(last updated 12 March 2000)


The quality of one's skies are typically expressed with two factors. One is sky transparency, which is a measure of sky haze, whether that haze is due to clouds or light pollution. A good way to measure it reasonably objectively is to give what is known as the zenith limiting magnitude, or ZLM for short. This is the magnitude of the dimmest star that you can make out by the unaided eye, using whatever techniques at your disposal (e.g., averted vision, dark adaptation, etc.). Sometimes, for observations concentrated in an area of the sky away from the zenith, you may also give the limiting magnitude in that area.

The other factor is atmospheric turbulence, commonly called seeing. Seeing is what makes many objects, particularly the planets, look as if you were observing them from the bottom of a swimming pool. In contrast to transparency, there are many different ways of expressing seeing, none of which have gained full acceptance. There is an Antoniadi scale from I to V, with somewhat subjective descriptions. There is a Pickering scale which focuses on the condition of the Airy disc and diffraction rings of an in-focus star. There is a way to measure it in arcseconds, but what it is precisely that you're measuring in arcseconds is somewhat vague—sometimes it's the fluctuations in the image, sometimes it's the minimum detail resolution you can make out. Clyde Tombaugh devised a scale which translated arcseconds of seeing into a 0–10 scale. Finally, there are subjective 0–10 scales which give some idea of the relative change from night to night but yield next to no information about the actual quality to someone who doesn't live in the area. (Despite that, I used such a personal scale until just recently.)

My First Attempt: The New Scale

My first attempt at an objective scale for describing the seeing was a combination of the size of the image movement in arcseconds plus the frequency of the movement in oscillations per second. I felt this was necessary because slow seeing movements might harm detail hardly at all, while fast seeing movements would destroy the same detail. A typical expression might be M3F1, which would indicate movement of 3 arcseconds in magnitude (amplitude) about once a second. Some of my log entries from January/February 2000 use this rating format. It was also possible for there to be more than one seeing movement, superimposed on top of each other. I posted my thoughts to sci.astro.amateur and asked for feedback.

The Bubl Scale

Brian Skiff was kind enough to respond with a list of double stars to use for determining the amplitude of the movement; if you could resolve the double, then the fast seeing could be no worse than the separation between the two stars. He also gave me a crucial insight—that there are really only two speeds of seeing to be worried about: seeing slow enough for you to see the movement, which he called bulk seeing; and seeing too fast for that, which he called blur seeing. In other words, one might have many different modes of bulk seeing, but the overall effect would be qualitatively no different from just one mode. The blur, as before, could be treated separately.

With that in mind, here's my second attempt: the Bubl scale. I'm calling it that because it's a combination of Bulk and blur seeing. So if the bulk movement is 2 arcseconds, and the blur is 1 arcsecond, I would write that simply as Bubl 2/1. One thing is that the rating depends on the aperture of the scope—generally speaking, as the scope gets larger, more of the bulk seeing becomes blur seeing. Out of my charming egoism, I'll assume a 5-inch SCT and only indicate the aperture if it's different. Let's try that for a while and see how it goes…

Copyright (c) 1998, 1999, 2000 Brian Tung