How do I - Magnitudes of DSO's?

What is generally more useful is what is known as the mean surface brightness
figure, usually quoted in "magnitudes per square arc minute". This is
generally calculated by a formula based on the angular size of the galaxy down
to some brightness level ("isophote"), which allows one to calculate the
surface area.

I think I more or less understand now how it is calculated, and
perhaps understand why. My big questions now a

1. "Magnitudes per square arc minute." Does this mean that something
with a surface brightness of 12.5 would have an average magnitude of
12.5? Or that it's average over any square arc minute would be be an
average of 12.5? In other words, is the "per square arc minute" a sum
of the total brightness in that area concentrated into a point source
or does that area glow with that magnitude?

2. I think I know the answer to this one but... A difference of 1 in
Surface Brightness is the same as a difference in 1 of magnitude
(2.5x)?

Thanks for your help.

Clear, Dark Skies

Mark

Edward Smith asked:

1. "Magnitudes per square arc minute." Does this mean that something
with a surface brightness of 12.5 would have an average magnitude of
12.5? Or that it's average over any square arc minute would be be an
average of 12.5? In other words, is the "per square arc minute" a sum
of the total brightness in that area concentrated into a point source
or does that area glow with that magnitude?


The object would have an average brightness over its entire area of magnitude
12.5 in an area of one square arc minute. This average brightness level would
be about like taking a magnitude 12.5 star and defocusing it until it was a
dim uniform disk of light 1.128 arc minutes in diameter (ie: a disk with the
area of one square arc minute). Of course, most galaxies aren't very square
and they have widely varying surface brightness, so the figure for surface
brightness will be an average over the entire galaxy. However, the surface
brightness figure is sometimes a more useful indication as to the visibility
of a galaxy than the galaxy's magnitude alone. There are some simple ways of
getting the average surface brightness, but the most accurate ones are a bit
more complex than just dividing a galaxy's magnitude by its area.

2. I think I know the answer to this one but... A difference of 1 in
Surface Brightness is the same as a difference in 1 of magnitude
(2.5x)?

Yes, the units are in magnitude, so if two objects have the same area and
differ in mean suface brightness by one magnitude per square arc minute, the
brighter one would probably be close to 2.5 times brighter than the fainter
one. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Edward Smith asked:

1. "Magnitudes per square arc minute." Does this mean that something
with a surface brightness of 12.5 would have an average magnitude of
12.5? Or that it's average over any square arc minute would be be an
average of 12.5? In other words, is the "per square arc minute" a sum
of the total brightness in that area concentrated into a point source
or does that area glow with that magnitude?


The object would have an average brightness over its entire area of magnitude
12.5 in an area of one square arc minute. This average brightness level would
be about like taking a magnitude 12.5 star and defocusing it until it was a
dim uniform disk of light 1.128 arc minutes in diameter (ie: a disk with the
area of one square arc minute). Of course, most galaxies aren't very square
and they have widely varying surface brightness, so the figure for surface
brightness will be an average over the entire galaxy. However, the surface
brightness figure is sometimes a more useful indication as to the visibility
of a galaxy than the galaxy's magnitude alone. There are some simple ways of
getting the average surface brightness, but the most accurate ones are a bit
more complex than just dividing a galaxy's magnitude by its area.

2. I think I know the answer to this one but... A difference of 1 in
Surface Brightness is the same as a difference in 1 of magnitude
(2.5x)?

Yes, the units are in magnitude, so if two objects have the same area and
differ in mean suface brightness by one magnitude per square arc minute, the
brighter one would probably be close to 2.5 times brighter than the fainter
one. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"David Knisely" wrote in message
...
The object would have an average brightness over its entire area of
magnitude
12.5 in an area of one square arc minute. This average brightness level
would
be about like taking a magnitude 12.5 star and defocusing it until it was
a
dim uniform disk of light 1.128 arc minutes in diameter (ie: a disk with
the
area of one square arc minute). Of course, most galaxies aren't very
square
and they have widely varying surface brightness, so the figure for surface
brightness will be an average over the entire galaxy. However, the
surface
brightness figure is sometimes a more useful indication as to the
visibility
of a galaxy than the galaxy's magnitude alone.

David,

Just to make sure I can relate this to something I know... is this the
reason the Helix nebula is so dim, even though it is huge (and relatively
close). Because its surface brighntess spread over a really wide area?

"David Knisely" wrote in message
...
The object would have an average brightness over its entire area of
magnitude
12.5 in an area of one square arc minute. This average brightness level
would
be about like taking a magnitude 12.5 star and defocusing it until it was
a
dim uniform disk of light 1.128 arc minutes in diameter (ie: a disk with
the
area of one square arc minute). Of course, most galaxies aren't very
square
and they have widely varying surface brightness, so the figure for surface
brightness will be an average over the entire galaxy. However, the
surface
brightness figure is sometimes a more useful indication as to the
visibility
of a galaxy than the galaxy's magnitude alone.

David,

Just to make sure I can relate this to something I know... is this the
reason the Helix nebula is so dim, even though it is huge (and relatively
close). Because its surface brighntess spread over a really wide area?

Kilolani posted:

Just to make sure I can relate this to something I know... is this the
reason the Helix nebula is so dim, even though it is huge (and relatively
close). Because its surface brighntess spread over a really wide area?

Yes, the Helix is perhaps nearly tied with M27 for the brightest planetary
nebula (total brightness), but it has a rather low surface brightness due to
its very large size. Still, I have seen the Helix fairly easily without
filters in a pair of 7x35 binoculars, so while its surface brightness isn't
exactly stunning, its still a bright planetary. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Kilolani posted:

Just to make sure I can relate this to something I know... is this the
reason the Helix nebula is so dim, even though it is huge (and relatively
close). Because its surface brighntess spread over a really wide area?

Yes, the Helix is perhaps nearly tied with M27 for the brightest planetary
nebula (total brightness), but it has a rather low surface brightness due to
its very large size. Still, I have seen the Helix fairly easily without
filters in a pair of 7x35 binoculars, so while its surface brightness isn't
exactly stunning, its still a bright planetary. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"David Knisely" wrote in message
...
Kilolani posted:

Just to make sure I can relate this to something I know... is this the
reason the Helix nebula is so dim, even though it is huge (and
relatively
close). Because its surface brighntess spread over a really wide area?

Yes, the Helix is perhaps nearly tied with M27 for the brightest planetary
nebula (total brightness), but it has a rather low surface brightness due
to
its very large size. Still, I have seen the Helix fairly easily without
filters in a pair of 7x35 binoculars, so while its surface brightness
isn't
exactly stunning, its still a bright planetary. Clear skies to you.

Interestingly I have the same experience, in that it much easier for me to
find in binoculars than any other way. Although it seems that both (or
should I say all three, counting Caroline) Herschels missed it, so I assume
it's because they weren't expecting anything quite so large... or its size
and surface brightness make it relatively tricky in large scopes.

"David Knisely" wrote in message
...
Kilolani posted:

Just to make sure I can relate this to something I know... is this the
reason the Helix nebula is so dim, even though it is huge (and
relatively
close). Because its surface brighntess spread over a really wide area?

Yes, the Helix is perhaps nearly tied with M27 for the brightest planetary
nebula (total brightness), but it has a rather low surface brightness due
to
its very large size. Still, I have seen the Helix fairly easily without
filters in a pair of 7x35 binoculars, so while its surface brightness
isn't
exactly stunning, its still a bright planetary. Clear skies to you.

Interestingly I have the same experience, in that it much easier for me to
find in binoculars than any other way. Although it seems that both (or
should I say all three, counting Caroline) Herschels missed it, so I assume
it's because they weren't expecting anything quite so large... or its size
and surface brightness make it relatively tricky in large scopes.

Kilolani wrote:
Interestingly I have the same experience, in that [the Helix nebula] is
much easier for me to find in binoculars than any other way. Although
it seems that both (or should I say all three, counting Caroline)
Herschels missed it, so I assume it's because they weren't expecting
anything quite so large... or its size and surface brightness make it
relatively tricky in large scopes.

Size was probably the deciding issue. Sir William Herschel's 18.7-inch
telescope typically operated at 157X, which produced a 15' diameter true field
of view. The Helix (NGC 7293) is about 16' in diameter.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
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