Part 2: Seeing Measurement Methods
Throughout the 19th century, visual astronomers communicated their observations of astronomical seeing as a qualitative judgment, recorded in language that combined the physical fact with the astronomer's emotional reaction to the fact. Adjectives such as "glorious," "excellent," "good," "passable," "difficult" or "terrible" were typical; most astronomers had a preferred set of descriptive phrases, or used common terms in idiosyncratic and unsystematic ways.
Often a judgment of "good seeing" was the declaration by an astronomer of his confidence in observations made under familiar conditions. The observational aims of the astronomer and his experience of what was relatively good or bad seeing at his habitual observing site significantly affected his appraisal of turbulence. Commonly, the evaluations made by visiting astronomers differed markedly from the resident observers.
As corrective to that disorderly tradition, several more objective seeing scales were proposed around the turn of the 20th century; some of those have come into common use. I describe the most common seeing scales below, with a few examples of abandoned methods.
Scales are grouped according to the criterion used as the test of seeing subjective image quality, task specific resolution, star diffraction artifact, double star separation, and CCD light distribution. The issue is what the observer is asked to observe as a sign of the atmospheric turbulence, and the validity of that criterion as a basis for judgment.
The following five issues should be kept in mind:
The perceived effects of atmospheric turbulence are significantly affected by the aperture of the observing instrument. Turbulence that produces a boiling, inflated star image in large apertures will appear as an oscillating but perfect star image in smaller apertures, and at the extreme as "twinkling" in the smallest aperture we use the naked eye.
As discussed in the previous page, atmospheric turbulence varies both in the scale or amplitude of optical distortion and in the energy or frequency of the distortion. This is often described as the contrast between oscillation and scintillation, but often both forms of turbulence are apparent, and one can be much more severe than the other. If a rating scale for atmospheric turbulence combines the two, for example as the rating description "frequent large undulations in the image", it makes the application of that rating ambiguous. If the motions are frequent but small, or infrequent but large, does the description still apply?
The reliability of a rating scale means the consistent application of the scale both by different observers at the same time and by the same observer across time: presented with exactly the same conditions on two different occasions, an observer will give the same rating as he did previously and his rating will agree with the ratings given by other observers. The validity of a rating scale means that different ratings actually reflect some objective and variable attribute of the atmospheric turbulence. Both attributes must be present in a useful rating system.
Turbulence becomes more or less troublesome depending on the visual or photometric task and on the magnification or image scale required. To ensure reliability, seeing should always be evaluated by means of a specific optical test using the same reference magnification (at least 20 per centimeter of aperture) or measuring instruments, even when images are quite acceptable at a lower power.
A long research literature in psychology has confirmed that people cannot reliably use more than about 5 rating intervals when asked to communicate subjective or perceptual judgments. Discriminations more nuanced than that are clouded by random error and "guesstimation".
As we'll see, seeing scales generally emphasize the degree of optical distortion present and minimize or ignore the frequency or time scale of variations in optical quality. In effect, they ask the observer or light sensitive instrument to make a time exposure image of a star, and then rate the quality of that static image. But many astronomers have emphasized that frequency is also important, and recommend rating both attributes.
Subjective Image Quality
The Association of Lunar & Planetary Observers Scale
We start with the seeing scale adopted by or attributed to the ALPO. In its briefest form, this is simply communicated as:
"... a numerical sequence ranging from 0.0 (worst possible seeing) to 10.0 (absolutely perfect seeing)".
Observers adopt the lay culture convention of a 0 to 10 scale ("so how did you like the movie, on a zero to ten scale?") and pick a number that corresponds to their impression of the night's seeing in comparison to their personal experience of seeing across all previous occasions. No other guidance is provided, no consideration is made of aperture or image quality: the observers simply state their reaction to what they see based on what they are used to seeing while doing what they normally do. This is essentially the 19th century approach to measurement, in which the expertise of the individual superseded objective facts and consensus judgment.
Sometimes an attempt is made to salvage the scale by providing verbal descriptions of the image quality. The most detailed wording I can find for the ALPO scale as one of the scales for reporting atmospheric conditions at the Clear Dark Skies web site is given below.
1 Very poor images, impossible to see details or to sketch.