Pi HD Camera Sky Brightness Project

[nexusjeep]

So playing around with a pi5 and pi HD camera not as an allsky camera but as a metering device so what I am doing is taking a 10s 400 iso shot of the sky with a 180 Lense and then masking the sky area to discount the houses / trees and also dynamically masking the moon as well to remove it from the calculation and then calculating the average pixel brightness in the shot which is then graphed to show the sky quality.

The large red circle if the area that is being averaged with the darker line around the moon being what is masked out of that average, the mask is dynamic for the moon so will look for it and mask 15 pixels clear so will track its shape form round to crescent. The graph will run for the duration of time that the camera is deemed to be night so currently set to sunset + 1hr to sunrise - 1hr the graph is accessible until the next evening at which point it is rewritten.

This is just a project to tax the brain cells a bit and to see how well it actually functions, basically as the sky gets better the value gets lower. Still need a really good clear night to ascertain reference points as well nights with moon and without moon to see how things stack up. So far in testing it looks like once it starts dropping to 0.04 and below the sky is relatively clear and perfectly clear looks to be around 0.033 but will update this thread as things progress in case anyone is interested.

SKy_Moon_Mask.png (646.37 KiB) Viewed 79 times

Cheers
Nick

[admin]

Hi Nick,

an interesting project, particularly as I have just recently added a tool to measure sky brightness in SharpCap. I don't think that tool will work with the PI camera (even if you could get it talking to SharpCap), as it requires a high bit depth mode and sensor analysis data (along with the f-ratio of your optics). It should be fairly accurate except that the QE factor of the sensor has to be estimated, but still should give you a value within 0.2 magnitudes I expect (0.2 magnitudes corresponds to a 20% uncertainty in QE).

You could use the SharpCap tool (in 4.2 beta) with any astro camera that you have to try to calibrate your other data without needing a hardware SQM device.

cheers,

Robin

[nexusjeep]

Hi Robin,
Will give it a look longer term but still fleshing out things at the moment, now have the pi pulling the meteo API data for the cloud base speed and direction plus calculation of the seeing based on the different atmospheric layers on a 15min cycle in the day.

The plan for tonight is to split the large sky circle into 16 quadrants and 8 concentric rings sized to take account of the compression of the fisheye as it nears the horizon, each of the created matrix squares will be analysed for brightness to look for clouds so at night I can mathematically calculate how fast the clouds are coming through and in what direction they are traveling and if I set the target location being imaged should be able to calculate whether they miss it entirely or if not how long I have until they obscure the target. There will be a fallback if there is heavy cloud as the difference between the matrix squares will be minimal at which point it drops back to data from the meteo api, as well as for a completely clear sky it will do the same.

The middle chart is empty in the image as I had to reboot the Pi to unlock the SDCard as the whole system runs as a read only overlay so never writes to the SD card and does everything in RAM. The middle chart optimistically will populate tonight if not there will be swearing .

daytime_meteo.png (851.5 KiB) Viewed 59 times

Cheers
Nick