[Prev][Next][Index][Thread]
Re: (meteorobs) Re: The Leonid Meteor Outburst of 1997
Hi Tony,
thanks for your discussions on this topic.
> While it is certainly true that the field of view of the human eye is
> greater than that of the video cameras we discussed, the low-light video
> cameras are far more sensitive to the quantity "meteors per steradian."
> Consider the geminid shower you observed. If you divide the number of
> meteors you counted visually by the field of view in square degrees, you
> will probably find that it is a lower number than the same ratio for the
> video camera.
That's no wonder since the limiting magnitude of the video camera was
much better. Dividing the number of meteors by the size of the fov can
only be done when the limiting magnitude is the same. Then I agree that a
video system will be superior, since it has no 'dropouts' - it records
everything (you only have to find the meteors on the tapes, which may
bring you back to the original problem - visual meteor detection :-).
> Of course, in the case of showers that might be dominated by
> bright meteors (the 1998 Leonids perhaps?) the human eye could win by
> virtue of its greater field of view. When the magnitude distribution is
> dominated by faint meteors, a video camera with 6+ limiting magnitude and a
> reasonable field of view is likely to be superior.
You have to think about which meteors you are interested in. Most
visual meteor showers (not just the Leonids) have population indices well
below 3, which is the r-value for the sporadics. That is, they are more or
less obvious in the visual magnitude range, but they get almost lost in
the sporadic backgroud once you come to magnitudes +6, +7 or +8.
Hence, it's the large field of view of visual observers that counts for
most meteor showers, not the better sensitivity of small-angle video
cameras.
You can modify the behaviour of a video system easily by changing the
lens. When I put an 2.8/20mm wide angle lens at my camera, the fov is
about 60 deg with lm near 5 mag. My 2.0/35mm lens gives an fov of 40 deg
with lm near 6 mag. Finally, with the standard lens of my camera
(0.75/65mm) I achieve about 8.5 mag at 20 deg fov. Thus, I can record in
many different magnitude ranges. However, I made the experience that the
overall number of meteors recorded per time interval is in the same
order regardless of the lens, since the loss in fov levels out the gain in
lm.
Again, if you do not look at the overall meteor numbers but at 'ordinary'
shower meteors (not telescopic showers) shorter focal lengths give more
detections. Even at the 1996 Geminids with visual ZHRs well above 100 the
percentage of non-Geminids on the video tape is relatively large, whereas
the sporadics are virtually non-existant in the visual data.
Regards,
Sirko
--
**************************************************************************
* Dipl.-Inform. Sirko Molau * *
* RWTH Aachen, Lehrstuhl fuer Informatik VI * __ *
* Ahornstr. 55, D-52056 Aachen, Germany * " 2B v 2B " *
* * *
* phone: +49-241-8021615 * Shakespeare *
* fax : +49-241-8888219 * *
* email: molau@informatik.rwth-aachendot de * *
**************************************************************************
* www : http://www.informatik.rwth-aachendot de/I6/Colleagues/molau *
**************************************************************************
To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html
Follow-Ups:
References: