A visitor to the site recently posted a question, asking what system I use to measure seeing and transparency. Currently, I use the Pickering Seeing Scale to describe deep-sky seeing conditions and use a Sky Quality Meter to measure sky brightness as a gauge of transparency.
The Pickering Scale
To describe seeing with the Pickering scale, I line my scope up on a bright star and insert a high-magnification eyepiece that gives around 200X or greater. I observe the appearance of the diffraction pattern around the star and compare that to the visuals and descriptions on this excellent page hosted by Damian Peach: The Pickering Seeing Scale.
Naked Eye Limiting Magnitude
For transparency, I used to examine Ursa Minor to determine the faintest star I could see with the naked eye and recorded that as the Naked Eye Limiting Magnitude (NELM). You can find a magnitude reference chart for Ursa Minor (and the Hyades) here: Weasner’s Meade ETX Site: Seeing Conditions and Transparency. This does take some time out of an observing session to make the estimate. Also, the perceived NELM value at the same observing site would easily vary from observer to observer depending on each person’s naked eye astigmatism, myopia, experience, and plain doggedness in going after the faintest possible star. So as a baseline, it only applies consistently to a single observer’s personal experience at a site.
Bortle Dark Sky Scale
Some observers use the Bortle Dark Sky Scale to rate the sky quality from 1 to 9. I’ve had difficulty applying it to my observing circumstances. For example, at pristine observing sites in Northern Arizona where the Zodiacal Band and Gegenschein are visible, the Bortle scale says that surroundings should be basically invisible. However, in my experience when dark adapted, the Milky Way, airglow and overall starlight are sufficient to illuminate my surroundings and make them visible—I can walk around and navigate the observing area without much difficulty and distinguish terrain albedo on more distant surroundings. The Bortle Scale also uses light domes as an indicator of sky quality; however, at high altitude in dry climates, the extent and affect of light domes near population centers is not as significant as it is for observers at lower altitudes or under more humid conditions. So, that’s a long way of saying I don’t use the Bortle Scale.
Sky Quality Meter
A couple years ago, I invested in a Sky Quality Meter to measure sky brightness in magnitude per square arcsecond. It’s a bit of an investment, but it provides a quick, consistent way to measure the observing site, and it removes a lot of subjectivity. So you can compare with other observers regardless of who has better night vision or perhaps a different interpretation of the Bortle Scale. The main limitation is it measures light, but can’t measure extinction. So in an extreme example, if you were at a pristine site with overcast skies, it would measure darker values than if the sky were clear and the Milky Way and skyglow were blazing overhead. So it still requires some caveats. But that’s what I use these days and how I describe things in my more recent observations.