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(meteorobs) Moonlight observing (Leo)
Hello,
Upcoming Leonids will see severe moonlight. So I thought I might give
some advise about do's and don'ts and pittfalls in moonlight observing to
those who have never done so. Even experienced observers generally do not
know some of the particular pittfalls.
Some things are obvious: don't go observing into the direction of the
moon, but take the opposite viewing direction. george wrote me that he
likes northwest most.
Also, it is worth moving to a dark spot on the countryside. A common
mistake made is that people think that is no use because the moonlight is
so severe that citylight is no longer a problem. But that is not true.
These things work cumulative: so moonlight+citylight means you are much
worse off then moonlight alone.
Much depends on the sky condition. Slight haze is deadly with moonlight.
But with a clear moistfree air, things are not necessarily bad.
Now, the moon is much less restricting than most people think. There is a
notorious problem among observers with moonlight conditions: they start
to UNDERESTIMATE the limiting magnitudes. This is psychological: they
think that the LM cannot be high, so they put less effort in counting or
are suspective of 'high' Lm counts. So, you frequently see Lm's reported
of 4 or 3.5 notwithstanding a clear sky.
In reality, Lm's with moonlight easily reach 5.0, and often higher. 5.3
is my typical count, but under very clear conditions I have also got to
5.9. When you have clear moist-free skies, be suspective about Lm's lower
than +5.0:these are most probably underestimated.
Another common thing is reduced attention to faint meteors. Agian, people
think that if conditions are worse, there will be little fainties seen,
so they try less hard.
Especially observers at the US west coast should take extreme care for
faint Leonids. Somewhere near 12-13h UT, they might see the narrow peak
of faint meteors superimposed on the brighter background that was seen in
1996 from Europe.
Here's another pittfall: last year(s), theLeonids produced many bright
meteors (that is :true fireballs). The risk is that observers become so
preoccupied with these that they start to neglect the faint meteors. I am
convinced this happened with some observers in Europe (and outside) last
year. You need to be a little bit cool-headed.
The stream will most likely produce many very bright meteors again. like
last year(s). That will mean that even with moonlight the show might be
very attractive!
All this is not to say that moonlight doesn't restrict the quality of
observations and the observed levels of activity of course. But with some
care for the psychological pittfalls, the quality of observations is
certainly improved. We have some experience with moonlight observing
here. people who are aware of the mentioned pittfalls usually produced
data that agreed quite well with multiyear averages for annual streams.
Those who where not aware sometimes didn't. I am convinced that it is the
Lm problem which is behind the apparent 'change in perception' that is
sometimes reported for moonlight data. It is not a true change in
perception, but faulty Lm estimates. So, be warned!
Marco Langbroek
PS: sorry for the typoes, but the University hardware is rubbish here...
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