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Re: (meteorobs) Meteors S FL USA, GRALE, 10/11 Aug 2001
On 18 Aug 2001, at 17:12, peter.atanackov@guest.arnesdot si wrote:
> missing here is a Bortle Class estimate. Remember, one man's LM 5.5 sky
> can be another one's 6.3 (in our observing group the difference can amount
> to almost a full magnitude between certain observers!). Do you regularly
> see the Zodiacal band and the Gegenschein from Haleakala?
Yes I can. When conditions are good I can see the ZB practically
all the way around from ZL to Gegenshcein. Still, the faintest star I
can see naked eye is about 6.7 mag. However, with my 25cm
reflector I can see slightly fainter than mag 15.0 under such
conditions.
> Also, Haleakala is 3000+m high. Perhaps oxygen deprivation is already
> playing a role here. I remember a similar comment from Marco Langbroek -
> when he observed from Mongolia in 1998 (Leonids) he couldn't get past LM
> 7.1
I dont think so. I also observe from the lower level at the Haleakala
Ranger station (2100m) and I dont see any fainter stars from there.
However, on Mauna Kea (4000+ m) I do get into trouble with O2. I
get periods of "gray-out" where the background seems to wash out
the stars. On the contrary, when I observe from our club's sea level
dark sky site on Oahu's north shore, my LM is typically about 6.3
which is about what to be expected from the extra 3km of
atmosphere and greater light pollution. Yet the humidity is
generally low and clarity of the air in the mid pacific high, so I
suspect Dillingham has better atmosphere than Florida Keys, but
no doubt our light pollution here is worse, being only 30km or so
from a large city. So, overall I would expect the conditions at Long
Key and Dillingham to be pretty similar. I dont think anyone here
has seen even mag 7.0 though...
> Significant disparities between perception of meteors and fixed stars do
> occur. Take Norman for example, he *routinely* sees LM 7.3 from his
> location, yet his sporadic counts are relatively low regarding the LM.
One area of concern may be the disparity between reporting the
absolute best LM from a site, (by using a star count region near
the zenith) and the actual mean LM over your FOV. Given a typical
observer looking at 60° elevation, and a 100° field diameter, that
corresponds to an average weighted airmass of 1.45. For a typical
sea level humid condition, thats 0.45 * 0.32mag = 0.14mag loss in
LM thats not accounted for properly. If the observer looks even
lower, the airmass effects increase rapidly, like 0.5mag at 50°
elevation.
Its important to use a reference star count area at or below your
average FOV elevation to properly account for this effect. It is
particularly important for humid, sea level sites such as this one
used by Lew.
Mike Linnolt
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