Quote:
Originally Posted by ausastronomer
Well I sort of agree with that, but not quite.
If you're at a true dark site with no background lights, or any light domes, pointing it at the zenith shouldn't matter, if the Milky Way is not a factor.
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Yes of course, with the added caveat that the sky is clear of cloud which can diffusely reflect any moonlight and that there's not too much moon (better none if you seek a true background measurement).
The "requirement" to measure at the zenith really depends on what you want the SQM data for. Possibly:
1. If you want it to compare different sites (dark and otherwise) across the world, Australia whatever, using resources such as
lightpollutionmapinfo, then it's important to measure at the zenith, since that is how all the data is presented there (pic below). Of course not measuring at the zenith will effect PERFECT dark sites less than urban or suburban locations readings.
2. If you want it to help with imaging to for instance achieve a similar signal/noise ratio in comparison with another location then one might even aim the SQM at the target sky area.
3. If you want it to measure local changes at a fixed location (say home) then provided you always point it appropriately away from light sources and possibly the same area then sure.
The zenith also provides an easy reference point for most and is often the darkest part of the sky, especially in light polluted areas.
Best
JA