RE: Light pollution monitoring using all sky camera

[Jan Hollan <jhollan@amper....muni.cz>]

> > - to get Milky way really impressive and at the same time to get light
> > pollution very impressive. This cannot be done do by using 100 ASA or some
> > additional additional processing would be necessary.

Andrej,

I was wrong saying that 1000 ASA brings nothing new over 100 ASA. As I
read the report on 5D by Christian Buil now, standard deviation of
darkframe values does not rise with sensitivity appreciably, unlike with
another cameras (I don't know why).  He recommends ISO 1100 to get maximum
signal/noise for faint light and may be true. So, for faint stars and sky
luminance, 1000 is a good choice for this camera.

Of course, earlier saturation of bright stars, of distant lamps etc.
holds for large ISO. A more detailed series of shorter exposures might be
then needed, using a factor of, say, 8 instead of 30, to cover all
luminance range, from 0.2 mcd/m2 (or even less, for ground) to tens of
cd/m2 (these may need low ISO anyway). But this concerns terrestrial
photometry of man-made light, not that of celestial lights, apart from
Venus, Moon and Sun.

My recommendation for darkframes and raw imaging holds. But the jpegs can
be really made from raw images, taken directly with ISO 1000. Good news,
for EOS 5D. No need to image twice, without and with raw setting. Once is
enough, for all purposes.

A good reason to use raw format is vignetting compensation: zenith images
are much vignetted beyond 80 degrees, this reduces apparent luminance of
the sky there. Compensating for that (with IRIS probably, but I might
improve my software too, I'm not sure if it does it now for jpegs) shows
the real pollution, which can be almost twice larger than that apparent at
firmware jpegs made with fish eye aimed at zenith.

Christian has nice August views from Pic du Midi, by the way, even with
zodiacal light (and with Uranus and Neptune in the last all sky image!
try to find them...) on his site
   Linkname: Spectroscopy, CCD and Astronomy
        URL: http://astrosurf.com/buil/

cheers, jenik