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These words bring sighs to the romantic but, to the astronomer, they can mean a lost night of useful work. Contrary to popular belief, crisp, clear Winter nights, with the stars twinkling like Christmas lights, are the worst possible for serious observing.

Twinkling is caused by masses of air of different temperatures passing between the observer and the object being observed. They have the same effect as poor quality window glass when trying to look through it. The image waves around and the atmosphere acts like an out of control lens that keeps changing the focus.

To the unaided eye, stars twinkle or change brightness irratically. Through a telescope, twinkling stars dance around like a drop of water in a hot frying pan. Planetary detail is smeared and fuzzy and little of interest can be seen on such a night.

This phenomonon is called "SEEING" and varies considerably from place to place and from time to time. In general, the higher up in altitude, the better the seeing because there is less atmosphere to see through. This is one reason professional observatories are usually located high up on mountain tops. However, some sea level locations (southern Florida for example) can be nearly as good at certain times and some locations such as the Midwestern U.S. are nearly always bad.

In order to discuss seeing intelligently and to help in site selection of large observatories, seeing must be quantified. It is also important to know what the seeing is when evaluating a telescope. Many new scopes and their manufacturers get a bad rap for "lousy optics" when, in fact, the best optics on Earth could not produce a better view because of poor seeing.

To quantify seeing, one could simply judge the amount and amplitude of twinkling but it would be to hard to get a good handle on the range or value. Much better is the system developed by William H. Pickering of Harvard at the turn of the century. The popular Pickering 1 to 10 scale is in common use by professionals and amateurs alike. The Pickering scale is based on what a highly magnified star looks like when carefully focused, in a small telescope.

Pick A star at high magnification, under perfect seeing (P-10) looks like a bull's eye. A small central disk surrounded by one or more concentric rings. At P-1, it is just an amorphous blob. The central disk is known as the Airy disk and it's size in inversely proportional to the size of the telescope objective. That is why a large telescope can see more detail under perfect conditions than a small one. Because of physical limits the Airy disk is the smallest detail that can be seen at maximum magnification and the smaller it is, the less it intrudes on the detail. Makes little difference when looking at a star which can never be resolved because of distance but when looking at the surface of Mars or the Moon, every feature is just a lot of Airy disks all jumbled together and the larger they are, the fuzzier the image.

One fact little understood by purchasers of new telescopes is that the effects of poor seeing increase dramatically as the size of the telescope is increased. This is simply because a small telescope has to look through a much smaller column of air than a large one. A fairly good night with a small scope might be not worth taking out a large one. Pickering established his system using a 5" diameter telescope and his scale would have to be fudged when used with a scope of larger or smaller aperture.



The small scope in the foreground was designed specifically to evaluate seeing at my site and is the same size as the one used by Pickering. The larger one in the background is the 16".

For more details on the "Pickering 5"...... [Telescopes]

The "bull's eye" or diffraction pattern as it is known starts to appear at about P-4 in the 5" and is distinct but unstable at P-7 (a very good night for the Midwest). At P-7, the 16" is about the same as the 5" at P-4. I have yet to see anything better than P-7 here which points out the fact that the 16" has yet to be used to it's fullest capability.

It should be pointed out that I am referring here to the ability to resolve detail and not just the ability to see dim objects. The large scope always prevails in the latter but when viewing the surface of Mars or the Moon, for example, no more detail can be seen on a poor night with a larger scope.

By now it should be clear why, in spite of it's modest size, the Hubble Space Telescope has produced photographs that exceed the reach of even the largest Earth based instruments. Being above the atmosphere, it is immune to the effects of seeing and its resolution is only limited by its size.

For these and other reasons, it is very difficult to photograph the diffraction pattern but there are more pragmatic ways of demonstrating the effects of seeing. Because seeing not only varies from location to location and from night to night but also changes drastically from moment to moment, particularly on poor nights. A P-3 night can have instants of P-6 and a patient observer can often snatch good views if persistant enough and does not blink at the right moment.

Because of the fast and continuous frame capture of video, it is very easy to demonstrate these moments just by attaching a video camera to a telescope and pointing it at the moon.

The following images were captured only seconds apart and by stepping through the tape, a moment of good seeing was found.

[Copernicus] [plato]



We now come to the details of Pickering scale as described in a S&T article from April, 1995.

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 following is a summary of the seeing conditions at my location about 50 miles NW of Chicago since Oct 1997.

Total Nights................ 213
Clear Nights............... ..71


The 71 clear nights out of 213 tell another story about the effects of El Nino on observational astronomy. It is however, only part of the story. Most of those 71 nights were only clear enough, long enough to make the evaluation. The actual number of useful clear nights during the period was a small fraction of that number.

I do not have a feel for what is "normal" for this area but it is not hard to understand why no new observatories have been built in the Midwest in the past several decades and the older ones are either decomissioned or virtual museums.

If anyone has or wants to gather info on their site, I would be be glad to post it here for comparison.

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