The determination of the extinction effects inducedby the atmosphere on the irradiance of externalsources is a basic problem of the atmospheric sciencesthat finds applications in a large variety of studiesfrom climatology to astroparticle physics. The opticalthickness is a measure of the integral of extinctionalong the line of sight due to contributions from mol-ecules and aerosol. In this work, a digital camera hasbeen used at night as a photometer to determine on awide angle the optical thickness of the atmospheresimultaneously from a number of stars. The observa-tion of a number of selected stars at different zenithangles is obtained by using objective lenses of ade-quate sensitivity and relatively large fields of view(FOV), and through a sequence of shots carried outwith the camera pointing at different angles. Addi-tional information on the aerosol conditions may beprovided by the skylight background as detected bythe camera, particularly in urban surroundings.
At visible wavelengths, the role of the aerosol, whichinclude thin clouds, is predominant. Thus a quasi-
instantaneous map of the inhomogeneity of the atmo-spheric aerosol content over a definite portion of thesky can be provided, overcoming to a certain extent theproblem of time variability of the signal implied bysequential observations of a single light source.
Considerable advantage by the use of the camera isobtained if a conventional lidar system is simulta-neously operational. While the camera may providethe columnar value of the extinction, the lidar pro-vides the vertical structure of the volume backscattercross section of aerosol layers and thin clouds. Forwell-defined layers, such as a cloud, a direct measureof the contribution of the layer to the extinction canbe obtained and it is thus possible to separately ac-count for the extinction due to the planetary bound-ary layer (PBL). The extinction-to-backscatter ratio(EBR) provides a useful insight into the nature of theparticulate and may, to some extent, make it possibleto distinguish between the separate contributions toextinction by absorption and scattering. These quan-tities in general have a different dependence onheight due to the large space and time variability ofthe composition, size, and shape of the particulate.
The use of the camera is of advantage at nightwhen Sun photometers cannot be used. A historicalaccount of the use of Sun photometers for aerosol
climatology, starting from the early work by Roosenet al. [1] and Volz [2], is given by Holben et al. [3] intheir comprehensive description of the results of theAeronet program. Combined use of lidar data andstar–Sun photometry during the ACE2 campaign hasbeen reported by Ansmann et al. [4], including resultsrelated to the aerosol EBR: for this research a singlephotometer was used, pointing at two stars near thezenith and the horizon, respectively. The photometerwas located at a height of 900 m to avoid observingthe PBL.
Use of a camera as the detector of a bistatic lidarsystem has been demonstrated by Barnes et al. [5]. Inthe present paper, the camera is utilized as a passivesystem providing additional yet important informa-tion complementary to the one primarily provided bya lidar. To summarize, use of a digital camera as avisual nighttime radiometer, prompted by the rela-tive low cost, versatility, and reliability of the basicinstrument, can be considered (a) as a single instru-ment, to obtain measurements of atmospheric trans-parency over relatively large fields of view, throughthe observation of stars as well as for measurementsof the skylight background, particularly in cities; (b)in connection to a lidar, and, in addition to the pre-vious point, (b1) to provide data on atmospherictransparency in the beam direction to be related tolidar backscatter data to obtain EBR and, (b2) to beutilized as a sensor for bistatic observation of thelaser beam.
2. Instrumentation: The Camera
Images of the nighttime sky are obtained with a digitalsingle-lens reflex (DSLR) Nikon D1 camera supportedby an altazimuthal mount assembled with standardmotorized laboratory hardware (Fig. 1): both camera
(Nikon software) and mount are computer controlled.The camera has a CCD of 2000 � 1312 pixel of size0.01185 mm � 0.01189 mm, amounting to an arrayof dimensions 23.7 mm � 16.6 mm. The camera istypically equipped with a 50 mm focal-length lensof f number 1.4, providing a FOV of approximately27° � 19°, equivalent to 0.24 � 10�3 rad�pixel.Exposures are generally performed at the nominalsensitivity of 200 International Organization for Stan-dardization (ISO), providing the best image definition,with an integration time of 20 s.
The CCD sensor, which is not cooled, has an effectivedynamic range of 8 bit. Working temperature range is3 °C–35 °C. The camera operates in the red–green–blue (RGB) color modality, but only the green channelhas been utilized: the G spectrum �500–600 nm� over-laps with the wavelength range of the Johnson’s Vfilter utilized in the Tycho Star Catalogue [6]. Inthese conditions, a portion of the sky extending for atotal width of 27° across the 0°–90° zenithal arc canbe covered with six shots in approximately 5 min,thus acquiring useful star signals of visual magni-tude, mV, between 2 and 6.
The output raw image file format is converted in bitmap format for analysis purposes: this format guar-antees no alteration or loss of the original informa-tion from the CCD. The analysis software has beenwritten by the authors in interactive data language(IDL), except for some parts pointed out below.
The experiment was set up inside a dome on therooftop of the Physics Department at the University’smain location, in a highly urbanized part of the city.A zenith pointing monostatic pulsed lidar, based on aNd:YAG laser running at 532 nm, can be operationalat the site simultaneously with the camera.
Since the camera was not originally intended to bea scientific instrument, a preliminary survey of itscapability and reliability was carried out, startingfrom its nominal characteristics to confirm the cam-era specifications. Particular attention was placed onretrieving the angular resolution, the color response,and the dynamic range.
The angular resolution and FOV were verified byretrieving couples of stars on the edge of the image anddetermining their angular distance, thus confirmingthe camera specifications. While the camera operatesin the RGB modality, no specific data on the filtersrange and transmission were available. On this issue,we assumed that the specifications would follow thestandard CIE 1964 10°. This would imply an accept-able superimposition with the Johnson V filter utilizedin the Tycho Star Catalogue: the check was carried outby examining the brightnesses of several stars in clearsky conditions, expecting that they would fit a Langleyplot. This was verified for the G channel but not for theR and B channels. The response of the CCD was ver-ified by exposing the camera to a diffuse source, utiliz-ing diffusing filters and verifying through differentand repeated exposures that the response was linearwithin one digit and that the saturation would ensuebeyond 250 digits.
Fig. 1. (Color online) Digital camera and its altazimuthal mountinstalled in the dome.
The retrieval of the atmospheric radiative conditionsat night is based on measurements of star irradiancesat the ground: inside the whole FOV several stars ofknown magnitude were selected to be used as extra-terrestrial calibrated sources. The choice of the usefulstellar sources is dictated by criteria of stability andunambiguity: variable stars and stars so close to eachother as to produce overlaps have to be excluded. Theuseful star magnitude range depends on a compro-mise between the CCD gain, the time exposure, andsensitivity; the 8 bit dynamic range and attendingsaturation problems; the range of zenithal angles andthe observed region of the sky; and the image reso-lution between near stars and the expected variabil-ity of sky opacity conditions.
In the present observational setup, the altazimuthmount maintains, for the duration of the shot, thecamera axis fixed at one direction in the sky on ameridian plane close to the celestial North Pole. Sinceno equatorial tracking capability has been utilized,the star image is spread out proportionally to itsdistance from the celestial North Pole and to theexposure time. Thus, the star image occupies severalpixels due, in addition to the star motion on the focalplane, to the seeing associated to atmospheric turbu-lence, to the camera resolving power, to vibrations,etc. The most luminous sources utilized may occupy atotal number of �80 pixels for a mean radius of �5pixels. Although the individual pixels only have adynamic range of 8 bits, the retrieval of the receivedstar irradiance, distributed as it is on a large numberof pixels, allows for a much higher resolving power.
The analysis software is able to find a star fromthe signal peak and its characteristic form (sectionof software developed starting by free online IDLLibrary [7]), then, a specific procedure selects in eachphotograph a square of 14 � 14 pixels around eachuseful star (Fig. 2a) to analyze the signal in it. Oneach section, the skylight background is evaluated onthe edge pixels and then subtracted from the signalon each pixel of the square: the integration of theremaining signal is taken as its equivalent irradiance(Fig. 2b shows a 3D example of the star signal).
Different stellar magnitudes imply that for the re-trieval of extinction it is necessary to normalize theirradiances at some magnitude; zero by convention.Besides, the magnitudes depend on the transmissionfunctions of the ultraviolet blue green red infraredwavelength filters used by a specific photometry.
Then, to have a correspondence between magnitudecatalog data and irradiance measurements providedby camera, the overlap between the transmissionfunctions of the photometric filters and the CCD fil-ters of the camera must be ascertained. As men-tioned, the transmission function of the V filter inJohnson photometry provides the right overlap withthe filter of the G channel in RGB modality.
The first step of the software procedures is to cor-rect for the barrel distortion of the lens, then to scanthe image to find all stars and to convert throughmapping their positions on the image to their celes-tial coordinates. To obtain the correct mapping foreach photo, the analysis proceeds after two stars, S1and S2, are manually recognized in the frame, andtheir position and sky coordinates [8] are manuallyintroduced. A sky-coordinate correction has been in-serted to take care of the position change with respectto the given ones by the catalog because of the equi-nox precession at the time of observation. Such dataare used into the equations for the mapping [9], al-lowing to retrieve the coefficients of a matrix throughwhich the sky coordinates of each star Si �i �1, . . . , n� are calculated. Subsequently the zenith an-gles and the stars identities (on the star catalog) areobtained by the knowledge of their coordinates on theimage and of the shooting time. The catalog providesthe visual magnitude mV of the sources and informa-tion on their variability. The mV is then used to nor-malize the IV irradiance measurement to I�V datumsource of mV � 0 through the Pogson law [10], ex-pressed for a generic wavelength �,
m� � m�� � 2.5 log10�I��I���. (1)
In this way, the stars become equivalent to a singlesource present simultaneously at different zenith an-gles. After these preliminary steps, the analysis pro-ceeds introducing a noise correction possibly inducedby dust on the CCD and by different sensitivity of itselements; then to extract the photometric informa-tion necessary to obtain the atmospheric extinction.Software has been developed to automatically obtainmaps of the atmospheric optical thickness and of the
Fig. 2. (Color online) a, Trace of a star on the CCD image plane.b, Representation in 3D of the trace in a after background noiseremoval.
Fig. 3. (Color online) Example of normalized irradiances versusthe secant of the zenith angle (Langley plot) in very clear skyconditions.
sky background based on a sequence of photos fromzenith to horizon.
4. Calibration
Generally the optical thickness at the zenith, �, isobtained by the linear form of the Beer’s law,
ln Ii�0� � ln I0��� � � sec �i, (2)
where �i is the zenith angle of the ith star; ln�Ii�0��,logarithm of the irradiance at the ground, is equal toln�Vi�0� � Fi� � ln�100.4mi�, where Vi�0� is the signalmeasured at the ground, Fi is the measured irradi-ance of the sky background and 100.4mi is a factor tonormalize the signal received by a star of magnitudemi to zero magnitude, by convention.
Equation (2), valid for a horizontally stratifiedatmosphere with constant values of the extinction,is the basis of a graphical procedure known as the
Langley plot, whereby the measured irradiances Ii�0�,shown as a function of sec �i, best fit a straight line ofangular coefficient � (the average optical depth); theextraterrestrial irradiance of the star of magnitudezero, I0���, is the intercept of the straight line.
A series of such “clear sky” Langley plots are thenused to provide the best estimate of I0���, the calibra-tion datum of the camera sensitivity to be used in Eq.(2); Fig. 3 is an example of real plot obtained in clearsky condition. In this way, the optical thickness in thedirection of each individual i star (without the effectof air mass) is given by the following expression:
�i � �ln I0��� � ln�Ii�0����sec �i. (3)
The variability of the directional optical thickness �i
is an indicator of the inhomogeneity of the sky trans-parency. Figure 4 shows a Langley plot in a cloudysky condition.
Finally a correction is applied to the zenith anglefor values larger than 30° to take care of the spher-ical geometry of the Earth and the effects of refrac-tion [11],
where ��i is the value of the zenith angle after correc-tion.
5. Results
Figures 5a–5e show, as an example, a typical imageand subsequent results of the data analysis; Fig. 5ashows the image obtained by combining six succes-
Fig. 4. (Color online) Example of Langley plot in the presence ofvariable sky conditions.
Fig. 5. (Color online) a, Image resulting from the composition of six photographs; b, celestial and local coordinates of a; c, map of the opticalthickness along the direction of observation (the useful stars are indicated by asterisks); d, same as c after removal of the air mass effect;e, relative irradiance of skylight background.
sive shots. Figure 5b depicts the celestial coordinates,the position of the North Pole and of the zenith, aswell as a trace of the meridian of the site. Figure 5c isa sky map of the optical thickness obtained by linearinterpolation of the data in the direction of each stel-lar source having removed the molecular contribu-tion. Figure 5d shows the same data in Fig. 5c,divided by the secant of the zenith angle, which givesevidence to the inhomogeneities in the atmosphericextinction. Finally Fig. 5e is a map of the skylightbackground.
Figure 6 is based on an extended series of obser-vations, each consisting of six photographs, carriedout with the camera during the night of 29–30 Sep-tember 2004, starting at intervals of 30 min, from20:30 to 5:00 local time. Figures 6a and 6b, respec-tively, are maps of optical thickness and skylightbackground that show the variable presence of aero-sol and clouds. In the interval between 22:30 and01:30, the cloud presence determined the reduction ofthe stellar irradiance and the simultaneous increaseof the skylight background at the zenith the opticalthickness reached the value of �1.6. At other angles,where the optical thickness exceeded the value of 2,its determination was no longer possible.
6. Combining Camera Data With Lidar Observations
Observations with a pulsed lidar were carried out atthe same location and time. As it has been said, thelidar wavelength �� � 532 nm� is within the overlaprange of the camera and of the photometric cataloguefilters. The lidar operates in a standard modality(e.g., monostatic, beam axis parallel to optical axis oftelescope) and is pointing to the zenith; a trace of itsbeam falls within the FOV of the camera but no anal-yses of the beam intensity as observed by the camerain a bistatic geometry have been utilized for thispaper. Typically a monostatic pulsed lidar providesmeasurements of the atmospheric backscatter vol-ume cross section by a comparison with the expectedmolecular signal, in a region where its transmittedbeam can be observed by the receiving optics; also,saturation at close range has to be avoided. Thus, fora monostatic system of simple design there is a region
close to ground where no measurement is possible.Techniques based on high spectral resolution or on adiversification of the receivers would satisfactorilyresolve this point but they would also increase com-plexity. In the presence of isolated aerosol layers thedetermination of the extinction is made possible by acomparison of the signal obtained, respectively, at thetop and bottom of the layer where the atmosphere isassumed to be clear.
Figure 7 depicts the vertical profile of the aerosol-to-molecule backscatter ratio, obtained with the lidarthrough the same time interval when the camera wasactive, showing the evolution of the aerosol in theboundary layer and the presence of thin clouds be-tween 9 and 12 km from approximately 22:30 to01:30. From such data, integral values of the lidaraerosol backscatter cross section have been obtained:Fig. 8a shows the integral value from the ground upto 12 km, while in Fig. 8b the integral is limited to thePBL, below 2000 m. It should be pointed out that thelidar was insensitive to the presence of aerosols belowapproximately 200 m; it has been assumed that theirpresence was homogeneous down to ground level.
The data in Fig. 7 can be combined with the ze-nithal optical thickness observed with the camerawithin an angle of 2.5° from the zenith shown in
Fig. 6. (Color online) Series of measurements, at half-hour intervals. a, Optical thickness corrected for air mass (see Fig. 5d); b, skylightbackground (see Fig. 5e) for the night of 29–September 2004.
Fig. 7. (Color online) Vertical profiles of the lidar aerosol-to-molecules backscatter ratio versus time indicating the presence ofaerosol layers in the boundary layer through the night and of acloud at �10 km in the interval 23:30–01:30 for the same night asin Fig. 6.
Fig. 9. The optical thickness in Fig. 9a is obtainedby the camera and represents the contribution ofextinction of the entire atmosphere, while Fig. 9bshows the thickness for the PBL; these are obtainedby the camera data after removal of the contribu-tion due to the cloud observed between 23:30 and01:30, directly obtained from the lidar signal.
The camera and the lidar results are used to obtainthe EBR in Fig. 10: the ratio shows values as large as160 sr in the first part of the night �20:30–22:30�decreasing to �50 sr in the last hours �02:30–04:30�.
The EBR obtained for the cloud in the interval22:30–01:30 has a maximum value of �20 sr. Thelarge values obtained in the boundary layer indicate,as expected, that the aerosol was strongly absorbingand that the amount of absorption changed drasti-cally throughout the night, while the value obtained
Fig. 11. (Color online) Standard meteorological observations ver-sus time: a, wind speed and direction; b, pressure; c, relative hu-midity; d, temperature.
Fig. 8. (Color online) Lidar integrated backscattering cross sec-tion: a, for the entire column; b, for the PBL.
Fig. 9. a, Optical thickness obtained with the camera at the ze-nith versus time; b, same as a but with the cloud thickness re-moved.
Fig. 10. (Color online) Evolution of the EBR in the PBL andthrough the cloud.
within the cloud was in a range expected for nonab-sorbing particles.
Some additional meteorological information helpsto better characterize the observations: Figs. 11a–11dshow wind speed and direction, relative humidity,temperature, and pressure measured at roof level bythe Ufficio Centrale di Ecologia Agraria observatorylocated downtown at Collegio Romano, at a distanceof �2.5 km from the University. The wind pattern istypical of the local sea breeze pattern, from the SEduring the afternoon up to 20 h and from the NNWafter midnight and up to sunrise. At 20 h, the effect ofdiurnal convection raising the mixed layer up to a fewthousand meters has long gone, an inversion at lowlevel limits the dispersion while in the late afternoonand early evening the local traffic is a conspicuoussource of heavily polluting particulate: thus theevening air is certainly loaded with aerosols rich incarbon, while the land breeze established in the earlymorning hours brings in cleaner air from the Tibervalley.
7. Conclusion
Radiometric measurements of the atmospheric ex-tinction have been carried out at night with a digitalcamera over a relatively large FOV: these measure-ments may be displayed to provide maps of the inte-grated aerosol content. These preliminary resultsprove that a digital camera can supply a simple andversatile solution especially when monochromatic in-formation is sufficient. The combination between theextinction data from the camera and backscatter datafrom a lidar allows for a determination of the night-time aerosol EBR, which is a sensitive indicator of theabsorption properties of the aerosol.
The technique provides an addition to the family ofradiometers used for the determination of atmo-spheric properties in the visible portion of the spec-trum particularly useful in filling a gap for nighttimemeasurements of the aerosol presence. As a possiblefuture work in this line, we should mention (a) thenecessity to make the system completely automaticand inside a proper housing and (b) the use of morethan one camera for the bistatic observation of laserbeams at different distances, angles, and polariza-tions, resulting in a considerable advantage for thecharacterization of the atmospheric aerosol. Also ofadvantage would be to extend the measurements tothe near-infrared windows through the use of an ap-propriate CCD camera. This region of the sky hasbeen systematically explored by the TMSS (Two Mi-cron Sky Survey) [12]. Possibly a relatively new areaof research requiring nighttime measurements of ex-tinction is related to energy calibration of cosmic ray
showers through the observations of fluorescence andCherenkov radiations. Data regarding extinction andabsorption are a necessary prerequisite in construct-ing a radiative model of the atmosphere and of thevarious classical applications to atmospheric radia-tion theory: ample reference is found in literature.
Thanks are due to F. Mangianti (UCEA, Roma)for providing meteorological data, to B. Bucciarelli(INAF, Osservatorio Astronomico di Torino), C. Rossi(Università “La Sapienza,” Rome), and R. Claudi(INAF, Osservatorio Astronomico di Padova) for as-tronomical information and advice on software devel-opment, and to M. Cacciani for lidar measurements.
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