NavList:

   

Reply

Thanks to Greg Rudzinski for his quick response to my questions, and for the
interesting selection from his photos of setting Sun.

As usual with good information, it raises further questions. And it may also
point to an effect that should be added to my list of drawbacks of a digital
camera, compared with eye-viewing in a sextant.

The big picture has a dark shade inserted in the Sun view, to limit
overexposure. The clouded pictire relies on that cloud to cut down the
brightness, and no shade is called for. But what about the others, imaging
the Sun alone? My guess, from the blueness of the surrounding sky, is that
the upper one (picture 11) has no shade, and that the Sun image itself is
vastly overexposed. Maybe the next one down (picture 17) was taken through a
shade, but I'm not certain; perhaps Greg will confirm.

If a sunspot could be picked out, on any of those images, it would confirm
that there was no severe overexposure, but I can see none. On the two
smaller pictures, much of the Sun is masked by an information panel, which I
can't see how to shift. But then, we're in a quiet period, for the Sun, at
present, and I don't know if any big spots were visible on that day (12 July
08).

The big surprise, to me, on all those pictures of the Sun, is the VERY
ragged nature of the Sun's edge. It looks a bit like those pictures we see
in a total eclipse, of prominences in the corona, but all around the limb. I
doubt if they ARE corona, which is many orders-of-magnitude down, in
brightness, on the Sun disc itself, so that you can see it only at a total
eclipse. Tentatively, I ascribe them to micro-turbulence in the air, and
thermal gradients across the light-path from the Sun, in which case they
would be constantly fluctuating; what an astronomer would describe as bad
"seeing". The image of the Sun that we see in a sextant, through a shade,
seems to have much smoother and sharper edges. Can we attribute that to two
factors, I wonder, as follows?-
1. The eye averages over a MUCH longer period than the 1/800 second, seen by
the camera.
2. There's a much wider spatial averaging, in the part of the light-cone
just near the observer, in that it takes in light over the whole width of
the telescope aperture, whereas the camera just uses the tiny central spot
of a highly stopped-down lens (f/14).

Those ragged edges of the Sun remind me of the pictures of the Sun that you
sometimes see in a television image from the African bush, in a long
telephoto shot, which often show a low Sun in an atmosphere that looks as
though it's boiling. Are Greg's shots showing a similar effect, but with the
time-slot restricted to be more momentary still? Would a following shot, a
few tenths of a second later, show a similarly ragged outline, that's quite
different in detail?

In addition, the Sun image that's seen by the camera through the shade has,
when you expand it, a VERY speckly appearance, rather like peering through a
car windscreen in the rain, without wipers. I have no theories to account
for that; does anyone else? Does that speckle-pattern shift, I wonder, if
you move the shade about?

Let me hazard a guess that any breeze or air-movement on the day of Greg's
observation was offshore. If so, it would be taking air, that had passed
over the Californian mountains, been heated by a summer Sun over the coastal
sands, and was now displacing, or mixing with air that had been in contact
with a cooler
sea; a perfect recipe for turbulence and thermal gradients. I'm guessing
that an onshore breeze would give rise to more placid images. Can Greg
recall the weather, that day?

This close-attention we have to give to the Sun image is solely because
that's the only yardstick we have to calibrate the overall scale, so any
errors in its diameter, in pixels, are greatly multiplied when assessing the
Sun altitude; by 12 times, or so, for the earlier shots in the sequence.
It's a consequence of using an otherwise uncalibrated camera to measure
with.

=============================

On a personal note, if I may add this tailpiece, on quite a different
matter-

I happen to be one of a large family, the oldest of seven siblings, normally
far-flung around the World. But this week, it's so happened that all are in
the UK at the same time, so we have just enjoyed a bit of a clan-gathering,
to celebrate. That doesn't happen often; the last time was in 2001.

In general, we're a rather fractious lot, all being about as argumentative
as I am. So, though it's good to meet up, it's best not to do so too often,
in the interests of goodwill and harmony. We reckoned, between us, that such
meetings, on a regular basis at 7-year intervals, would be just about
right...

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply