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Mr Q
sage
Reged: 02/25/08
Posts: 351
Loc: N Central New Mexico
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Looking through several DSO lists, I find that some use surface brightness magnitudes computed from square arc seconds and others square arc min. Is there a simple way (mentally) to convert the two; mag./sq. arc sec to mag./sq. arc min without any mathematics?
Since I usually observe faint extended objects 1.5'and larger, I'd like to convert the surface brightness mag. listings in arc sec to arc min. If I understand this right,professional astronomers generally use arc sec and amateurs use arc min in computing surface brightness figures. But some lists I've seen use arc sec and I would like to convert the S.B. mag figures to reflect sq. arc min. without any math; just a ballpark figure to the nearest magnitude. Of course I would use the S.B. mag./sq. arc sec for objects smaller than one arc min. in size; planetaries, etc.
Does anyone know what format the NGC/IC and S.A.C. databases use for surface brightness? I've checked their "field" and column notes/descriptions and neither says whether the S.B. mags. were based on arc sec or arc min.
Any help will be much appreciated
 Mr Q
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What goes around, comes around, eventually.
Meade DS-10(10" newt)
10x50, 10x70 binos
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timokarhula
member
Reged: 01/30/06
Posts: 87
Loc: Sweden
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Quote:
Looking through several DSO lists, I find that some use surface brightness magnitudes computed from square arc seconds and others square arc min. Is there a simple way (mentally) to convert the two; mag./sq. arc sec to mag./sq. arc min without any mathematics?
No, I'm afraid you must use mathematics! Just subtract with 8.89 to convert the surface brightness in magnitudes per square arc-second to magnitudes per square arc-minute, exactly. To convert to magnitudes per square arc-second, add 8.89. 
I'm pretty sure the surface brightness (S.B.) units are given in mag./sq.minute. Almost all galaxies have their average S.B. between 10 and 15 mag./sq.minute. If the values are fainter, the units must be mag./sq.sec. Small planetary nebulae can have considerably brighter S.B. than 10 mag./sq.minute.
/Timo Karhula
Edited by timokarhula (10/22/08 09:18 AM)
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astrokido
space wanderer
Reged: 06/09/08
Posts: 661
Loc: Phoenix, AZ
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I've been reading the recent SB posts here and have to admit I really like some of the points brought up by several members. This one I really want to know though: what subsitute for mathematics did you have in mind? That would be a dream come true for so many, tell us, please!
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- Gill C. - Celestron Cometron CO-100, 10x25, 20x80, Binochair, Nikon D40
The Night Sky Atlas: www.nightskyatlas.com
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Mr Q
sage
Reged: 02/25/08
Posts: 351
Loc: N Central New Mexico
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Timo Thanks for the conversion info. It's just what I was looking for; something simple to use in database lists. Now all I have to do is determine what each list uses - arc min or arc sec. I would assume that for galaxies any list would use arc min due to their sizes and arc min for smaller objects such as planetaries.
Astrokido - Timo came up with the solution to my problem of what value to add or subtract from the list's S.B. mag. figure.
Now all I have to figure out is what the S.A.C and NGC/IC lists use. Does anyone know? Does each list contain both, depending on the object in the list?
Another problem that confuses the matter is some lists have "peak" and "average" S.B. mag. values (for cores of galaxies and for the whole photographic extent of the galaxy).
Thanks Timo for half solving my problems! Mr Q
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What goes around, comes around, eventually.
Meade DS-10(10" newt)
10x50, 10x70 binos
Edited by Mr Q (10/22/08 02:10 PM)
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3463
Loc: Cambridge, MA, USA
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Quote:
Now all I have to do is determine what each list uses - arc min or arc sec.
It's pretty easy to tell just by looking, because 8.9 is such a huge difference. The ranges barely overlap -- the mag per square arcmin of even the faintest galaxy is going to be a lower number than the mag per square arcsecond of 99.9% of all DSOs.
Galaxy surface brightnesses in mag per square arcsec typically range anywhere from 21 to 23. In mag per square arcsec that would be 12 to 14. At the opposite extreme, the highly condensed planetary nebula NGC 7009, the Saturn Nebula, is listed around mag 15 per square arcsec, or mag 6 per square arcmin.
Professionals use magnitude per square arcsecond more frequently.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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Mr Q
sage
Reged: 02/25/08
Posts: 351
Loc: N Central New Mexico
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Tony - Thanks for the info! I assumed that most lists did use arc min and because of the great difference in mag. numbers between the both, that's why the lists don't say what they are using. I guess its one of those things that's implied unless you don't know (like me). Now it all comes together and makes sense. I figured most of the lists were in arc min. but had to ask to be sure. Thanks again, Mr Q
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What goes around, comes around, eventually.
Meade DS-10(10" newt)
10x50, 10x70 binos
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timokarhula
member
Reged: 01/30/06
Posts: 87
Loc: Sweden
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For those mathematically interested, it is quite easy to understand where this conversion term 8.89 comes from.
1 square arc-minute = 60 arc-seconds * 60 arc-seconds = 3600 square arc-seconds
To convert the ratio 3600 to magnitudes:
2.5 * log (3600) = 8.8907... [2.5 comes from (1/log 2.5118...) and 2.5118... = 1 magnitude or the fifth root of 100. The factor 2.5 is thus not an approximation of 2.5118...!]
When you know the brightness ratio it is simple to convert it to a magnitude difference.
A variable star varies its luminosity by 30 times. How much is that in magnitudes?
2.5 * log (30) = 3.7 magnitudes
/Timo Karhula
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astrokido
space wanderer
Reged: 06/09/08
Posts: 661
Loc: Phoenix, AZ
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Thanks, though I'd think many of the mathematically interested would never admit being too interested.
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- Gill C. - Celestron Cometron CO-100, 10x25, 20x80, Binochair, Nikon D40
The Night Sky Atlas: www.nightskyatlas.com
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