ASTMON - SkyBrightness Monitor

The company iTec Astronomica S.L. has developed an AllSky transmission monitor (hereafter ASTMON, see more pictures).  The main purpose of this instrument is to characterize and measure the light pollution of the night skies (See photos). It comprises of a detector CCD (3500x2500 pixels and 5.5 microns pitch), a filter wheel with 5 available positions and a fisheye that provides a full circular field of view 180 degrees in any direction.

An outdoor carrier contains all the stuff inside as a laptop with the automatic acquisition and reduction software, the main electronic, three fans, four thermostats spread around, the anti-humidity devices, the CCD, the filter wheel, the fisheye objective and the acrylic dome to protect the fisheye. An special custom solar shutter covers the aperture of the fisheye during the day to protect against the sunshine. Those stations are thought to be permanent installed in the same place and can resist any weather conditions. A new portable version of the instrument is now available.

Currently, there are some ASTMON devices working around Spain. They are periodically working every night at the astronomical observatory of Calar Alto (Almeria, Spain), National Park of Doñana (Sevilla, Spain) and at the Sciences faculty of the Universidad Complutense of Madrid. Those places are in fact some references for the measurement of the light pollution in Spain.

ASTMON uses a filter set calibrated in the Johnson photometry system that is normally used in the professional astronomy. The normal size of the filters are 1 1/4 inches. Any other photometry system is possible. Due to the known brightness of the stars in the mentioned photometric system, the sky brightness is calibrated by comparing with the flux of photons coming from the stars. In this manner, a full sky brightness map is obtained for all the sky, generating a image with the sky brightness for every pixel per every CCD shot.

All the processes are completely robotic and starts during the evening when the astronomical night begins. Then, an internal program of ASTMON starts the CCD up by cooling it down to a proper working temperature about 15C below zero. When this happens, the system generates a master dark that will be used by the system during the measurements. After this, the solar shutter is opened and the measurement process starts. The dark and the flatfields frames are corrected to the acquisition image. ASTMON has an internal photometric star catalogue and can identify automatically the stars in the acquisition frames. Due to this, the stellar extinction and a cloud map are obtained.  The sky fluxes are measured and converted to stellar magnitudes for every pixel so a full circular sky brightness map is created. This information is stored in the computer hard disk in FITS (standard files in the professional astronomy) and JPEG files (useful to create movies or to display in a webpage.). Some internal thermostats and fans control an adequate internal temperature inside the carrier during all the active processes. When the end of the astronomical night comes, the program warms the CCD up, close the solar shutter, and shutdown the system waiting for the next day, when all the cycle will be repeated.

 

Other additional features are included in the program as for example to define preferred position in the sky or auto calibrate mode. With this option, the user can define special position in the sky where the light pollution has to be measured periodically and always in the same position. The program will generate a file with only the sky brightness and the universal time of the measurements for every special position. This is very useful to study the evolution (in mid-long term time scale) of the light pollution in a special location. The auto calibrate mode allows the system to re-calculate the instrumental zero point when the atmospheric conditions are good enough.