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Topic: Measure nightsky with TSL237 (Read 12 times) previous topic - next topic

minaya

#10
Feb 26, 2013, 10:36 pm Last Edit: Feb 26, 2013, 10:47 pm by minaya Reason: 1
@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.

Corpze


@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?

GoForSmoke


@GoForSmoke, each star contributes to the 'total' flux on the sky with the same amount of photons no matter how the sky is polluted.


There must be something you say that I don't understand.

Parts of the sky, especially at low alt, do get a lot of particles/dust from natural and man-made sources. At night here we have electric generation stations do their stack cleaning (evidenced by the crap laid down for miles with lead, cadmium and mercury as well as sulfur compounds). For well over 10 years now the pall running from India clear across China has cut sunlight by 15% and more, reducing crop yields. Even CLOUDS reflect light back to space... water and dust pollution!

Somehow I don't think that all the photons from Sirius make it to Earth surface. Flying over Earth it's possible to see how much dust and humidity in the lower atmosphere blur details compared to up in the mountains from the same AGL. And up in the mountains where the air is thinner and doesn't have such a load of junk (first time I really understood was on Pike's Peak) objects miles away look very clear where at sea level they're blurred in less than 2 miles.

When a big volcano pops (Krakatoa around 1880) the whole planet cools just from the dust.

Does starlight get through all that with the same photon count? Sunlight doesn't and the Sun is a  star.

I've had the chance to see the night sky from 6500 ft (the saddle between Mauna Loa and Mauna Kea) and 8000-9000 (central north New Mexico) up away from any cities before and wow, the sky doesn't just look more clear but far brighter.

I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

Corpze

#13
Feb 27, 2013, 02:59 am Last Edit: Feb 27, 2013, 04:30 am by Corpze Reason: 1


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?



Oh, and one more question; The code i am gonna start out with is this: http://arduino.cc/forum/index.php/topic,21536.msg160200.html#msg160200

But will that work? it is written for a Arduino duemilanove?

minaya

@GoForSmoke.

I didn't mention the important 'extinction' effect (I was just talking about the 'light pollution' effect only, the photons added by human activities). Of course, dust particles extincts and scatters the radiation of objects that are outside the atmosphere, but normally the human effect here isn't as important as one thought (maybe in heavy polluted cities). The main effect of those particles is to reflect the scattered light that comes from cities to the outside.

In high altitude mountains like Mauna Kea, Atacama, Roque de los Muchachos aerosols (these atmospheric particles you mention) abundance is relatively low, and thus is the effect on the night sky.

But of course, if the sky is foggy/cloudy o very polluted (like in Pekin or even at desert with sand storms), you won't see anything. ;).

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