Towards a calibration of meteor counts with the Mintron
video system
Daniel Fischer, Königswinter, Germany
Summary: With the availibility of highly sensitive CCD
surveillance cameras around mid-2002, namely the
Mintron 12V1C-EX, video monitoring of
meteor showers has suddenly become an attractive option for
»the masses«. While not as sensitive as the
image-intensified systems used in the amateur community since ca.
1993, they are cheaper, easier to use and less easy to break. The
question naturally arises how the rate of meteors counted in
Mintron videos compares to visual rates recorded from the same
location as well als to the global ZHR profile calculated by the IMO.
Here preliminary results are presented from two observing runs
during the maxima of the Perseids in 2004 (from Romania) and
2005 (from Germany). On average it is found that the
Mintron-«Video-ZHR«, when the camera is equipped with a 6
mm f/0.8 lens and set to an integration factor of a few (to be able
to use their shortlived wakes for a slight increase in limiting
magnitude for meteors), hovers at about 1/3 of the visual
ZHR on average, regardless of whether the latter is calculated
globally by the IMO or by the DMS only from their
»own« observers. But the ratio IMO/video varies
widely, even for rather long intervals of ½ or 1 hour, raising
many issues ...
The 2004 and 2005 experiments
While the Mintron immediately showed its capabilities during
early experiments by the author in 2002 (during the rising flank
of the Leonids before their last storm) and 2003 (during the peak
of the Perseids at full moon), the first
systematic observing run - with the 6 mm lens - was performed in the
nights of 11/12 and 12/13 August 2004 during the international
meteor camp
by SARM in Romania: The circumstances
and results are described in a detailled report
on the web.
On 11/12 Aug. 2004 the sky conditions at Corbasca were good,
though inititally the field of view had to the moved several times to
avoid passing cloud fields. The next night at Darmanesti was cloud-free but more humid,
leading to frequent fogging problems.
The Perseids of 2005 were a major washout in central Europe,
with the clouds finally clearing less than one hour before dawn at
the author's home in Königswinter, Germany, in the night
of the predicted maximum. In the 20 minutes (!) that could be used
- after scrambling together the equipment - for video
observations, the skies were perfect after all the rain, almost
matching the conditions in Romania, and 8 Perseids were
recorded - not bad since this was already many hours after the
peak. Follow-on observations a few days later when the Perseids
activity had basically collapsed also demonstrated that useful
cross-calibration beween one individual video system and visual
rates is only possible during well-observed meteor showers: Only
4 meteors in total were recorded in one hour, with at most 2 of
them being Perseids.
Visual versus Video: early comparisions
The following table summarizes the 2004 and 2005 Perseids
observations which were binned into ½- and/or 1-hour
intervals which were then compared to ZHR values calculated for
(or could be reliably interpolated for) the middle of the respective
interval. The counts from the videos (done visually, mostly on
large TV screens with the contrast set very high) were only
corrected for the radiant elevation (i.e. multiplied by factors
continuously dropping from 2.4 in the earliest Romanian counts
to 1.1 later in the nights).
Paretheses around the ZHR figures indicate interpolation by me,
double-parentheses a risky extrapolation. »EVZHR« is
the effective video ZHR I got from (visual inspection of) my
Mintron tapes, ZHRIMO and ZHRDMS
are the global ZHRs calculated by IMO and published in the
4th Shower Circular of 2004
(the most complete analysis published so far
by the IMO) and the
1st Shower
Circular of 2005 (the only one distributed so far over the IMO list)
and on the web
site of the DMS, respectively (which is based on 2004 observations
from Western Europe).
Night 11/12 August 2004, Corbasca, Romania
| UTC from - to
| UTC central
| length
| EVZHR
| ZHRIMO
| ZHRDMS
| ZHRIMO/EVZHR
|
| 19:30 - 20:00
| 19:45
| 30 min.
| 24
| 85
| -
| 3.5
|
| 20:30-20:55
| 20:45
| 25 min.
| 72
| 176
| 185
| 2.4
|
| 22:30-23:00
| 22:45
| 30 min.
| 60
| 82
| 70
| 1.4
|
| 23:15-00:15
| 23:45
| 1 hour
| 43
| 105
| 90
| 2.4
|
| 00:30-01:30
| 01:00
| 1 hour
| 28
| (130)
| 100
| 4.6
|
The average ZHRIMO/EVZHR for this night is
2.9, but note the the scatter - and the strange V shape of
the factor as function of time over 6 hours ...
Night 12/13 August 2004, Darmanesti, Romania
| UTC from - to
| UTC central
| length
| EVZHR
| ZHRIMO
| ZHRIMO/EVZHR
|
| 19:30-20:30
| 20:00
| 1 hour
| 15
| 90
| 6.0
|
| 20:00-21:00
| 20:30
| 1 hour
| 16
| (87)
| 5.4
|
| 23:00-23:30
| 23:15
| 30 min.
| 34
| ((65))
| 1.9
|
The extreme fluctuations of the calibration factor - the reasons for
which could be manifold - do not seem to justify the determination
of an average value.
Night 12/13 August 2005, Königswinter, Germany
| UTC from - to
| UTC central
| length
| EVZHR
| ZHRIMO
| ZHRIMO/EVZHR
|
| 2:30-2:50
| 2:40
| 20 min.
| 24
| 75
| 3.1
|
The calibration factor of 3.1 is indistinguishable from the
result of the maximum night one year earlier (based on many
times the number of meteors). And so - ignoring for the time being the strange experience of the
post-max. night of 2004 - I boldly propose the conversion formula
EVZHRMintron, 6 mm, f/0.8 =
ZHRIMO / 3
for the peak night of a major shower and the Perseids in
particular.
The figure shows how the video and the visual ZHR
(multiplied by that correction factor) refuse to fall onto a straight
linear correlation line (or any other obvious mathematical
function) - perhaps plotting the results from the table this way
inspires the search for influencing factors ...
Where next?
The experience outlined above - that the video rate falls only a
factor of about 3 short of the ZHR calculated for perfect visual
conditions, but that the actual conversion factor jumps around
without any obvious reason - calls for further work in many
respects:
- It has to be established - preferrably from statistical first
principles - of which length the time intervals should be in which
the calculated EVZHR is compared to the visual ZHRs. (Here the
choice was largely dictated by the availibility of my own
observations - and by the fact that the ZHR in 2004 was changing
pretty rapidly in the maximum night.)
- The Romanian data of the max. night, covering by far the
most time (3½+ hours of raw data), should be compared to
reliable video and visual recordings from Corbasca or at
least from the region, to see if there are local deviations from the
global ZHR profile (as they existed e.g. during the 1999 Leonids
storm). There must be 2+ good visual records from Romanian
observers near the camera plus one deep video by C. Terkuile - of
which the DMS website only presents only a
plot
of a small part, unfortunately.
- The complete analysis of the IMO visual data for the 2004
and 2005 Perseids data must be obtained - and perhaps average
ZHR values could be calculated explicitly for the time intervals
when the Mintron ran without interruption. Also more precise
»individual« ZHR values from DMS or other
organizations might be of use (in the max. night of 2004,
ZHRIMO/ ZHRDMS was fluctuating
between 0.95 and 1.3, with an average of 1.15, with a much better
correlatin than seen between the individual EVZHR and either visual ZHR).
- The influence of various factors that could influence the
detection efficiency of the Mintron system must be studied: The
changing distance of the field center from the radiant, the
changing integration times, the changing sky quality (from day
to day). Systematic experiments to test this effect would also be
advisable during future good showers, with similar video systems
sitting side by side but with different settings!
- And more data - if simply for better statistics - are needed in
any case. Unfortunately they will not come soon, as a look at the
IMO Calendars for 2005 and
2006 shows that the next moon-free
major shower (well-placed for Europe at least) will not come until
the 2006 Geminids! And these meteors from an asteroid-like
body, lacking significant wakes and looking very differently to the
Mintron system (as a minor test with the 2004 Geminds had
revealed), perhaps cannot even be compared directly to the
comet-born Perseids ...
And thus the road to fully calibrated video meteor rates that could
be fed directly into the calculation of universal ZHR profiles will
probably be a pretty long one. (The same ist true, by the way, for
comet magnitude estimates for which still no rule is known to
turn CCD in to visual photometry!) But it will thus be a motivation
to continue experimenting with the new technology in many
years to come and will keep us busy long after the exciting Leonids
years (which came just a tad too early for the Mintron) have come
to an end ...
Daniel Fischer, Im Kottsiefen 10, D-53639 Königswinter,
Germany,
dfischer@astro.uni-bonn.de
This poster - with many hyperlinks! - is available at
www.astro.uni-bonn.de/~dfischer/mintron/perseids.html
(where also a link to a more complete paper for the proceedings
will appear)