Re: Data Volume

[jg <jg@jgws.freeisp.co.uk>]

Tom Droege wrote:
> 
> I am getting fewer measurements this summer than last winter.  Expected, of
> course.  But I notice that I do not measure as faint as in the winter, and
> this has a big effect on the number of stars seen.  Some of the scans are
> now in the Summer Milky Way.  So far I see about 1/3 the star detections as
> in the winter Milkey Way.  Either the stars are less dense, or there is
> something else going on.
> 
> I mostly suspect that the hazy conditions in the summer cut down just
> enough light that I get a mag or so less sensitivity.  Anyone with
> experience on this want to comment.  I suppose all surveys have this
> problem.  Some areas of the sky just get better coverage.
> 
> Tom Droege

Okay, some of this you might know as obvious anyway, but just in case.

There's civil twilight (sun only down to six degrees below horizon),
nautical [I think called "military" in USA] twilight, 6 to 12 degrees,
and astronomical twilight, 12 to 18 degrees.  Lower than 18 degrees it's
"night".

At my latitude, +52 degrees, we never really get long at much darker
than astronomical twilight all night long during June, July and August. 
For a short while around June we don't even manage astronomical
twilight.  Things'll be better at +41.5 degrees lat, but still, it just
isn't as dark.

On the other hand, I dunno how your camera setup works, but star
brightness is a limiting factor to the eyeball.  I've been less dark
adapted at the end of some observing runs than I was to start with due
to too much Milky Way in the sky.  This too increases the brightness of
the background sky, reducing limiting magnitude.  There are a lot more
faint on the edge of detection stars in these Milky Way fields.

Winter Milky Way fields are towards outer spiral arms from your
latitude, on the whole, whereas Summer ones are towards inner spiral
arms and the Galactic Centre, usually far more dense in both stars and
extinction-causing dust (reddens stars as well as making 'em fainter).

I don't know if "airglow" is more prominent in Northern Summer months,
I'd have thought that was more connected to the seasons of the sun than
the earth though.  This is generally due to flourescence and
recombination in the upper atmosphere due to incident solar radiation. 
I suppose there's more radiation for longer in Northern Summer during
the day, but I can't remember if recombination is "realtime" or occurs
more at night when the upper atmosphere is cooling off, ie less
"energized". Basically, I can't barely remember how airglow works.

The latter few are fairly small in effect, but they add up to a brighter
sky background.  Some, all, or none of the above may be at play here,
but the twilight one for definite I feel.

Hopefully someone'll be able to elaborate on this more jargony like ;)

Maybes you're more likely to find airglow and starglow mentioned in
weather books than astro books.  All pale into insignificance relative
to light pollution nowadays, so don't get mentioned much.

Cheers

John G.