEGRADE" on the NASA-Ames CDC 7600. The
3
program assumes a single layer atmosphere an y, the Curtis-Godson approximation
for temperature and pressure (Augason et al., 1975; Augason and Burnes, 1977).
The H2O and G3 line list of McClatchey et al. (1973) is used.
In order to determine the water vapor absorption from the observed spectrum
of any given astronomical source, one must have a good idea of the true source
spectrum and the instrument response. For the lunar spectrum we assume a gray
body at a temperature appropriate to the location of our beam on the lunar surface
relative to the subsolar point (Lipsky, 1973). For Mars, Jupiter, and Saturn,
brightness temperatures as a function of wavelength described by Erickson et al.
(1978) were used to compute the source spectra. The instrument function is deter-
mined by iteration from the comparison of the measured spectrum with a synthetic
spectrum. The initial instrument function is determined from laboratory measure-
ments made with a blackbody in vacuum. The instrument response function is then
revised for those wavelengths where the synthetic spectrum disagrees with the
observed spectrum and the transmission is good enough that water absorption is
not the cause of the disagreement.
To determine the amount of water vapor in the line of sight, synthetic spectra
are computed for various values of W. Each synthetic spectrum is compared to the
observed spectrum and the sum of the squares of the residuals is computed for
wavelengths where the instrument response function times the source spectrum is
greater than a certain amount. The finite spectral range is chosen to minimize
the effects of noise where the signal is very low, but otherwise, the widest pos-
sible spectral range is used. The "best fit" value of W corresponds to the mini-
mum in the sum of the squares of the residuals. The accuracy of this procedure
depends upon the resolution of the spectra. For 5 cm 1 resolution W can be
obtained to about 1.0 precipitable microns. For 2.5 cm 1 resolution the water
vapor can be determined to about 0.25 u. For 10 cm 1 resolution, the procedure
4
H2O. For larger amounts of water the accuracy decrease. In fact, the accuracy
seems to be proportional to the log of the water vapor column density.
Some examples are plotted in Figures 1 and 2. Both figures show a measured
spectrum of the Moon and the beat fit synthetic spectrum at 5 cm 1 resolution.
The small deviations throughout the spectra may be due to noise and to the fact
that the instrument response function is tabulated only every 10 cm 1 . The RMS
average deviation for each point is 2.2% for scan 9 from 28 June, 1977 and 1.8%
for scan 49 from 1 July, 1977, relative to the measured peak intensity.
Comparison of the Two Experiments
Determinations of the water vapor column density in the line of sight for
36 scans from 5 different days are plotted in Figure 3. The abscissa is the value
of W determined by the Ames Michelson interferometer (WM) and the ordinate
is the value determined by the NOAA radiometer (W R) for the same times during
the flights. The astronomical sources were the Moon, Jupiter, and Mars for Nov.
17, 1977 and Nov. 21, 1977, the Moon for April 21, 1978, and Mars, Jupiter, and
Saturn for January 21, 1976 and January 26, 1976.
There is seen to be a strong correlation (0.90 for all 36 points) in the
data, with the radiometer values (WR) consistently (with one exception) higher
than the Michelson values (WM). A quadratic equation fitted by the method of
least squares to the data gives
W  - 0.526 W  (1 + 0.016 WR)	 (1)
Submillimeter spectroscopy has also shown a good correlation with the radio-
meter measurements (Nolt et al., 1979).
5
The differences between the values of W determined by the two methods could
arise from a number of causes. However, the important question for an astronomer
using the telescope is how to calculate an atmospheric absorption correction to
his data given a value of WR. From Figures 1 and 2 it appears that the atmospheric
transmission is modeled to ti1X by the computer program DEGRADE using the value WM.
The following simple procedure then provides a reasonable correction: using
equation 1, calculate a value of W  from the measured W R . Then use this value of
W  in program DEGRADE---or its ADAMS equivalent, CDC21 (Augason and Burnes, 1971)--
to compute the transmission over the wavelength range and at the spectral resolu-
tion of interest. The error in the correction may be obtained by estimating the
limits on W  from Figure 3, and using the program to recompute the transmission
for these values of WM.
Programs DEGRADE and CDG21 use a number of parameters, in addition to W,
to computer the transmission. The following parameters should be used when
applying this correction technique:
V1 and V2, the wavenumber limits of the spectrum to be calculated.
Type of slit function and full width at half max for triangular (CDG21
only) or equivalent interferometer mirror travel for a Michelson
(DEGRADE only).
Pressure - 0.126 atmospheres and temperature - 216.6 0K (These are for the
Curtis-Godson approximation (see Augason and Burnes, 1977).
Wavenumber intervals (bin sizes) for calculating and for plotting.
The DEGRADE data cards require a number of other parameters that are the
same for every run involving H2O.
It should also be mentioned that an appropriate instrument response function can
and should be used with DEGRADE or CDG21 to compute the average transmission.
6
7	 ./^
REFERENCES
Augason, G.C. and Burnes, N.S.: CDG21: A Computer Program for Computing Atmo-
spheric Transmission Using the C-141 HP-2100 Computer. NASA TM X-73,194,
1977.
Augason, G.C., Mord, A.J., Witteborn, F.C. Erickson, E.F., Swift, C.D., Caroff,
L.J., and Kunz, L.W., Water Vapor Absorption Spectra of the Upper Atmosphere
(45-185 cm 1). Appl. Opt., 14, 2146 (1975).
Erickson, E.F., Goorvitch, D., Simpson, J.P., and Strecker, D.W.: Far Infrared
Spectroscopy of Jupiter and Saturn. Icarus, Vol. 35, 1978, p. 61.
Erickson, E.F., Strecker, D.W., Simpson, J.P., Goorvitch, D., Augason, G.C.,
Scargle, J.D., Caroff, L.J., and Witteborn, F.C.: Spectrum of the Kleinmann-
Low Nebula from 29 to 125 Micrometers. Astrophys. J., Vol. 212, 1977, p. 696.
Kuhn, P.M., Magaziner, E., and Stearns, L.P.: Stratospheric Areal Distribution
of Water Vapor Burden and the Jet Stream. Geophysical Research Letters,
Vol. 3, 1976, p. 529.
Kuhn, P.M., Stearns, LP., and Lojko, M.S.: Latitudinal Profiles of Stratospheric
Water Vapor. Geophysical Research Letters, Vol. 2, 1975, p. 227.
Linsky, J.L.: The Moon as a Proposed Radiometric Standard for Microwave and
Infrared Observations of Extended Sources. Ap. J. Suppl. 25, 1973, p. 163.
McClatchey, R.A., Benedict, W.S., Clough, S.A., Burch, D.E., Ca'fee, R.F., Fox,
K., Rothman, L.S., and Garing, J.S.:	 AFCRL Atmospheric Absorption Line
Parameters Compilation. Air Force Cambridge Research Laboratory Rept.
AFCRL-TR-73-0096, Bedford, Mass., 1973.
Nolt, I., Radostitz, J., Donnelly, R.J., and Stearns, L.P.: A Comparison of
Stratospheric Water Vapor Measurements by Spectroscopic and Radiometric
Methods. National Oceanic and Atmospheric Administration Report, 1979
(in publication).
8
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..O^
11


Determination of the Telluric Water Vapor Absorption Correction for Astronomical Data Obtained from the Kuiper Airborne Observatory

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