How Dark is the Night Sky ?

How dark is the night sky? To this question we tried to give an “exact” answer by trying to measure the brightness of the night sky using a sensor based on a photomultiplier. Of course it is mandatory, if you want to get significant results, position yourself in places not reached by artificial lighting and take measurements in moonless nights.

The use of a pointing device will allow us to measure at different points of the sky in order to evaluate the brightness of the area covered by the milky way and to highlight the phenomena of zodiacal light in a qualitative / quantitative manner.


The apparatus is mainly composed of a Hamamatsu R647 photomultiplier with an anode connected to ground and a cathode connected to a negative high voltage. The connection diagram with the voltage divider and the coupling capacitor is described in the following post: Photon Counting. High voltage is obtained with the HV generator Matsusada J4-3N-LX. The normal working voltage of the PMT is 1000V. The image below shows the PMT and the generator.

The PMT is placed in a light-tight plastic tube about 1m long. The PMT is held in place by a layer of foam rubber. The PMT connector, with the dynode voltage divider, protrudes beyond the foam to be connected to the SHV (for high voltage) and BNC (for the signal) connectors. The images below show these connections.

The tube with the PMT was placed on a camera tripod, together with the HV generator, so that it could be aimed at different points of the sky. The image below shows the complete apparatus.

To evaluate the brightness measurements it is necessary to know the solid angle subtended by the photocathode of the PMT, the geometrical scheme shown below shows the position of the PMT inside the pipe, where d = 850 mm, the pipe diameter is 63 mm and therefore the surface of the tube opening is 3117 mm2.

The solid angle is calculated : Ω = A / d2 = 0,0043 sr


The PMT has been DC coupled with the “universal” PMT amplifier from Micod described in the post : Micod PMT Universal Amplifier. The light intensity levels are very low, but not so low that you can use the PMT in “photon counting” mode. The photocurrent produced by the PMT is sent to the CSP to be amplified and produce an output signal proportional to the intensity of the light reaching the PMT photocathode. Actually the output signal is produced by the sum of all the single pulses that overlap each other.
The image below shows, as an example, the value obtained (348 mV) pointing the PMT towards the night sky inside a built-up area.



The apparatus we made is rather rudimentary. However, it allowed us to evaluate quantitatively the different brightness of the night sky measured in the vicinity of a city from that in a mountain location relatively free of light pollution. The difference is considerable, going from a value of 350mV to a value of 100mV (value measured at the amplifier output).
In conditions of low light pollution it is possible to clearly appreciate the increase in brightness obtained by pointing the instrument towards the Milky Way.
The table below summarizes the values ​​obtained during some measurement sessions.

Type of Measure Value (mV)
Background 45
Dark Sky near a City 350
Dark Sky without light pollution (on Mountain) 100
Dark Sky without light pollution – Milky Way 150

To make measurements with greater precision it is necessary to equip the instrument with a suitable mount (equatorial or altazimuth) and make measurements at different coordinates in order to evaluate the brightness in different points of the sky.
In optimal conditions it would also be possible to quantitatively assess the brightness due to the zodiacal light.

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