M48/NGC 2548 - Great seeing but poor transparency

Brian Tung wrote:

Andy Hewitt wrote:
Yes, much the same here really, I usually measure the clarity of viewing
by seeing how much 'twinkle' the stars have (when I can see them at all
that is).

Twinkling is an indication of atmospheric turbulence, but it doesn't
generally correlate very well to telescopic seeing. The stars can seem
perfectly still to the unaided eye, yet the planets will seem very fuzzy
at best focus in the telescope; and correspondingly (though less often,
it seems to me), the stars can twinkle noticeably, and yet the planets
will seem pretty sharp in the telescope.

How often either of these situations happens seems to vary considerably
with your observing site, judging from comments on SAA over the years.

So in short, just point your scope at the sky, and see what you can see?
;-)

--
Andy Hewitt
http://web.mac.com/andrewhewitt1/

On Fri, 23 Feb 2007 10:27:15 -0800 (PST), (Brian Tung)
wrote:

Twinkling is an indication of atmospheric turbulence, but it doesn't
generally correlate very well to telescopic seeing. The stars can seem
perfectly still to the unaided eye, yet the planets will seem very fuzzy
at best focus in the telescope; and correspondingly (though less often,
it seems to me), the stars can twinkle noticeably, and yet the planets
will seem pretty sharp in the telescope.

How often either of these situations happens seems to vary considerably
with your observing site, judging from comments on SAA over the years.

Indeed. Twinkling at my location is a fair (but far from perfect)
indicator of visual seeing. I'd add, however, that it's a very reliable
indicator of _photographic_ seeing. When you are imaging, slow image
wander destroys resolution. Visually, that same wander may have little
effect (the eye integrates at about 100mS, compared to minutes when
imaging). The nights I get the highest resolution images are always the
nights that the stars are steady.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:
On Fri, 23 Feb 2007 10:27:15 -0800 (PST), (Brian Tung)
wrote:


Twinkling is an indication of atmospheric turbulence, but it doesn't
generally correlate very well to telescopic seeing. The stars can seem
perfectly still to the unaided eye, yet the planets will seem very fuzzy
at best focus in the telescope; and correspondingly (though less often,
it seems to me), the stars can twinkle noticeably, and yet the planets
will seem pretty sharp in the telescope.

How often either of these situations happens seems to vary considerably
with your observing site, judging from comments on SAA over the years.


Indeed. Twinkling at my location is a fair (but far from perfect)
indicator of visual seeing. I'd add, however, that it's a very reliable
indicator of _photographic_ seeing. When you are imaging, slow image
wander destroys resolution. Visually, that same wander may have little
effect (the eye integrates at about 100mS, compared to minutes when
imaging). The nights I get the highest resolution images are always the
nights that the stars are steady.

I always rub my hands in anticipation when I see a lot of humidity
and/or slight fog, for this is when I have my best seeing for imaging.

Anthony.

On Fri, 23 Feb 2007 21:42:59 +0200, Anthony Ayiomamitis
wrote:

I always rub my hands in anticipation when I see a lot of humidity
and/or slight fog, for this is when I have my best seeing for imaging.

I remember we had a little fog here one night a few years back. But when
we get some serious humidity, maybe 40%, it doesn't seem to correlate to
seeing g.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Fri, 23 Feb 2007 19:17:24 GMT, Chris L Peterson
wrote:

On Fri, 23 Feb 2007 10:27:15 -0800 (PST), (Brian Tung)
wrote:

Twinkling is an indication of atmospheric turbulence, but it doesn't
generally correlate very well to telescopic seeing. The stars can seem
perfectly still to the unaided eye, yet the planets will seem very fuzzy
at best focus in the telescope; and correspondingly (though less often,
it seems to me), the stars can twinkle noticeably, and yet the planets
will seem pretty sharp in the telescope.

How often either of these situations happens seems to vary considerably
with your observing site, judging from comments on SAA over the years.

Indeed. Twinkling at my location is a fair (but far from perfect)
indicator of visual seeing. I'd add, however, that it's a very reliable
indicator of _photographic_ seeing. When you are imaging, slow image
wander destroys resolution. Visually, that same wander may have little
effect (the eye integrates at about 100mS, compared to minutes when
imaging). The nights I get the highest resolution images are always the
nights that the stars are steady.

I used to believe this too Chris. Unfortunately it's not as reliable
as you would think. I've had plenty of nights with steady stars which
have got me very excited. Sometimes they are good for high resolution
imaging but only about 50% of the time.

In the UK at the start of February we had a very promising start. High
pressure and a favourable position for the jets stream. A number of
nights were crystal clear with very steady stars. Unfortunately the
seeing for imaging Saturn at an altitude of ~50 degrees was very poor
indeed.
--
Pete Lawrence
http://www.digitalsky.org.uk
Last updated June 2006

Pete Lawrence wrote:
On Fri, 23 Feb 2007 19:17:24 GMT, Chris L Peterson
wrote:


On Fri, 23 Feb 2007 10:27:15 -0800 (PST), (Brian Tung)
wrote:


Twinkling is an indication of atmospheric turbulence, but it doesn't
generally correlate very well to telescopic seeing. The stars can seem
perfectly still to the unaided eye, yet the planets will seem very fuzzy
at best focus in the telescope; and correspondingly (though less often,
it seems to me), the stars can twinkle noticeably, and yet the planets
will seem pretty sharp in the telescope.

How often either of these situations happens seems to vary considerably
with your observing site, judging from comments on SAA over the years.

Indeed. Twinkling at my location is a fair (but far from perfect)
indicator of visual seeing. I'd add, however, that it's a very reliable
indicator of _photographic_ seeing. When you are imaging, slow image
wander destroys resolution. Visually, that same wander may have little
effect (the eye integrates at about 100mS, compared to minutes when
imaging). The nights I get the highest resolution images are always the
nights that the stars are steady.


I used to believe this too Chris. Unfortunately it's not as reliable
as you would think. I've had plenty of nights with steady stars which
have got me very excited. Sometimes they are good for high resolution
imaging but only about 50% of the time.

In the UK at the start of February we had a very promising start. High
pressure and a favourable position for the jets stream. A number of
nights were crystal clear with very steady stars. Unfortunately the
seeing for imaging Saturn at an altitude of ~50 degrees was very poor
indeed.

Pete et al,

For my location, nights which are CRYSTAL clear invariably translate to
VERY poor seeing. In fact, it is usually one or the other right now
(winter): I can wish for great transparency which comes with very poor
seeing (FWHM greater than 4.5") or great seeing (FWHM around 2.2") but
with average to poor transparency. For the latter, I must have high
humidity and/or some slight fog which of course impact transparency.

Summer time is another story.

Anthony.

On Fri, 23 Feb 2007 20:35:48 +0000, Pete Lawrence
wrote:

I used to believe this too Chris. Unfortunately it's not as reliable
as you would think...

Well, that just goes to show that this rule is variable with location
both for visual and imaging. Certainly, at my site, the degree of
twinkling correlates almost perfectly with long-exposure FWHM.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:

On Fri, 23 Feb 2007 20:35:48 +0000, Pete Lawrence
wrote:


I used to believe this too Chris. Unfortunately it's not as reliable
as you would think...


Well, that just goes to show that this rule is variable with location
both for visual and imaging. Certainly, at my site, the degree of
twinkling correlates almost perfectly with long-exposure FWHM.

I also use twinkling as a gauge and which I have found to be accurate as
well (for imaging).

Anthony.

On Thu, 22 Feb 2007 20:19:05 +0000,
(Andy Hewitt) wrote:

Anthony Ayiomamitis wrote:

Dear group,

The skies finally cleared last night for the first time in about three
weeks.
The open cluster M48 in Hydra is most impressive and I am disappointed
the poor transparency did not permit for an even better result. For
those interested, the result is available at
http://www.perseus.gr/Astro-DSO-NGC-2548.htm .

Clear skies!

Nice pic. It's just not fair that I can't get them like that!

I agree; and I've recently added the website to my own page list of
links. Sometimes the pages don't come up too fast out here in
California but it's worth the wait to see such excellent photos.

The viewing of late has been appalling hasn't it? I got a new 150mm Dob
last week, and my 105mm Mak arrived yesterday. I reckon I've got about 2
hours viewing in the last two weeks.

Normally winter seeing is not consistently very good for me out here
on the Pacific coast. But, amazingly: during clearings for as much as
a week at a time, going back to December, I've had the most amazingly
GOOD seeing out here, at 3,400 feet elevation, not far from Santa
Cruz. I have had numerous nights of 8-9 Pickering scale seeing, and
fantastic sky transparency too. The last time I remember such good
winter seeing was back before Mt. Pinatubo erupted. I've done more
winter observing this year than on any single year during the winters
of the seventies and eighties. Of course it's cyclical,
unpredictable, and regionally localized so one can't extrapolate
anything from my experience here.

However, the other night I saw a show on the Discovery Channel about
"chemtrails" and then subsequently heard much discussion of it on talk
radio. It seems to me, though, that there can't be any 'seeding' of
the high atmosphere with reflective particles according to the
conspiracy theries, or by now it would be utterly impossible for me to
see things like Minkowski 1-18 and large diameter faint Abell
planetaries -- like Abell 7 -- that I've looked at this winter. I am
sure that the infrared sky survey data would show a trend one way or
another to give some evidence; after all, if light is being allegedly
reflected away from earth, then it would dim, obscure, or blur
astronomical observations: and from my own narrow personal experience
this winter, it isn't happening!

AstroApp