[SQM] Updated version of the community standard for skyglow measurements

Christopher Kyba christopher.kyba at wew.fu-berlin.de
Mon Jun 11 09:47:27 UTC 2012

Hi Stefan,

Thanks for your comments,

> The proposed file format looks promising. I just have two comments:
> - A field of view is usually two-dimensional. If there is only a single
> number, it has to be a characteristic angle like a diameter. What would you
> take if you had a rectangular field of view? You could improve the
> documentation here.

Most of the current lightmeters just look at zenith, so their field of
view can be defined by a single number.  But you're right, someone
could install a camera that looks at the sky and then integrates the
photos to get a skyglow measurement.  I've added the following to the
field of view section:

Some devices may have a two dimensional field of view (e.g. cameras).
In this case the two angles should be specified separated by a comma,
and the user should describe the orientation of the device in the new
header entries specific to the new device. Note that this format is
only intended for devices with a small amount of data. (In the case of
a camera's pixels being summed, it would be possible to avoid this
problem by only selecting the pixels enclosed up to a certain zenith

> - I can understand if the primary data units are "counts". However, it would
> be very helpful if less instrument-dependent brightness units could also be
> provided like mag/arcsec^2 in example_sqm_file.txt. This is important for
> users who aren't familiar with the instrument or don't want to perform the
> data reduction/unit conversion. Providing data also in physical units makes
> the files more valuable. I would be pleased if this was a recommendation in
> the documentation.

I understand your concern.  At the CLIC workshop we discussed this,
and the major problem with doing this is that if the conversion
depends upon some calibration factor and that factor changes or is
unknown at the data taking time, then someone would have to reprocess
their "raw" data.  The consensus view that emerged at the workshop was
that the raw data should never be re-processed, and if you convert
lightmeter "counts" to "lux" you should store the data in a new
"level1" file, which would have a different format.

The idea behind standardization is to make it possible to create a
database of lightmeter measurements world wide.  When the database is
complete then everyone would be able to download processed data
reported in standard units rather than instrumental specific units.  I
also hope that by this time next year we will have defined a natural
night sky brightness unit, and then data from different devices (and
different spectral ranges) will be much easier to compare.

> I will endorse the final version.

Great!  I hope that we will have a final version very shortly.  I will
release version 0.1.3 today for the Starlight conference attendees to
look at.

Best wishes,

Christopher Kyba
Institute for Space Sciences
Freie Universität Berlin

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