Re: full moon and observing

On 2008-02-21, brucegooglegroups <brucegooglegroups@xxxxxxxxxxx> wrote:
On Feb 21, 5:48 pm, Andrew Smallshaw <andr...@xxxxxxxxxxxxxxxx> wrote:

It depends on the target. Stars (binary or otherwise) and planets
are still viewable with the full moon just as any other source of
light pollution - just up the magnification a little to dilute the
effect of the skyglow. The faint fuzzies are much more difficult
under a less than dark sky. The fact that the light in question
is from the moon doesn't really make much difference, aside from
the fact that the full moon is really very good at destroying night
vision, much more so than skyglow.

Also, what about star clusters and galaxies in a moonlit sky?

Galxies and nebulas, no they will be adversely affected by light
pollution. Most clusters will be too - it depends to what extent
you can resolve the cluster into individual stars. Basically
anything that is a point source of light can be observed through
light pollution if you increase the magnification.

Consider for a moment viewing an area of sky equal to the smallest
possible area you can see with the naked eye - this is something
on the order of 1 arcminute but the precise figure varies depending
on the circumstances and who you ask. For ease of analysis we'll
say that this area is a square of side 1 arcminute. Within that
square there is going to be some skyglow - we'll say 10 units of
light due to skyglow is coming from that area of the sky (it doesn't
matter what the units are since this is just a thought experiment).

Also in that area there is a dim star, which gives additional light
worth an extra one unit. The star isn't likely to be noticeable
because the light from that area is 11 units (skyglow + star)
compared with 10 units from all the surrounding equal sized areas
that do not contain a star (skyglow only). Put simply the star is
drowned out by the skyglow.

Now go from a naked eye view to 10x magnification. The same area
of sky the is now viewable as 100 smallest visible areas (a grid
arrangement 10 high and 10 across). The skyglow is evenly distributed
among these areas, so you have 0.1 unit of light in each of them
attributable to skyglow. However, the star will not resolve into
a disk, it is still a point source of light. It can only be in
_one_ of those areas, still giving the same 1 unit of light. This
means that area of sky emits 1.1 units vs 0.1 unit for the areas
that don't contain a star - this is obviously much easier to pick
out compared to the naked eye view because it is 10x brighter than
the surroundings instead of only 10% brighter.

However, for anything that isn't a point source of light this
doesn't work. By increasing the magnification you spread the same
amount of skyglow over a wider area, however you do exactly the
same to the nebula, galaxy or whatever. If the object being viewed
does not fit into one of our imaginary squares then the light is
being spread at the same rate as the skyglow. The ratio between
the two does not vary and you may actually find the object _less_
visible as its apparent surface brightness is now lower.

--
Andrew Smallshaw
andrews@xxxxxxxxxxxxxxxx
.

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