[Prev][Next][Index][Thread]
(meteorobs) Limiting magnitude question
Hi!
I've read some reports of extreme naked eye deep sky object
sightings and a pronounced difference between limiting magnitudes reported
by meteor observers and deep sky observers struck me. Most meteor
observers use the IMO star count method to determine their limiting
magnitude. This is done by counting stars in predefined areas. Here the
entire count is usually less than 1 minute in duration (unless your LM is
7.5 and you're counting in area 11...), so you only count the stars you
readily see, Lew suggested a 50% perception threshold. Even from under the
darkest skies reports of LMs exceeding 7.3 or so are relatively rare.
On the other hand there are frequent reports of LMs in excess of
of 7.5, sometimes even 8.0 from deep-sky observers under dark skies. Just
as an example - I chanced upon a naked eye sighting of M81 by Brian Skiff
where he saw several 8.0 magnitude stars and a report from Nebraska Star
Party in 1995 where an observer (forgot his name) saw 8.2 magnitude stars.
Now there is an obvious difference between these limiting
magnitudes. While those determined by IMO method represent 50% perception
level, others represent much lower perception levels. How do these two
methods compare? If someone reports LM 7.5 from say Long Key National Park
and another observer reports LM 8.2 from NSP, does this mean the second
location really has 0.7 magnitude better skies? I think not. Would this
mean thet two classes of limiting magnitudes should be introduced?
Comments? I'm sure this topic is of interest to both meteor and deep sky
observers!
Just a thought: if a meteor observer using IMO method determines LM 7.4,
and another observer using the other method at another location determines
say LM 7.6, does this mean they're both under Bortle Dark Sky class 1 sky?
Is the first one under class 2 and the second under class 1?I guess this
question would best be answered by Mr. Bortle himself : )This is just a
thing that bothers me with the Bortle Dark sky scale.
Clear skies!
Jure A.
To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html