RE: Light pollution monitoring using all sky camera[Scanned]

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

> We will also try to take raw images in the future. I had problems how to
> convert raw to jpg. I used Capture One demo software which cost 1500+
> EUR. I still don't know for more simple solutions for non-experts.
> Professionals say that by using raw it is possible to get much more out
> of any picture. From wedding pictures to darky sky.

Dear Andrej,

any modern image processing software accepts raw formats as an input
(relying on dcraw by Dave Coffin). Gimp, which is free (GNU license), for
example. It shows histograms, gamma curves (you can create any curve for
12bit to 8bit conversion) and offers further options for the 8-bit output
for monitors. Adobe Photoshop does it surely too, but it's not free -- and
Gimp works on all platforms as well.  Perhaps even some new image viewers
can compute jpegs from raw (or at least extract the embedded jpegs of
smaller size). My recommendation is, install Gimp and try it.

IRIS is probably the best tools for sky images, using darkframes etc. See
again http://astrosurf.com/buil/ for the link. IRIS can do even imaging
for you, it controls EOS cameras via USB. But, it's not for linux... so
not easy for me to use.

Myself, I use only dcraw for getting images from raw formats quickly.
http://www.cybercom.net/~dcoffin/dcraw/ is the page on this programme,
windows executables are available at

   Linkname: Bennis und Karins Welt >> dcraw
        URL: http://www.lebsanft.org/blog/index.php?cat=9

A couple of commands I use is:

  dcraw -e *.crw
 extracts the embedded jpeg from all *.crw very quickly. For web, it's
mostly enough -- horizontal size is 2048 for D60 instead of the full 3088.

  dcraw -i -v
 says the exposure settings, but "exiftool" says more (and reports focal
length). I'm still using "jhead" applied at *.thm (small jpegs stored
separately by EOS D60).

  dcraw -v *.crw
 computes *.ppm from all images, these are easy to convert to jpegs even
by image viewers.

To make all large jpegs at once (and discard *.ppm) I write

  for F in *.crw ; do dcraw -v $F ; convert ${F/crw/ppm} ${F/crw/jpg} ; rm ${F/crw/ppm}; done

 as a bash command. It's surely possible to store analogous msdos command
as a *.bat and use it after that (I know nothing on batch processing in
windoze).

For sky images, the automatic scaling by dcraw (percentil 99 is considered
as white) is unrealistic, using -b 0.2 or so helps, as "man dcraw" says,
if installed:
    -b brightness
          By  default,  dcraw writes 8-bit PGM/PPM/PAM with a BT.709 gamma
          curve and a 99th-percentile white point.  If the result is too
          light or too dark, -b lets you adjust it.  Default is 1.0.


> It is incredible how much light pollution is visible on all sky photo at Pic
> du Midi. This location is one of the darkest in Europe. They get light
> pollution from Barcelona which is 200+ km away. And don't forget - Pic is
> 2800 m above see level!

I'll write to the author to put raw images somewhere, so that I could
compute log-coded luminance images from them.

cheers,
 jenik

PS. I'm putting this message into a public archive as well, so that
anybody can be said an URL or gugle it out:
  http://amper.ped.muni.cz/jenik/letters/radiometry/