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Re: (IAAC) How Bright are the Galaxies - Really?
This was Chris Marriot's response vis Skymap
Pro 8 use of the Revised NGC Catalogue
Brian said
"It looks as though Marriott has (mistakenly?) adopted blue
magnitudes
rather than V."
Chris
replies,
"All that I've done - needless to say! - is to use the Revised NGC
catalog.
What that catalog contains is completely beyond my
control.
It might be interesting to put the points raised by Brian to
Wolfgang
Steinicke, the compiler of the catalog, and see what he has to
say."
Regards,
Chris
----- Original Message -----
Sent: Friday, December 07, 2001 1:36 AM
Subject: Re: (IAAC) How Bright are the Galaxies -
Really?
Here is a very helpful reply from Brian Skiff on the topic of
"what magnitudes
mean to the galaxy observer", and specifically on naked-eye
visibility of M81.
Enjoy folks, and clear skies this weekend!
Lew
Gramer
------- Forwarded Message
Date: Thu, 6 Dec 2001
13:21:48 -0700 (MST)
From: Brian Skiff <bas@lowell.edu>
To: dedalus@latrade.com
Subject: Re: (SNES)
How Bright are the Galaxies - Really?
>> SkyMap Pro, is now
available. Version 8...
>> I find the numbers for M81 to be of
particular interest (6.9 vs. 7.8)...
It looks as
though Marriott has (mistakenly?) adopted blue magnitudes
rather than
V.
>> Overall I find the new higher magnitudes to be much more
realistic.
>> M33, as an example, makes much more sense at magnitude
6.2 than at 5.7...
[Here we] are confusing total
magnitudes of extended objects with what
is perceived. Simply lowering
the total brightness to conform with the
perceived
brightness-plus-surface-brightness impression won't work since
galaxies vary
in mean surface brigntness. You can of course adjust different
galaxies
by different amounts to conform to what you think you see, but
that isn't
very helpful. You need to consider both total brigntness and
mean
surface brightness and most importantly how the eye detects stuff at
low
light levels, largely a contrast phenomenon. In other words it's has
as
much to do with brightness _differences_ as with light-levels per
se.
All this stuff is gone over in excruciating
detail by Nils Olaf Carlin
at Mel Bartels' Web site starting here:
http://www.efn.org/~mbartels/aa/visual.html
When
you read the numbers out of a catalogue---assuming they are really
total V
magnitudes and mean surface brightnesses (in units of mag/square
arcminute
usually), you will simply have to adjust your personal scale
of "this mag/sfc
br means about this difficult to see". Despite the
apparent conflict, I
would adjust your scale to V-sub-T, i.e. total V
magnitudes, not to B or some
other (particularly non-total) magnitudes.
Because most people fail to take
this perception effect into account,
recent reports about Comet
LINEAR's brightness are also too faint.
Comparing the comet to open clusters
(having bright backgrounds) to the
comet in another part of the sky, where
the natural and local backgrounds
are different will almost guarantee the
magnitude estimate will be wrong.
>> Doing a search for M81's
"photometric data points"...
That's the place
too look. Note that the NED header pages for an
individual galaxy
usually list the total B magnitude, or some other blue-light
magnitude that's
not necessarily an integrated value. This isn't specified
and is a
known problem, as it were. One of these days we'll get to
fixing
that.
>> ...inclusion of the revised New General Catalog
(NGC) database,
>> published earlier this year...
>>
...which Doug mentions from the revised NGC...
There is no such revision that I know of. What is the
source?
>> I'd imagine that for those
>> folks trying to
compile databases for amateur use, this must lead to
>> all kinds of
nasty confusion...
Right, especially if they
don't know anything about photometry!
>> Is it possible that NED
has simply
>> not been updated with more recent total V magnitude
measures for M81...?
It's been a couple of
decades since anyone actually measured M81 for
total brightness. The
numbers are bascially unchanged since the RC2 from
1976. Same goes for
nearly all of the brighter few thousand galaxies,
whose magnitudes (in
general) are well in hand at the couple-tenths of a
magnitude
level.
>> ApJ costs a few
thou/year!
"Only" $1500/year, and papers more
than two years old are available
for free on-line back to issue number 1 in
1895.
>> ...to magnitude estimates in OTHER BANDS such as 'I', 'V',
etc...
>> In other words,
>> all the recent research I could
find only relates to how bright
>> the galaxy M81
>> would
appear in infrared, ultraviolet, x-ray or other non-amateur
telescopes!
Just ignore all that stuff.
Again, no one has measured M81 in the
manner of interest in a long
time.
>> I suspect the V magnitude value of "about 6.9" may have to
stand...
The figure from RC3 is Vt = 6.94 +/-
0.03. That uncertainty is a
formal one, and in fact total magnitudes of
galaxies seem to have a minimum
error of around +/- 0.15 or
thereabouts. But, yes, V=6.9 will stand until
someone shows that the
raw data that went into that integrated value is
somehow wrong---not likely,
since the data were compiled from many mutually
consistent
sources.
>> And what *is* the "experienced
>> amateurs'"
consensus on M81....
You mean about its total
magnitude? Doesn't matter, since there is
real data to go
on.
>> ...and in particular its naked-eye
visibility?
Several observes have found it
reliably, particularly indicating in
their reports the process of sorting out
the neighboring mag. 7 and 8 stars
in order to isolate the galaxy. Most
who attempt this don't make this
effort, so their reports aren't
reliable. I've copied out an old s.a.a.
post about finding M81 to the
Lowell ftp area:
ftp://ftp.lowell.edu/pub/bas/deepsky/m81.msg
So basically folks need to calibrate their eyeballs a little more
carefully
to account for surface brightness. Read the Carlin/Bartels/Clark
stuff
at Mel's Web site. OBserve galaxies noting their apparent
difficulty
as compared to the combination of total brightness and the
mean surface
brightness.
\Brian
------- End of Forwarded Message
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