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Re: (meteorobs) LM Star Count vs Milky Way Perception
Hi George,
The Lm of 6.0 necessary to see the milky way in my case only applies to
skies with some light pollution background, e.g. moonlight or citylight.
It is true for example for my home at Voorschoten, in a suburban area.
Under dark conditions, things are different. The point is, that for fuzzy
glows over an extensive area (that is, low surface brightness objects)
such light interference (e.g. light pollution) takes a larger toll than
on stellar objects. That means, that when I am observing under Lm 6.0
from Voorschoten (with light pollution), seeing the milky way is more
difficult than when observing with a similar Lm 6.0 from Biddinghuizen
(where light pollution is much less, i.e. the skies are darker). The same
applies to moonlight
conditions. Light pollution just affects starry objects and fuzzy objects
differently. As far as I can remember, that was our discussion last
November (triggered by the Leonids under moonlight -we had very clear
skies with LM near 6.0 notwithstanding moonlight, and even milky way).
When you see the milky way with lower Lm, this can be because of 2 or 3
reasons:
1) although the Lm is modest at 6.0, your sky background remains dark due
to favourable non-light pollution conditions. That is what I have
outlined above.
2) you slightly underestimate the true Lm, for example by counting too
much with the center of your vision.
3) the physiology of your eye and eye-brain system is for some reason
very favourable for fuzzy objects. This can happen. Some people, even
those who have good eyesight for faint stars, are just not good in seeing
low surface brightness objects. In my case, I am quite sure though that I
have better than average eyesight also for such objects (deep sky
observing background -have had my own column on deep sky in Zenit (kind
of Dutch S&T) for years). But this might explain for example the
difference between you and Bob.
I am not sure at all what is the reason in your case. Perhaps, you just
have good eyesight for low surface brightness objects.
Perhaps interresting, how do the Lm's of Bob and you compare when you are
both observing from the same location (and thus with similar sky
conditions)? Some of my Dutch friends consistently arrive at Lm that can
be up to 0.5-0.6 lower than me or Koen. Casper for example reaches much
lower Lm's (but in his case, his eyes might not be too good of quality,
and moreover he is not a regular visual observer) That hints at
differences in the
way the Lm is estimated -to some extent, these things are just very personal
too: a Lm is not something fixed, just like perceptions, differences in the
number of meteors seen by different persons.
In other words: Lm's are not a strict measure of sky conditions -people
estimate different Lm's under similar conditions. Moreover, the limiting
brightness of starry objects behaves different from those of extensive
fuzzy objects and are more influenced by the balance in sky background
brightness.
-Marco
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