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An infrared hygrometer for astronomical site testing
In/rarrd Phwic P Vol 20, pp 321 to 325 0 Pergamon Press Ltd 1980. Printed m Great Brltam
AN INFRARED
0020
0891.80!09014321I02.00/0
HYGROMETER FOR ASTRONOMICAL SITE TESTING E. B~SCHERand D. LEMKE
Max-Planck-Institut
fiir Astronomie, (Received
Heidelberg,
10 March
West Germany
1980)
Abstract-This paper describes an i.r. hygrometer which measures the absolute water vapour content in the atmosphere. The instrument is easy to handle, it can be used with either the sun or the moon as a radiation source, and its design minimizes errors due to atmospheric aerosols. The instrument has already been used for astronomical site testing.
INTRODUCTION Infrared (i.r.) and submillimeter observations with ground-based telescopes are strongly affected by atmospheric water vapour, which limits the sensitivity of astronomical measurements by extinction, background emission and sky noise. The water vapour content and its annual variation are important criteria when selecting a site for an i.r. telescope. At existing observatories the continuous monitoring of the H,O content allows statistical and short term predictions about the expected quality of i.r. observing nights. Several i.r. hygrometers have been built during the past two decades at various observatories and meteorological stations. Most of these operate by measuring solar radiation intensity (1) within a water vapour absorption band, and (2) in an adjacent absorptionfree part of the spectrum. The ratio of these two measurements is then calibrated in terms of water vapour column density. Often these instruments can only be used during the day time, and suffer from aureole effects. Other instruments are designed for night observations using stars as the illuminating source, but they are more complex and require mountings. DESIGN
OF
THE
INFRARED
HYGROMETER
It was intended (i) to make the instrument simple, small and light-weight for easy hand-carrying and application, (ii) to facilitate day- and night-measurements, (iii) to avoid aureole effects, (iv) to allow corrections for aerosols, (v) to calibrate the instrument accurately in terms of absolute water vapour column densities and (vi) to allow a quick check of the calibration. In order to achieve both goals (i) and (ii) we restricted the night measurements to those nights on which the moon could be used as a radiation source. Figure 1 shows the hygrometer, while Fig. 2 gives the optical and electronics layouts. In order to overcome the large brightness ratio (- 105) between sun and moon a neutral density filter can be placed in front of the lens. Four band pass filters can be selected by means of a filter wheel. The filters were chosen to match the atmospheric bands selected. Their band passes were checked in the laboratory using a convergent beam as produced by the instruments objective lens. A photovoltaic silicon-diode (PIN 5DP, United Detector Technology) was chosen as the detector since this is linear over a factor of 10” in intensity variation. The simple electronics consists of 3 operational amplifiers with the potentiometers allowing a gain variation of 106. Figure 3 shows the H,O-absorption bands, which are available to the detector.‘2’ The strongest absorption band (apz-band) was chosen in order to allow measurements at low column densities. No filter can be used on the longer wavelength side of the absorption band because of the rapid decrease of the detector responsivity. 321
E. B
322
Fig
D. LEMKE
I. Photograph of the ix. hygrometer showing the filter selector and the alignment on the top surface. the elevation angle indicator on the near side.
projector
filter at 913 nm are worse, because the average 1‘he results with the second absorption line : strength of water vapour at the wavelength of filter 2 is less than at that of filter 1. curves (Fig. 4). Thi s is also shown by the calibration
Fig. 2. The optical
and electrical
layout.
An infrared
5
z
0.6
hygrometer
for astronomical
‘E O.L-
Transmission of theEarth’s w=lOkgm-2
!J g 0.2-
323
w=
0.6 p
a
site testing
Atmosphere
02
I
0.7
0.8
Fig. 3. The atmospheric
11)
transmission
Table
with an H,O-column
1. The spectral
Filter 1 nm Peak wavelength Equivalent width
Filter 2 nm
940 18.7 Absorption
DATA
density
I of 10 kgm-*
bands Filter 3 nm
Filter 4 nm
913
869
757
14.3 filters
12.1 Reference
5.3 filters
REDUCTION
The ratio U1/U3 is a measure of the water vapour content w [marked as w(U,/U,)], where Ui (i = 1,2,3,4) is the measured voltage with the filter i in the ray path. Filter 1 passes radiation in the absorption band, while filter 3 passes non-absorbed radiation in an adjacent part of the spectrum. It is more accurate, however, to use two reference filters and to calculate the flux which would be measured with the absorption filter if there were no water vapour in the atmosphere. This calculation is easily done with the RayleighJeans-law and the calculated voltage is denoted by U1 (w = 0). The use of two reference
Calibration
1.0
of
0.8
0.6
9
16
25
0
t+O-column
Fig. 4. The calibration curves. Q is the ratio calculated from U3 and U4 and refers to the filter No. 1. The theoretical calibration curves of Q and a square root expression for low Q derived from the calibrations, and are used
I.P. 20/5--r
density
1 w[kg
L
9
16
25
me21
of the observed Li, and Liz (w = 0). Lil (w = 0) is unattenuated solar flux at the central wavelength of are described by a linear expression for large values values. The numerial values in these expressions are for the pocket calculator program. For details see Ref. (1).
E. BUSCHER and
324
D.
LEMKE
filters has the advantage of avoiding errors due to a change of the sun’s or moon’s spectrum caused by aerosols. The ratio between the observed U,(w) and the calculated CT, (NJ= 0) voltage at absorption filter 1 is a measure of the water content in the ray path, u’ (Q). Therefore Q
=
In G/13) - In (H4/H3) + In H3
-
Hexp
In (&/J3)
and (i = 1,2.3,4)
Hi = CiUj
(2)
ci are factors related to different equivalent widths of the filters and the different detector response at & They contain corrections for the true spectrum of the sun and the moon and also for the neutral filter transmission curve at Li. The ci values can be calculated and have been found to be for example cID = 0.6235, cIc = 0.6619. After calibration, the water vapour content in the ray path w(Q) follows directly from Q. Using the set z-law. w(Q) is reduced to zenith angle z = 0 [denoted wO(Q)]. With a programmable pocket calculator it is possible to obtain the water vapour column density we(Q) from the measured voltages U1, U3, U4 and from the zenith angle z using formulae (1) and (2) and the set z-law. THE
AUREOLE
Besides affecting the solar spectrum as mentioned above, the aerosols have a second effect on the measurements since they are responsible for the sun’s aureole, which is caused by Gil-s~ttering. Because of the longer path of the scattered light through the atmosphere the aerosols cause an erroneously high measurement of water vapour content. But this aureole effect decreases with decreasing field of view. We used field of view of 1”15’, which has been found to be the smallest useable in an instrument without a mounting. CALIBRATION
The i.r. hygrometer was calibrated, (1) in the laboratory to get a measurement without H,O-attenuation, (2) for low water vapour content at the mountain Zugspitze (2966 m), and (3) for higher contents in Stuttgart (330 m) (both in Germany). Figure 4 shows the calibration curves. The reference data of these field measurements came from radiosonde data. The launch site of the radiosondes and the place of infrared measurements were close together. The water vapour column densities are expressed in units of kg me2, but the values given can also be read as mm of precipitable water. For a quick check of the calibration it is sufficient to repeat the laboratory measurement, because all properties of the instrument are checked in this way. After one year’s use of the hygrometer no change in the calibration curve was found.
eh
lob
12h lb
16h 18h 2oh
22h
xh
Fig. 5. Typical measurements of the water vapor content w above the Zugspitze on 1978, using the sun and the moon. For comparison radiosonde data RS are indicated. of passing thin clouds can be seen at 22 hr.
11 October Influence
An infrared
hygrometer
for astronomical
site testing
325
CONCLUSIONS
During a one year test period the i.r. hygrometer described here has been found to be very useful for field measurements, in particular at the Calar Alto observatory. It allows quick measurements of the total water vapour content above the observer during both day and night. The field of view is a compromise in order to allow free-hand measurements but also to avoid aerosole effects. The accuracy of the measurements is about f0.5 kg me2 (or f0.5 mm precipitable water) over the 2-40 kg mm2 range. The instrumental calibration curve has been found to be stable over a period of at least 1 year. AcknowledgementsWe thank the Aerologische Station des Deutschen tut fir atmosphlrische Umweltforschung der Fraunhofer-Gesellschaft radiosonde measurements. REFERENCES 1. B~~SCHERE., Staatsexamens-Arbeit, Univ. Heidelberg (1979). 2. KOEPKE P. & H. QUENZEL, Appl. Opt. 17, 2114 (1978).
Wetterdienstes Stuttgart and the Instiin Garmisch-Partenkirchen for the
AN INFRARED
0020
0891.80!09014321I02.00/0
HYGROMETER FOR ASTRONOMICAL SITE TESTING E. B~SCHERand D. LEMKE
Max-Planck-Institut
fiir Astronomie, (Received
Heidelberg,
10 March
West Germany
1980)
Abstract-This paper describes an i.r. hygrometer which measures the absolute water vapour content in the atmosphere. The instrument is easy to handle, it can be used with either the sun or the moon as a radiation source, and its design minimizes errors due to atmospheric aerosols. The instrument has already been used for astronomical site testing.
INTRODUCTION Infrared (i.r.) and submillimeter observations with ground-based telescopes are strongly affected by atmospheric water vapour, which limits the sensitivity of astronomical measurements by extinction, background emission and sky noise. The water vapour content and its annual variation are important criteria when selecting a site for an i.r. telescope. At existing observatories the continuous monitoring of the H,O content allows statistical and short term predictions about the expected quality of i.r. observing nights. Several i.r. hygrometers have been built during the past two decades at various observatories and meteorological stations. Most of these operate by measuring solar radiation intensity (1) within a water vapour absorption band, and (2) in an adjacent absorptionfree part of the spectrum. The ratio of these two measurements is then calibrated in terms of water vapour column density. Often these instruments can only be used during the day time, and suffer from aureole effects. Other instruments are designed for night observations using stars as the illuminating source, but they are more complex and require mountings. DESIGN
OF
THE
INFRARED
HYGROMETER
It was intended (i) to make the instrument simple, small and light-weight for easy hand-carrying and application, (ii) to facilitate day- and night-measurements, (iii) to avoid aureole effects, (iv) to allow corrections for aerosols, (v) to calibrate the instrument accurately in terms of absolute water vapour column densities and (vi) to allow a quick check of the calibration. In order to achieve both goals (i) and (ii) we restricted the night measurements to those nights on which the moon could be used as a radiation source. Figure 1 shows the hygrometer, while Fig. 2 gives the optical and electronics layouts. In order to overcome the large brightness ratio (- 105) between sun and moon a neutral density filter can be placed in front of the lens. Four band pass filters can be selected by means of a filter wheel. The filters were chosen to match the atmospheric bands selected. Their band passes were checked in the laboratory using a convergent beam as produced by the instruments objective lens. A photovoltaic silicon-diode (PIN 5DP, United Detector Technology) was chosen as the detector since this is linear over a factor of 10” in intensity variation. The simple electronics consists of 3 operational amplifiers with the potentiometers allowing a gain variation of 106. Figure 3 shows the H,O-absorption bands, which are available to the detector.‘2’ The strongest absorption band (apz-band) was chosen in order to allow measurements at low column densities. No filter can be used on the longer wavelength side of the absorption band because of the rapid decrease of the detector responsivity. 321
E. B
322
Fig
D. LEMKE
I. Photograph of the ix. hygrometer showing the filter selector and the alignment on the top surface. the elevation angle indicator on the near side.
projector
filter at 913 nm are worse, because the average 1‘he results with the second absorption line : strength of water vapour at the wavelength of filter 2 is less than at that of filter 1. curves (Fig. 4). Thi s is also shown by the calibration
Fig. 2. The optical
and electrical
layout.
An infrared
5
z
0.6
hygrometer
for astronomical
‘E O.L-
Transmission of theEarth’s w=lOkgm-2
!J g 0.2-
323
w=
0.6 p
a
site testing
Atmosphere
02
I
0.7
0.8
Fig. 3. The atmospheric
11)
transmission
Table
with an H,O-column
1. The spectral
Filter 1 nm Peak wavelength Equivalent width
Filter 2 nm
940 18.7 Absorption
DATA
density
I of 10 kgm-*
bands Filter 3 nm
Filter 4 nm
913
869
757
14.3 filters
12.1 Reference
5.3 filters
REDUCTION
The ratio U1/U3 is a measure of the water vapour content w [marked as w(U,/U,)], where Ui (i = 1,2,3,4) is the measured voltage with the filter i in the ray path. Filter 1 passes radiation in the absorption band, while filter 3 passes non-absorbed radiation in an adjacent part of the spectrum. It is more accurate, however, to use two reference filters and to calculate the flux which would be measured with the absorption filter if there were no water vapour in the atmosphere. This calculation is easily done with the RayleighJeans-law and the calculated voltage is denoted by U1 (w = 0). The use of two reference
Calibration
1.0
of
0.8
0.6
9
16
25
0
t+O-column
Fig. 4. The calibration curves. Q is the ratio calculated from U3 and U4 and refers to the filter No. 1. The theoretical calibration curves of Q and a square root expression for low Q derived from the calibrations, and are used
I.P. 20/5--r
density
1 w[kg
L
9
16
25
me21
of the observed Li, and Liz (w = 0). Lil (w = 0) is unattenuated solar flux at the central wavelength of are described by a linear expression for large values values. The numerial values in these expressions are for the pocket calculator program. For details see Ref. (1).
E. BUSCHER and
324
D.
LEMKE
filters has the advantage of avoiding errors due to a change of the sun’s or moon’s spectrum caused by aerosols. The ratio between the observed U,(w) and the calculated CT, (NJ= 0) voltage at absorption filter 1 is a measure of the water content in the ray path, u’ (Q). Therefore Q
=
In G/13) - In (H4/H3) + In H3
-
Hexp
In (&/J3)
and (i = 1,2.3,4)
Hi = CiUj
(2)
ci are factors related to different equivalent widths of the filters and the different detector response at & They contain corrections for the true spectrum of the sun and the moon and also for the neutral filter transmission curve at Li. The ci values can be calculated and have been found to be for example cID = 0.6235, cIc = 0.6619. After calibration, the water vapour content in the ray path w(Q) follows directly from Q. Using the set z-law. w(Q) is reduced to zenith angle z = 0 [denoted wO(Q)]. With a programmable pocket calculator it is possible to obtain the water vapour column density we(Q) from the measured voltages U1, U3, U4 and from the zenith angle z using formulae (1) and (2) and the set z-law. THE
AUREOLE
Besides affecting the solar spectrum as mentioned above, the aerosols have a second effect on the measurements since they are responsible for the sun’s aureole, which is caused by Gil-s~ttering. Because of the longer path of the scattered light through the atmosphere the aerosols cause an erroneously high measurement of water vapour content. But this aureole effect decreases with decreasing field of view. We used field of view of 1”15’, which has been found to be the smallest useable in an instrument without a mounting. CALIBRATION
The i.r. hygrometer was calibrated, (1) in the laboratory to get a measurement without H,O-attenuation, (2) for low water vapour content at the mountain Zugspitze (2966 m), and (3) for higher contents in Stuttgart (330 m) (both in Germany). Figure 4 shows the calibration curves. The reference data of these field measurements came from radiosonde data. The launch site of the radiosondes and the place of infrared measurements were close together. The water vapour column densities are expressed in units of kg me2, but the values given can also be read as mm of precipitable water. For a quick check of the calibration it is sufficient to repeat the laboratory measurement, because all properties of the instrument are checked in this way. After one year’s use of the hygrometer no change in the calibration curve was found.
eh
lob
12h lb
16h 18h 2oh
22h
xh
Fig. 5. Typical measurements of the water vapor content w above the Zugspitze on 1978, using the sun and the moon. For comparison radiosonde data RS are indicated. of passing thin clouds can be seen at 22 hr.
11 October Influence
An infrared
hygrometer
for astronomical
site testing
325
CONCLUSIONS
During a one year test period the i.r. hygrometer described here has been found to be very useful for field measurements, in particular at the Calar Alto observatory. It allows quick measurements of the total water vapour content above the observer during both day and night. The field of view is a compromise in order to allow free-hand measurements but also to avoid aerosole effects. The accuracy of the measurements is about f0.5 kg me2 (or f0.5 mm precipitable water) over the 2-40 kg mm2 range. The instrumental calibration curve has been found to be stable over a period of at least 1 year. AcknowledgementsWe thank the Aerologische Station des Deutschen tut fir atmosphlrische Umweltforschung der Fraunhofer-Gesellschaft radiosonde measurements. REFERENCES 1. B~~SCHERE., Staatsexamens-Arbeit, Univ. Heidelberg (1979). 2. KOEPKE P. & H. QUENZEL, Appl. Opt. 17, 2114 (1978).
Wetterdienstes Stuttgart and the Instiin Garmisch-Partenkirchen for the