minaya:
@GoForSmoke, each star contributes to the 'total' flux on the sky with the same amount of photons no matter how the sky is polluted. In fact, in urban observatories it's very possible to do some limited photometry with objects that are bright enough. You just need to 'subtract' the background flux to the total (background object) flux and you get the net object flux.The problem with light pollution is the 'photonic' noise it adds. This photonic noise, that scales with the root square of the flux (in photoelectrons) tends to make the observation of a particular object in the sky just impossible if it is dimm. This idea applies to digital astronomical images, but with your eyes the idea is somewhat similar.
@Corpze, the zero point needs to be measured for each individual photometric system, no matter if it's a telescope ccd, a camera with an objective lens or your little TSL photometers with IR-cut filter and lens. One way (maybe the recommended one) to measure it is using calibrated lamps and calibrated photodiodes as reference. But in the end, you can approximately calibrate it if you know someone who has a SQM or you have a digital camera capable to take fixed exposures images (you calibrate the camera filter lens system with absolute photometry as usual in astronomy, then determine the Night Sky Brightness at some point in the sky with your already calibrated camera, take some measures with the TSL pointing at that direction and derive an approximate Zero Point to match both measures).
As I said, this ZP calibration is (repeat after me) hard if you want to make your instrument scientific-grade. Depends of course in your particular interest, but sometimes people (even in scientific projects) just use them 'differentially'. If you just need to measure how the sky at some location compares to the sky at other locations (or how it changes over the time), you can even use the direct 'frequency' as your variable, no need to mess around on these strange unit systems hehe.
The frequency measured with these photodiodes increases linearly with the number of photons.
Thanks for your very good answer, i will use the meter for my own, to measure different sites where i do my astrophotographing.
For the ZP i have a luxmeter that i can use, it will not be scientific grade, but good enough for me!
It´s just one thing left, does anyone know what kind of "lensmodule" i have marked on the picture?
