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Re: (meteorobs) 1st Attempts at logging data, QUESTIONS
[This is another LOOOOOONG one - sorry! :>]
Howdy, John - I wanted to say *congratulations* on making that first attempt at
logging what you see! From those two short nights, you've now learned more
about meteor observing than 99% of the amateur astronomical community knows -
let alone the rest of the world.
Also, thanks for posting some GREAT questions to the list: you'll probably hear
soon from our most experienced readers. In the mean time though, I'll try to
answer your questions too...
LIMITING MAGNITUDE:
First off, your question about measuring Limit Magnitude is a tricky one. To
summarize John's question as I understood it: he's been trying to count the
number of stars he sees inside some of IMO's "Star Areas". From these counts,
IMO tables then allow him to read off his all-important Limiting Magnitude
("LM") for a session. Performing counts in one or more of these 30 little areas
of the sky is *the* accepted IMO method for measuring LMs.
My personal counting technique (based loosely on long-ago advice from Norman
McLeod and more recent advice from George Zay), is to only count those stars
inside the area which I can see MOST of the time. In other words, count all
stars I see "normally" with direct vision, and also count any stars that I see
consistently with averted ("peripheral") vision, but ALSO count all stars which
I can perceive *at least 50% of the time* with averted vision!
SHOWERS TO WATCH:
Regarding what showers to watch for, I suggest at first that you just try to
identify meteors from ONE major shower at a time; then after a little observing
experience, try adding more showers to your "watch-list" each night. Just my
opinion, though: more experienced observers may disagree!
SHOWER ASSOCIATION:
Your question about how to tell which meteors are and aren't Perseids is a good
one! The three key factors to always keep in mind when associating a particular
meteor with a shower are: trace back to the radiant, apparent relative speed,
and meteor trail length versus radiant distance. It's hard to keep all this in
mind at first - the thrill of seeing a meteor may overcome your attempts to
remember all three criteria. But with practice, it's really not hard!
These three criteria are all explained in detail in the "NAMN Observing
Handbook" (available on-line or from Mark Davis), or George Zay's new booklet
(email George about this). But the basic idea is that a meteor radiant has a
certain (often small) size: getting a meteor to aim back to the entire
constellation is usually not good enough. And of course, that radiant circle
will be on the move every night! That's why (when counting) you need to know
the radiant's EXACT position in the sky each night you watch the shower.
For the Perseids for example, keep in mind that right up into early August, the
radiant will actually appear closer to the "W" asterism in the constellation
Cassiopeia, than to the "hero" figure of Perseus!
Second, your meteor has to match the shower's expected SPEED, relative to other
meteors: e.g., the Perseids are considered a "fast" shower. If you see a meteor
that lines up well with the radiant, but still appears slower than the average
for most meteors, you have to disqualify it as a Perseid!
Finally, your meteor can't be too LONG: if the meteor's trail (path) starts
just 10 degrees from the radiant area, and the meteor is CONSIDERABLY more than
5 degrees long, you ought not to count it as a member.
CENTER OF FIELD OF VIEW:
On what area of the sky to watch in, there are many opinions, John. But the
general rule *I* use (again based on advice from this list), is to choose a
central point to look that's at least 50 degrees above the horizon (ideally
about 70o), and gives you some reasonable distance (20-60 degrees) from all the
shower radiants that you want to watch that night. But you're already doing the
MOST important thing: you're LOGGING where you center your view!
VALUE OF WHAT YOU DID:
Last but MOST important, keep in mind that even if you *don't* feel like you
applied all of the above in your most recent Perseid watch, your observing was
EXTREMELY valuable - to you! You might try reporting a log of what you saw to
Mark Davis of NAMN (he's "MeteorObs@Charlestondot net" to his friends). He's a
mighty nice guy, and should have even more constructive comments. But even more
important than producing some NAMN reports, you began the process of learning!
And perhaps most significant of all, you've begun experiencing the real joy
that can only come from coupling thoughtful scientific observation with a love
of the night sky... :)
Keep the above (and other responses you get) in mind next time you try watching
meteors: then, if you run into problems, write down your questions or put them
on tape, and post more on 'meteorobs'! That's EXACTLY how many of us learned to
record meteors - and it wasn't TOO painful after all. :>>
Clear skies and many (easy-to-record) meteors, John!
Lew
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