[SQM] Directional Accuracy of SQM-L

Anthony Tekatch anthony at unihedron.com
Thu May 14 15:39:57 UTC 2009

Hello Tony,

The sensor and lens are not precisely registered to the face plane, so
variations in bright-spot sensitivity location between units will be
seen. Your calculation of up to 8 degrees off the box plane seems quite
possible. We currently do not have a spec on optical axis variations.

For full-sky accurate point-mapping to less than about 10 degree
increments, possibly a different method could be employed:
1. Test the bright-spot angular-displacement for the associated meter
   with a point source lamp, protractor, and tripod. An SQM-L or SQM-LE
   would be more appropriate for applications with a high sensitivity to
   pointing because of their more narrow field of view over the SQM.
2. Another method would be to use a fish-eye lens on a camera with long
   exposure shots. This should provide a good overview of the sky.

To ensure repeatability for full-sky tests down to the horizon; the meter
should be mounted on a tripod with a plumb-bob ensuring precise matching
of zenith positioning, and always face a certain direction (like North).

The SQM meters were designed for generally measuring sky brightness.
Surrounding light or shadows will affect the reading. Taking measurements
in urban areas with street lights and buildings close by, or readings off
zenith will make the angular displacement issue more pronounced.

Did you also notice the same issue of reading variations when pointing at
the zenith on a clear night and rotating the meter?

Best regards,
Anthony Tekatch

More information about the sqm mailing list