[AZ-Observing] Re: Double stars for seeing test
- From: Brian Skiff <Brian.Skiff@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Tue, 24 Jun 2003 19:24:34 -0700 (MST)
Tom and Bill posted the URL for my list of roughly-equal pairs
suitable for seeing estimates. The original of that list was published
in Chris Luginbuhl's and my book way-back-when, so it's been around
awhile.
The URL for the list of my own seeing estimates maybe isn't useful
except to show what sorts of numbers to expect.
Bill Wood also mentioned the very nice site that has the simulations
of the Pickering scale, which I've seen a fair number of people use---
though Australian amateurs seem to go for a similar scale by Antoniadi.
Remember that both the double-star type estimates and the Pickering
scale will be aperture-dependent. From the mid-range upwards, the
Pickering scale depends a lot on having a well-defined Airy disc, which
most telescopes above 12-inches or so simply won't show either because
the optics aren't good enough or because the seeing just won't allow it
(at 12-inches aperture the Airy disc is only 0".43 or so across).
Thus the simulations are about right for the 6- to 10-inch range.
Similarly, the mag. 6-7 stars in my doubles list are optimal for
this medium-aperture range. It turns out that the image you see for
this magnitude range and in this aperture range is reasonably close to
"full-width-half-maximum" of the profile. On the same stars a bigger
telescope shows more of the fainter outer parts of the blur, so the
number you get will seem worse. With my 70mm Pronto, the Airy disc is
already 1".7 across, and close mag. 7 stars are pretty tough, so the
seeing almost always seems "perfect", when it's actually just smaller
than the telescope can resolve---the problem is finding third magnitude
pairs that fit the bill for seeing tests!
BUT...as long as you use the same telescope for all the estimates
at different sites, etc., then you'll build a consistent set of data
after doing it each session. Perhaps the main value in doing it will
be forcing yourself to pay attention to detail in the image structure,
and "notice stuff" kinda generally, as a result of which you'll find
that the visual skills gained will make you a better observer---which is
what all this is about after all.
\Brian
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