[Prev][Next][Index][Thread]
(meteorobs) More on LM estimates and sources of error in ZHRs
Asaf wrote:
>How did you get to such precision of finding the
>limiting magnitude? Anyway, I believe it's impossible
>of viewing over 6.5 magnitude, which is the top limit
>of human eyes.
Asaf asks a great question, following up on a post by Rob McNaught last week on
the errors inherent in visual meteor observing...
The first answer is that the LMs usually quoted by IMO visual observers are
AVERAGES, derived from as many as 6 LM estimates per hour taken using the IMO
Star Count method. So you will very often see LM averages being quoted in tenths
or hundredths, though as Rob would be quick to point out, the standard error
might really be half a magnitude, even for careful observers.
Second, the old presumption of 6.5 being a hard physical limitation of the human
eye has long ago been disproved: there are double-blind trials done in the past
20 years which have convincingly recorded human faint-star perceptions down to
*MAGNITUDE 8.9*! (Refer to experiments publicized by Stephen James O'Meara, Fred
Schaaf and others in Sky & Telescope.)
Note by the way, Mike L., that some of these experiments were done on Mauna Kea,
as well as West Texas and other dark places... Of course, as mentioned in the
past, NONE of these were valid LM estimates from a meteor observer's point of
view: they were attempts to map or describe all the very faintest stars in a
given field, seen by ANY visual method at all. This is very different from the
IMO's requirement that observers ONLY count stars seen with "slight averted
vision" in the designated Star Count Field...
Asaf, if you're interested in learning the IMO LM technique, which would be very
handy for your new program of meteor recording in the New Year, check out the
following unofficial Web site for IMO's Star Count Area charts:
http://www.seds.org/billa/lm/rjm.html
==================
Note also that though LM estimates ARE highly subject to error, it would not be
strictly correct to call them "subjective" as Rob does. Nor would I expect
shower association to be quite as significant an error source as Rob suggests,
at least under conditions where many meteors per hour were being recorded and
they were predominantly from ONE shower...
However, his point is very strong regarding meteor magnitude estimates: these
are highly subjective, and their accuracy probably varies from hour to hour and
from sky region to region for a given observer. That is why making significant
use of magnitude distributions in ZHR calculations seems like an inherently
problematic - but also a probably necessary - endeavor?
Clear skies all,
Lew
To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html
Follow-Ups:
References: