How do professional astronomers measure and describe (ambient) observation conditions?

I know smog, light contamination from the city lights, fog, clouds, rain/snow, humidity etc all contribute negatively to quality of the image, obscuring the celestial objects more or less, but I'm fairly sure there is a more precise measurements of viewing capability for any given location or time than "good weather and far from cities" or "Kinda can see M42 in Orion if you squint."


I'm not an observer, but there are a couple of quantities I know:

  1. seeing: is a measure of atmospheric turbulence and airmass. Its units are arcseconds and is the measured size of a point source, typically a star, as it appears in the image. In short: atmospheric turbulences shift slightly the position of the star, so if you observed it for long enought, the star will appear blurred. Good places for telescopes have seeing of few arcsecond; Excellent ones reach about 0.6 arcsec. This link can give some more info
  2. fog, cloud, rain, snow: you don't even bother opening the telescope domes as you won't be able to observe much
  3. humidity: I think that part enter in the seeing. If you do just imaging what you get is some extra blurring of the image. If you do spectroscopy you will see a lot of atmospheric lines (due atoms and molecules, e.g. water, in the atmosphere) that you want to remove from the measured spectrum.
  • 1
    $\begingroup$ Perhaps add Bortle scale measurements for a quantification of how dark it is? $\endgroup$
    – Jeremy
    Sep 25 '13 at 11:48
  • $\begingroup$ Feel free you add your answer or edit mine. The best I can do is look up in the internet and recap what I've understood. $\endgroup$ Sep 25 '13 at 12:24
  • $\begingroup$ @Jeremy The Bortle scale is not used by professional astronomers. We use the sky brightness in mag per square arcsec. $\endgroup$
    – ProfRob
    Aug 16 '16 at 12:47

I think about 3 factors that impact the quality of observing and astrophotography.

  1. Seeing - as mentioned before, the turbulence in the atmosphere and ultimately, determines the resolution you can image or resolve by eye. I would kill for 1 arc-second seeing at my site :-(
  2. transparency - humidity in the air, dust, fog clouds - all these things soak up photons as they travel across the universe and before they hit your eyeball. Note that at some sites, poor transparency due to humidity, etc. might produce very good seeing because the atmosphere isn't mixing very much and is stable. Similarly, if you observe nearer to the horizon vs. overhead near the zenith, you look through more atmosphere which makes your transparency and seeing problems worse.
  3. light pollution - from nearby (and maybe not so near..) lights emitting light up into the air. As it scatter of dust, water, vapor, etc, it makes it difficult to see dim objects. Some interaction with transparency - if there's a lot of water vapor in the air, it also scatters more light back into your instrument.

At my observatory site, I had so-so seeing, but being out in the sticks, not much of a light pollution problem. All these things are trade-offs..


Site quality is typically measured and stated in the following way.

  1. Fraction of photometric nights (really no clouds/cirrus) per year and/or the fraction of night where spectroscopic observations can be done (some clouds, thin cirrus). The best sites are up at the 60% and 80% level for these. You would also include time lost due to high winds

  2. The median astronomical seeing and its distribution from night to night - the full width half maximum of astronomical images. At the best sites this is around 0.5 arcsec on the best nights.

  3. Median extinction. The amount the atmosphere absorbs for a star at zenith. Typically quoted in the V-band and just below 0.1 mag at the best sites.

  4. Brightness of the dark, moonless sky in mag per square arcsecond. Often quoted in the V or B bands. A value of 21.5-22 mag/sqarcsec is a good site.

There are some other numbers to do with adaptive optics correction or observing in the infrared that I might add later, but the above are the main ones.


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