I agree with the other posters. Don't stress too much over "what is the right number". Stress over applying
"YOUR" assesment criteria, consistently from observing session to observing session.
There are two common scales for assessing seeing conditions.
The Pickering Scale and the Antoniadi Scale.
Pickering Scale
P-1 Star image is usually about twice the diameter of the third diffraction ring (if the ring could be seen.
P-2 Image occasionally twice the diamteter of the third ring.
P-3 Image about the same diameter as the third ring and brighter at the center.
P-4 The central disk often visible; arcs of diffraction rings sometimes seen.
P-5 Disk always visible; arcs frequently seen.
P-6 Disk always visible; short arcs constantly seen.
P-7 Disk sometimes sharply defined; rings seen as long arcs or complete circles.
P-8 Disk always sharply defined; rings as long arcs or complete but in motion.
P-9 Inner ring stationary. Outer rings momentarily stationary.
P-10 Complete diffraction pattern is stationary.
The Antoniadi Scale
The scale is on a 5 point system, with one being the best seeing conditions and 5 being worst. The actual definitions are as follows:
I. Perfect seeing, without a quiver.
II. Slight quivering of the image with moments of calm lasting several seconds.
III. Moderate seeing with larger air tremors that blur the image.
IV. Poor seeing, constant troublesome undulations of the image.
V. Very bad seeing, hardly stable enough to allow a rough sketch to be made.
Note the scale is usually indicated by use of a Roman numeral.
The Antoniadi scale is gaining widespread acceptance and is almost used with equal predominance as the pickering scale these days. This is likely because it is easier for less experienced observers to apply on a consistent basis than the pickering scale.
I use a 1-10 scale based on the pickering scale, making my own mental adjustments based on the resolving capability of the telescope under the prevailing conditions.
There is no defined scale for measuring transparency alone. Many people just use an estimate of the zenith limiting magnitude to determine sky glow effects and transparency. This is very subjective because not everyones eyes are the same and an experienced observer will see far deeper naked eye than a beginner. The Bortle Scale, published in Sky and Telescope (Feb 2001)is a combined scale which reflects both sky transparency and latent sky glow effects to determine
"sky quality". It tries to set some standard parameters making estimates between different observers a little more consistent.
Bortle Scale
http://www.skyandtelescope.com/resou...tml?c=y&page=1
http://en.wikipedia.org/wiki/Bortle_Dark-Sky_Scale
Some of the very best dark sky sites in Australia will rate a 1 to 2 on the Bortle scale. NB This is not within 200 km of any of the major cities in Australia. In addition, just because a site is a long way from civilisation doesn't necessarily mean it will be exceptional because it may be affected by airborne particulates, which affect tranparency. Most of the good dark sites in Australia will rate a 2 to 3 on the Bortle scale. The two best dark sites that I have seen in NSW are Coonabarabran and Mt Kaputah, which rate a 1 on the Bortle scale. Centaurus A which is a mag 6.7 extended galaxy was visible naked eye last new moon at Coonabarabran, when we had 30+ US visitors up there. Ilford (ASNSW site) would rate similar to IISAC 2008 site at Lostock in the NSW Hunter Valley and would be rated almost a 2 on good clear nights, although you do get some very low sky glow at Ilford from Sydney and Newcastle to the East and South East. I have observed the Gegenshein from both Ilford and from Lostock.
Cheers,
John B
