[Prev][Next][Index][Thread]

RE: (meteorobs) Limiting magnitudes and more



At 12:34 14-12-96 UT, you wrote:
>Hello everybody,
>
>
>Great to see all those replies and discussions, both via Meteorobs and in
>private mails, about ZHR-calculations, correction factors etcetera! This is
>actually what I like about Meteorobs: that it is lively and people respond
>to each other, and that constructive debate is possible.
>
>To some of you, these discussions might be all abracadabra. Don't bother
>about that. This is something for freaks (and freaks are odd, as you know.
>They need a psychiatrist). But still, if you try, it is all less esoteric
>than it sounds, and it concerns things fundamental to meteor science,
>though you can perfectly enjoy your hobby without it, let me add. So don't
>bother if you think you don't understand, or if you think that it just
>doesn't interrest you.
>
>There are two contributions to the discussion to which I want to answer in
>particular: that of Rainer Arlt and of Jim Richardson. Both were very
>constructive and I enjoyed them very much.
>
>I fully agree with most of Jim Richardson's statements, also his statements
>about the nature of science (this is riding my hobby too). Indeed, in
>science all 'truths' are provisional and models are not reality itself.
>'Objectivity' does not exist, it is all a matter of conviction, culture and
>attitudes (more about that later, in my reply to Rainer actually). So I
>should have been more carefull in my statement, though I also think that
>Jim takes it a little bit different than it was meant. What I wanted to
>make clear is that in my opinion the problems Norman touched upon in his
>critique on Joseph were in this particular case not so much due to the
>nature of the limiting magnitude correction itself, but to other elements
>in the reduction procedure, most notably perception differences and a wrong
>applyance of a particular element in the Lm correction (the r-value). This
>is something different than to say that the Lm correction equation itself
>is beyond discussion, though I admitt that my statement can be read that
>way.
>
>There is one particular passage in Jim's response that I don't quite get
>and in which he, I think, might misinterpret some things. Jim wrote:
>
>> "This brings up one of the problems which Norman pointed out. If your
>limiting magnitude is 6.5 or less, than the correction factor is 1.0 or
>greater. If you have extremely dark skies, as he does, then the correction
>becomes less than 1 and reduces the total number of meteors seen to match
>the standard conditions. Marko [sic!] corrected for this effect by assuming
>[sic!] that Norman had less than standard perception, multiplying the total
>by 2.5 to bring the number back up again."
>
>Now, I don't see the point about the fact that for Lm larger than 6.5 the
>correction REDUCES the number of meteors. Because that is what it actually
>SHOULD do indeed! With the radiant in zenith and being the 'average'
>observer, you SHOULD see more meteors than the ZHR with Lm above 6.5. The
>reason that you don't in most cases, is that you seldom have the radiant in
>the zenith. Also, it is certainly not true that I corrected for THIS when I
>applied that Cp-factor amounting to a factor 2.5 upgrading to Norman's
>data. I should have applied that same 2.5 factor when he had Lm 6.0, so a
>Lm lower than 6.5. Also, I didn't 'assume' this factor 2.5. I INFERRED it
>(calculated it) from calibrating his sporadic rates to match those of the
>'median' observer observing with a similar Lm. So it has a base, in his
>data. Rainer already commented that IMO found a similar result for Norman's
>data. It shows that Norman, as every other observer, has a SYSTEMATIC
>difference in observed numbers compared to the 'median' observer. This
>systematic difference should be more or less independant of the Lm (though
>there are some pittfalls in this, which I will not discuss now for reasons
>of space): it is the same with Lm 6.0 or 7.0. And it is for this that I
>corrected, not for the reduction due to Lm larger than 6.5 'in order to
>bring the number back up again' as Jim commented. In this aspect, I feel
>that Jim might use the term 'perception' different from what I do in his
>contribution. In the way I (and Peter Jenniskens) use it, it has nothing to
>do with shifting detection probabilities for magnitude classes with
>shifting Lm, but with systematic differences between observers. Be carefull
>not to confuse things up in this respect.
>
>I fully agree that many things can be improved, also concerning r-value
>determinations. But for the time being we have to work with the current
>methods. Also, I don't see things so pessimistic as Jim. Peter Jenniskens
>recently correlated visual and video magnitude distributions to obtain r-
>values (for the alpha Monocerotids and Quadrantids) and they were well in
>line. Also, there are other techniques for establishing r-values than
>probability functions (and for establishing detection probability functi-
>ons).
>
>To turn back to the original subject: I think (and this is what I meant in
>my comment to Norman) that there are elements in the reduction procedure
>which are more uncertain and are more messing up the results than the Lm-
>correction that Norman deemes the cullprit: for example the radiant
>altitude correction (another cullprit Norman rightly deemed), but also
>systematic under- or overestimates of the Lm by some observers and certain-
>ly the whole issue of observer perception. Rainer already made some very
>valuable remarks in his contribition regarding radiant altitude correction,
>so I won't go into that again. But I do want to add something about
>observer perception:
>
>There ARE differences in perception between individual observers (put two
>obsevers together at one site and you'll note) which are in principle Lm
>independant. They are roughly systematic and I think you therefore can and
>should correct for them. One means of doing this is by using the sporadics,
>because you can reasonably assume that there is a correlation between your
>observed 'fraction' of sporadics and streammembers compared to the 'median'
>observer. There are other ways too, of which Rainer advocates the combina-
>tion of many observers to counter perception effects. Largely, this is a
>matter of conviction and scientific culture and there probably is not one
>'sacred' way of doing it. But personaly I have some doubt if the power of
>statistics is in sheer numbers, as Rainer argues. Me thinks some idea and
>control of the quality of your data (by normalizing for effects known to
>influence them as good as possible) is more important than sheer numbers.
>Anyway, I don't think the 'right' values pop up by combining many obser-
>vers, because of the thing put forward by Peter Jenniskens in his paper on
>annual stream activity (A&A 287 (1994), 990-1013): he notes 'a skewness of
>the Cp to high values'. Anyway, Rainer already notes that a scarceness of
>data can cause problems. Systematic differences between different parts of
>a global activity profile due to group-based perception differences become
>particularly risky. This is one of the reasons why I (and Peter) have so
>little fate in minute, small scale detail in activity curves (subtle sub-
>peaks etcetera) and tend to talk more in the line of large scale profile
>trends in our analysis.
>In addition, I want to make clear that I by no means pretend that the DMS
>system of accounting for perception differences is fool-proof or the
>ultimate 'all time all weather' solution. They do not perfectly align
>results (Jenniskens states that they alligns results to 'roughly within
>20%'), but when applying them you can just note that the scatter indeed
>becomes considerably less which for me is a sign that things work at least
>to some extend, which is better than nothing.
>
>Best wishes to everybody,
>
>
>Marco Langbroek
>(the Netherlands)
>
>
>PS: To be fair, problems with using the sporadic rates for calibration
>might be for example:
>
>- differences in r-value between sporadics and stream meteors. This becomes
>a concern when an observer under- or overestimates his Lm. On the other
>hand: while not fully correcting it, our Cp system takes of the sharpest
>edges of any systematic under or overestimate of Lm by an observer!
>- It might be that a faint meteor coming from a radiant has more chance to
>be judged as a genuine meteor than a faint sporadic. This is psychology
>involved. It leads to different perceptions for sporadic and stream
>meteors.
>
>(note to Rainer: I think a similar effect, though the other way round,
>caused that 'hick-up' in my sporadic rates in my Nov. 17 Leonid data. When
>you positively see many faint meteors coming from a radiant, you might be
>more easily convinced that that other faint flash that did not come from
>the radiant was a genuine (sporadic) meteor too. I think this might have
>triggered my high sporadic detection in that sample of data).
>
>
>*--------------------------------------------------------------------------*
>|     Casper ter Kuile, Akker 145, NL-3732 XD De Bilt, the Netherlands     |
>|    Phone: (31)-30-2203170; Fax: (31)-30-2202695; GSM: (31)-6-54723974    |
>|                         E-mail: pegasoft@cc.ruudot nl                       |
>|         DMS-website: http://www.pidot net/~terkuile/meteors/dms.htm         |
>*--------------------------------------------------------------------------*
>
>
>
>What is the correction factor formula?
>
>Jonathan
>


This is great!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


-Marco

*--------------------------------------------------------------------------*
|     Casper ter Kuile, Akker 145, NL-3732 XD De Bilt, the Netherlands     |
|    Phone: (31)-30-2203170; Fax: (31)-30-2202695; GSM: (31)-6-54723974    |
|                         E-mail: pegasoft@cc.ruudot nl                       |
|         DMS-website: http://www.pidot net/~terkuile/meteors/dms.htm         |
*--------------------------------------------------------------------------*