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Re: (meteorobs) Observing Standards
Hello folks,
I joined the list some days ago, and I'd like to become an active
subscriber now as well. ;-)
Let me first shortly introduce myself. I'm a 25 year old computer science
student from Berlin/Germany. I have been visually observing meteor
for 10 years and became more and more active in this area of amateur
astronomy in the last few years. My speciality in video meteor
observation. We constructed our first video system in 1992 (MOVIE) and
have been using it on many occasions. This year is another 'milestone' in
our video work, since a friend of mine (Mirko Nitschke) built the first
series of 6 video meteor cameras for German meteor observers. Those systems
had first light during our Perseid camp a few day ago and turned out to
be *very* powerful devices.
You may already have come across all those activities in recent issues of
WGN, in IMC proceedings or at IMO's web site
(http://www.tu-chemnitzdot de/~smo/imo/index.html, if you didn't knew that
before :), which is maintained by me.
This short description will tell you already, that I'm heavily involved
in our German AKM meteor observers group, and that I have close links to
IMO. This is why I was especially interested in the point George touched
with his argument. Even though I agree with Lew, that Georges argument
was a little polemic and maybe not the best style for a discussion in a
forum like this, I very much appreciate the point of view George expressed.
Indeed, standardization is absolutely essential to make observations
comparable with one another, to be able to obtain *global* shower
activity profiles. It was one of the main reasons for the foundation of IMO,
that a standard method of observation is used to ensure, that not
everybody uses different methods. Even having IMO standards it is still
difficult enough to put together observations from different observers,
but it is almost impossible without them.
However, I don't know whether just the determination of the limiting
magnitude is something to be discussed in this context. The problem is
not so much the method actually used, but the consistency of the method!
It was already pointed out, that there are observers who have an extreme
wide angle view, whereas others refer more to a kind of 'tunnel vision'.
You can also express it with other words: There are guys, who obtain
dreamlike limiting magnitudes when counting IMO's polygons or using the
faintest visible star (FVS) method, but they do not see the equivalent
number of meteors. Others simply do not reach as high lm's, but see
amazingly many meteors (that's the group I belong to, for example). You
cannot correct the situation by pushing the one to see less stars or the
other to see more. The correct solution is the individual 'perception
coefficient', which was introduced to account for just that effect. This
coefficient simply tells you, whether you see on average more or less
meteors compared to your lm estimates, and so the results of different
observers become comparable with one another again. By the way, the
perception coefficient of an visual observer can only be determined
accurately, if a large enough sample of observations is available.
A few years ago I was worried, since my ZHRs where always much higher
than those of other AKM observers. However hard I tried, I could not see
more stars in the counting areas and hardly reached the 6.0 limit even
under good conditions, but I saw many meteors. Juergen Rendtel gave me
the advise, those check whether my results would be more persistent if I
tried the FVS method. I checked that out, but that method occured even
more difficult to me. However, I realized that some areas (especially the
already menitoned 14 in Cygnus) are simply unsuitable for lm
determination. I always found it an simple field to count since it is so
easy to find, but it was already well established that lm's of that area
are often much to low (by up to 0.5 mag). It is very difficult to
separate the many faint stars in the milky way of Cygnus, so we should
better leave that area out. However, I also found especially the Pegasus
and Cepheus areas very useful, and there I have been counting much more
'appropriate' numbers of star in the last few years. Now I often have
lm's up to 6.5, which are still a little low compared to my meteor counts,
but quite consistent. The remaining differences in ZHR are then considered
by the described perception coefficient.
To cut the long story short: I would not be too dogmatic about using the
IMO method of counting areas, much more important IMHO is the consistency
of lm determination. Personally, I use the IMO method and I feel more or
less comfortable with it (even though I also dislike starting to count
several times since often meteors appeared inbetween ;-). I can follow
the argument of Marko, who stops observation during his lm determination
for a minute or two (even though I could not convince myself yet to be that
accurate ;-) and I would suggest IMO's methods to new observers. However,
if somebody uses the FVS method consistently, that should be fine as well.
The question of the center of field of view has a little different impact
in my mind. Here I think it really makes a difference looking directly at
the radiant or some 30 degree away. I do not have so much observational
experiences that I could judge between the methods, but others have, and
they suggested the 30 deg since it was most effective for some reasons.
This is why I would use the proposed radiant distance. Again, I want to
stress the argument of George, that consistency of methods between
different observers is absolutely essential. As long as it is not proven
and generally accepted, that looking at the radiant *is* more efficient, we
should at least not recommend that method.
Some short comments on other stuff I read in the last days.
Yes, it is right that Ralf Koschak does have regularly lm's of 7.0 and
higher. In his case it is not only a question of the perception
coefficient, but he really sees that much meteors! This is why I would
never like to observe with him together, it would probably make me feel
blind... ;-))
I don't agree, that it is easier to see a faint meteor directly at
the radiant better than some 30 deg away, since the motion effect is very
important IMHO. However, that is not the main point of the discussion above.
There have been some posting on video meteor observation. Jeff Lashley
was a little disappointed, since his system did not work out to be as
effective as he thought. Jeff, maybe you should describe your system a
little more detailed (the lens, the type of image intensifier, which is
most important, etc.) either via private e-mail or in the mailing list,
that we can look for the reasons. Brian Nestel also reported about some
video activity, which does not involve any image intensifier. Brian: I think
it was one of our first results (bad experiences, so to say ;-), that it
is impossible to record meteors without an image intensifier. However
sensitive your camera is (btw, the lux number is only a joke from
advertisement as long as it is not supported by spectral response
funtions. You will not gain anything if the camera's sensitivity maximum
is somewhere in the far infrared, which is not unusual), you will not be
able to record many meteors. Don't be fooled by the sensitivity even
Camcorders show when using the full zoom power. Maybe you will be able to
see stars down to +4 mag or so. However, the field of view is only some
degrees then, and if you use the zoom lens in the wide angle mode, lm
goes down to 0 mag or even less.
Even not all types of image intensifiers are of use: first generation
devices with only 1 level of amplification (gain ~100) are worthless for
meteor observation. You need at least a gain of some 5000 or so,
usually achieved by either first gen. intensifiers with 3 amplification
levels, or preferably by decond gen. devices (MCP's).
Using photoelectric cells is a good idea, especially for accurately
timing bright fireballs. I remember a talk I had with Hans Bethlem from
the DMS in July. They operated such systems some years ago but not
anymore, since now they get the timing in a different way. Those systems
have been much more sensitive than discussed in this list. Hans said,
that you could even push the sensitivety up to meteors of +3 mag or so.
However, for determining zenith rates or flux rates, I would prefer video
systems... ;-)
They are simply much more objective (no calibration problems, no 'response
funtion' depending on the direction your are looking at, etc.), and they
aren't as expensive anymore! A single camera from our last series costed
less than $800 (see the last WGN issue or IMOs WWW video pages for details).
Finally one word about the new Visual Handbook of IMO. I find it a
wonderful source of information for meteor observers. I bought it
just when it came out, and even though I had already much experience in
meteor astronomy, I found many new aspects and things I didn't know
before in that book. By the way, I do not get any money for this ad ;-),
but I really appreciate this book. Just a warning at the end: There were
not so many new books printed from the last edition, and many of them are
already sold...
So long, Sirko
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* Sirko Molau * __ *
* Str.246 Nr.16 * " 2B v 2B " *
* D-13086 Berlin * *
* smo@informatik.tu-chemnitzdot de * Shakespeare *
* http://www.tu-chemnitzdot de/~smo * *
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