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NightMonDocNightMon - Allsky night monitor documentationCreative Commons Attribution-ShareAlike 4.0 International License IntroductionNightMon is designed to monitor the night time brightness over the upward hemisphere (astro model) or over the complete sphere (eco model). It uses a raspberry pi 4b and a HQ raspberry cam with a fisheye lens. There is also a RTC chip to keep the time in case of powering off the system. In output, NightMon produce 6 calibrated maps of the sky brightness in the R, V and B Johnson astronomical bands with and without the stars. When a list of azimuth and altitude angles is provided in the points_list file, the NightMon will extract the night brightness at the given angles from the starless images. Optical considerationsWe are using the Arducam 180 Degree Fisheye 1/2.3" M12 Lens with Lens Adapter for Raspberry Pi High Quality Camera (SKU: LN031). You can get that lens from uctronics.com . We keep the IR filter on the lens along with the one installed on the camera body. Spectral sensitivityThe HQ raspberry pi camera delivers RGB images using a Bayer matrix. In the analysis of the images, we combine 4 subpixels (R G G B) of the Bayer matrix to create a panchromatic pixel. But the combination depend on the spectral band to monitor. We searched the best correlation of a star field with the SIMBAD calibrated star catalogue by varying the weight of the linear combination of R, G And B bands. This way we can roughly simulate the Johnson R V and B bands out of the R G B bands. CalibrationThere are two calibration methods allowed with the NightMon (stars or fixed). The stars calibration analyse the starfield to identify tenth to hundreds of stars and search for the best correlation between the star integrated digital number and the calibrated star magnitude given by the SIMBAD star catalogue. We are doing that for the 3 Johnson bands R V and B. To be accomplished correctly, the star calibration method needs the Polaris star to be near the expected pixel position in the image. So the user of the NightMon needs to align and level the NightMon to achieve that. The process rely on the shift of the image to position Polaris at the correct position and then rotate the image if necessary. The correct X and Y shifts and the rotation angle is determined with the FindShiftRotate.py script. Once these values determined, they have to be writen in the nightmon_config file. The second method for calibration is the fixed method. It is the one recommended. It uses predetermined zero points values to convert the digital numbers into sky brightness. The zero points were determined with the star calibration method on a clear and stable night with atmospheric aerosol properties being stable for 2 days (the day preceeding the observing night and the day after). With stable values for the aerosol optical depth and angstrom exponents, we were able to retreive the atmospheric corrected digital numbers and then correlate them with the SIMBAD stars Johnson magnitudes. The zeropoints are provided by default in the nightmon_config file. 3D printed enclosure  You can download the STL files to proceed with their 3D pring. |