[Prev][Next][Index][Thread]
Re: (meteorobs) A view of Perception Coefficients from 3 years ago...
Hi Lew,
I do not know what is the "prevailing" view on Cp's. However, we employ Cp's
in calculating individual ZHR's since the middle '80-ies within DMS. And in
general, I feel employing Cp's is helpfull in getting better alligned
results (in the sense of: internally in the dataset). Note however that we
calculate Cp's distinctly different within DMS then the way Rainer wrote
you - but the results on observers for which both Rainer and I tried to
determine a Cp are the same, indicating that both ways yield apparent valid
and results, and consistent with each other. And that there is some grain of
reality in the method at least. Our way of calculating a Cp is that
described by Peter Jenniskens in Astron. Astroph. 287 (1994), 990-1013. We
basically calibrate observers against the sporadic background, using a large
sample of sporadic meteors over multiple nights for that purpose. Our
"standard" Cp=1.0 observer is defined as one that would see 10
sporadics/hour around local midnight in mid August with an Lm at +6.5. All
Cp's are calibrated towards that standard (which is purely arbitrary by the
way). NB: please note that Jenniskens' paper has a typoe in the ZHR formula
with regard to employing the Cp: it should be multiplying with1/Cp, not Cp.
Note that the Cp correction is a tool to REDUCE scatter between the results
of individual observers, but NOT one that reduces scatter due to perception
differences to zero! That is a goal which is unattainable. Yet, with Cp's
employed, the scatter is clearly more limited and the results internally
become more consistent. The positive effect of it is that when the make-up
of the observer population would be assymmetric with regard to perceptions
(meaning e.g.: a large number of very perceptive observers, artificially
"upgrading" observed rates when not corrected for, or vice versa), the Cp
largely corrects the negative side effects of that - merely averaging
without Cp's employed in such a situation would introduce a systematic error
to either the lower or higher side on the ZHR, which becomes notably
problematic when creating a profile with a heterogenous observer population
over time, creating artificial jumps in rate level if not taken into
account.
One drawback of Cp calculations and employment which I feel to be at work,
is that the method is not well suited for data of novice or casual observers
(casual meaning: those who observer only a small amount of hours per year,
e.g. during the Perseid maximum only). Perception is both a function of eye
quality, but also to some extent of experience. When a novice observer gains
experience usually his Cp will increase quickly. Only after some time it
stabilizes and if he keeps up his experience by observing regularly, his Cp
will be stable enough to employ it over a longer time. Note that over the
course of many years a Cp can gradually change, e.g. due to aging of the
observer which usually has some effects on the eyes as well.
- Marco Langbroek (DMS)
To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html
References: