CCD MEASUREMENT OF NIGHT SKY BRIGHTNESS

An IDA project

 

Fabio Falchi[1]

Pierantonio Cinzano[2]

 

 

 

We invite advanced amateurs astronomers to collaborate with us in a scientific project which has the aim to collect a large number of measurements of night sky brightness around the world in the main astronomical photometrical bands together with extinction data. The project is part of scientific activities of the International Dark-Sky Association – Italian section and is already started in this country.

Many studies of light pollution and artificial sky brightness require large quantities of measurements of sky brightness that, in order to be useful, need to be associated to the knowledge of the atmospheric conditions during the measurements. The vertical extinction is one of the parameters most easy to measure which allow to evaluate the aerosol content of the atmosphere.

We propose to advanced amateur which can have at their disposal a CCD device mounted on a small telescope and one, or more, standard U, B, V, R filters to participate to our group of study making accurate measurements of sky brightness together with extinction in a large number of nights and, if their instruments are transportable, in many different sites. This will allow to obtain for each site the relation between atmospheric conditions and sky brightness (see an example in fig.1). Measurements require registration of date, time, sky position in both celestial and altazimuthal coordinates. Could be useful if measurements will not be limited to zenith but will cover the entire hemisphere with one of the common sampling schemes.

Instead of the small telescope it is possible to use a wide-angle photographic lens (with, of course, the photometric filter between it and the CCD). This configuration will allow to obtain a larger field of view (10° or more) to have a more complete coverage of the night sky. The use of a wide angle lens permits to neglect the rotation of the Earth and to use a simple photographic tripod as a mount. All the operation of identification of the photometric stars can be done later on the images, provided that one marks for each exposure the approximate altitude and azimuth of the centre.

We plan to publish results in a professional journal with a paper at which all active observers will take part as authors, if  resulting measurements will have an adequate level and will be in a sufficiently large number.

 

For information please contact Fabio Falchi at  falchi@lightpollution.it

Look also at Light pollution in Italy Web Site - http://www.pd.astro.it/cinzano/

 

 

Operating technique:

 

Using a CCD, a small telescope and standard photometric filters you may follow this procedure to obtain the night sky brightness:

 

·         It is not necessary that the night be particularly clear, in fact we need several brightness measures taken in different transparency condition. The only request is that condition be constant during the measures and without clouds, veils and haze. The meteorological condition should remain constant and the Moon should be well under the horizon (h< -10°) during the measures.

 

·         Possibly avoid sites with lighting installations at distance under about 500 m. Inside cities a unlighted park could be a good choice.

 

·         Choose one or more sky zones, always including the zenith, where to measure the brightness. For example: the zenith, 8 zones at 45° altitude equally spaced in azimuth along the horizon and 12 zones at 20° altitude every 30° in azimuth.

 

·         Choose several  (a dozen, if possible) photometric standard stars (e.g. Johnson, H.L., 1963, in Basic Astronomical Data, ed. K. A. Strand, Univ. Of Chicago Press, p.204; or, if you need fainter stars: Landolt, A.U., 1992, The Astronomical Journal, 78, 959; on line at: http://www.cfht.hawaii.edu/ObsInfo/Standards/Landolt/ and at http://herbie.ucolick.org/techdocs/standards/Stds_Landolt_0to4.html ; you may use the Hipparcos Catalog, often included in planetarium software packages; in table 2 you can find a list of bright standard stars) having different altitude (from near the zenith to below 30° altitude). Particularly useful are the stars above 60° and those below 30°. Possibly choose a star near every zone where the brightness will be measured.

 

·         Take exposures of standard stars and determine their altitudes (e.g. using a planetarium software later). Take exposures in the chosen sky zones. If the time used to make the exposures is so long to suspect a change in sky  transparency you should measure again the standard stars count in order to determine a second extinction coefficient. The coefficient to use should be the average of the two.

Take care to record the observing site latitude, longitude and altitude above sea level as accurately as possible (an accuracy better than 15» would be appreciated), the date, the duration and time of each exposure (check if the CCD camera control software does it automatically), alt-azimuth and equatorial coordinates of the measured sky zones and stars (you could do this later using a planetarium software and knowing exactly the sky zone or the star exposed and the time). Keep a note on the meteorological conditions.

In detail:

a)             choose the photometric band(s) to use and mount the appropriate filter(s).

b)            Get reference flat frames and the dark frames for the exposure needed on the stars.

c)             Expose the standard stars. Pay attention NOT to saturate any pixel.

d)            Get reference dark frames for the exposure needed on the sky.

e)             Expose the sky zones. Do not include brilliant stars.

 

·         Reduce the images following the standard procedure:

a)             from every raw image subtract the dark frame taken with the same exposure time (the bias frame is assumed to be included in the dark frame). From the flat frame (It should be the average of several exposures) subtract the appropriate dark. Divide the dark-subtracted raw image by the flat normalised to the mean value (usually this normalisation is included automatically in the standard action of 'divide by flat' in the CCD control software)

b)            On the star exposures measure the total counts of the star in an area surely covering the entire star image (often the outer portion of a star image seems black even if it contain counts). Subtract the sky counts on an equal area of the same image where there is only the background sky. Scale the counts to the time of one second.

c)             In the sky images measure the total counts of the pixels in an area where no star are. Calculate the total sky area in arcsec² covered by these pixels. Scale the total sky counts in that area to that in a square arc second. Scale this number to one second exposure.

·         Get the instrument photometric scale factor and extinction coefficient in this way (for every band studied):

a)             for every standard star calculate the air masses: x=1/cos z [for very low altitudes (z>70°) this formula is more accurate: x=sec z (1 - 0.0012tan² z)] where z is the zenith distance and the variable y=m_cat+2.5log I_star, where m_cat is the magnitude of the star and I_star the counts in a second.

b)            Graph y over x and obtain the best fit line y = a + bx . It is better to compute not only a and b, but their errors too (e.g.fig.2).

c)             The photometric scale factor is a and the extinction coefficient is k = - b

d)            The star apparent magnitude «under the atmosphere» is m_app = m_cat + k/cos z = C –2.5log I_star where k is the extinction coefficient in magnitude per air mass and C is the photometric scale factor.  From this formula we obtain that C – k/cos z = m_app –2.5log I_star, so that y = C – kx and C = a and k = - b

 

·         Compute the brightness of the measured sky zones from: m_sky = C – 2.5logI_sky, where I_sky is the sky count in a second in a square arc second.

 

·         Every reported measure should include all the necessary information, such as: the telescope aperture and CCD used, geographical position (latitude, longitude and altitude) of observing site, date and time, extinction coefficient in each band measured, alt-azimuth and equatorial coordinates of the measured sky zones and their brightness in each band measured. Please, include also an estimation of the magnitude of the fainter stars visible in the sky images and the size of the area used for the brightness measures. Specify the snow covering of the site and the surroundings. If  available, give the solar cycle phase too.

 

·         A useful and free software to reduce photometric data is Christian Buil's IRIS. You can download it from his homepage at: http://www.astrosurf.com/buil/

 

 

 

 

Star	a (2000)	d (2000)	Spectral type	Magnitude B	Magnitude V
b Lib t Her a Ari a Ser g Peg d Cas b Ari g Ori bTau g Gem e Ori a Leo b Leo g Uma d Uma h Boo g Ser a Oph b Oph g Lyr a Aql b Aql a Del a Peg	15h 17m 0s 16h 19m 41.8s 19h 30m 43s 15h 44m 16.0s 00h 13m 14.1s 01h 25m 48.9s 01h 54m 38.4s 05h 25m  7.8s 05h 26m 17.4s 06h 37m 42.7s 05h 36m 12.6s 10h 08m 22.3s 11h 49m  3.5s 11h 53m 49.9s 12h 15m 25.4s 13h 54m 38.3s 15h 56m 27.1s 17h 34m 56.1s 17h 43m 28.3s 18h 58m 56.5s 19h 50m 46.9s 19h 55m 18.7s 20h 39m 38.3s 23h 04m 45.6s	- 09° 22’ 58” +46° 18' 45" +27° 57' 34" +06° 25' 32" +15° 11' 01" +60° 14' 08" +20° 48' 28" +06° 20' 58" +28° 36' 26" +16° 23' 56" - 01° 12' 06" +11° 58' 01" +14° 34' 18" +53° 41' 42" +57° 01' 57" +18° 24' 01" +15° 39' 41" +12° 33' 36" +04° 34' 02" +32° 41' 23" +08° 52' 06" +06° 24' 24" +15° 54' 43" +15° 12' 18"	B8 V B5IV K2 III K2 III B2 IV A5 V A5 V B2 III B7 III A0 IV B0 Ia B7 V A3 V A0 V A3 V G0 IV F6 V A5 III K2 III B9 III A7 IV-V G8 IV B9 V B9 V	2.50 3.74 3.15 3.82 2.60 2.81 2.78 1.41 1.52 1.93 1.51 1.25 2.23 2.44 3.39 3.27 4.33 2.23 3.93 3.20 0.99 4.57 3.71 2.44	2.61 3.89 2.00 2.65 2.83 2.68 2.65 1.64 1.65 1.93 1.70 1.36 2.14 2.44 3.31 2.69 3.85 2.08 2.77 3.25 0.77 3.71 3.77 2.49

 

Table 2. A list of bright standard stars.

 

 

Fig. 1. Example of  the dependence of the sky brightness with the transparency of the atmosphere in a site. In other sites very different dependence are to be expected.

 

 

Fig.2. In this example the extinction coefficient k =0.385 in magnitude per air mass and the photometric scale factor C = 16.19 magnitudes.