I can't find information that relates a scope's theoretical limiting magnitude to a corresponding theoretical limiting magnitude based on surface brightness. Maybe I'm missing something that is obvious but I can't put my finger on it. Can anyone help? Kim
Kim Not sure if this will help at all but below the calculator they discuss TLM vs TLM surface brightness and the impact of seeing conditions and magnification on said object. I'm sure those more verse in this will come along, just trying to see if that site has a quick answer for you. Oh, here is the site: http://www.stargazing.net/naa/scopemath.htm Jay On Sat, Apr 16, 2011 at 11:29 AM, Kim Hyatt <kimharch@cut.net> wrote:
I can't find information that relates a scope's theoretical limiting magnitude to a corresponding theoretical limiting magnitude based on surface brightness. Maybe I'm missing something that is obvious but I can't put my finger on it. Can anyone help?
Kim
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
-- Jay Eads
Thanks, Jay. I did look at this site and a few others like it but I didn't have my question answered. However, as I've thought about it some more, I've begun to wonder if there is any correlation at all. Perhaps it is more a function of one's own visual acuity. But hey, someone out there has to have a more definitive answer, so I'm still "listening." Kim -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Jay Eads Sent: Saturday, April 16, 2011 3:29 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Technical question Kim Not sure if this will help at all but below the calculator they discuss TLM vs TLM surface brightness and the impact of seeing conditions and magnification on said object. I'm sure those more verse in this will come along, just trying to see if that site has a quick answer for you. Oh, here is the site: http://www.stargazing.net/naa/scopemath.htm Jay On Sat, Apr 16, 2011 at 11:29 AM, Kim Hyatt <kimharch@cut.net> wrote:
I can't find information that relates a scope's theoretical limiting magnitude to a corresponding theoretical limiting magnitude based on surface brightness. Maybe I'm missing something that is obvious but I can't put my finger on it. Can anyone help?
Kim
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
-- Jay Eads _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php ----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1321 / Virus Database: 1500/3577 - Release Date: 04/16/11
Kim, Perhaps this will help. The total integrated magnitude figures listed for objects more extended than stars (DSOs, in other words) are calculated from the sizes to the magnitude 25 per square arc-second isophote sizes (the outline where the object reaches mag.25 per sq.arc-second). In other words, in order to see the size of object they list, and use to calculate brightness, your scope would have to see to magnitude 25 per square arc-second. Typical dark-sky sky brightnesses are magnitude 21 to 22 per square arc-second. In order to reach magnitude 24, you'll need a large scope at high power. The result is that the overall brightness (and size) of almost all DSOs are overstated. When the sizes are adjusted down to an actually visible isophote (say, mag.23.5), the overall brightness diminishes on most objects. Ergo, for a normal transportable telescope, the overall magnitude figures are a little brighter than reality. Surface brightness, on the other hand, goes the other way. If the size of the object is limited to a brighter magnitude, the remainder will have a HIGHER average surface brightness than the calculated one. The fainter sections, which drag the brightness average down, have been cut off because they're not visible. Ergo, the surface brightness figures are a little fainter than what you'll see in reality. What does this mean? Say you use an 8" telescope at a dark site, which might be expected to reach mag.15 on stars (at high power, where the sky is darker), but only 14 or so on deep-sky objects. Which figure to use? The SURFACE BRIGHTNESS figures will tell you more about what you will or won't see than the overall magnitude figures. Is that what your trying to figure out Kim? And remember, the surface brightness figures quoted are dimmer than reality, so you might see fainter "quoted" surface brightness magnitudes than you expect. The aforementioned M33, with an average surface brightness of 14.1 will actually appear brighter than that. An 8" scope will see several H-II "knotty" regions in the spiral arms and see a lot of galaxy here. M33 has so much low surface brightness area you can't/won't see, the SB figure is pessimistic. So don't automatically assume you can't see an object if you don't look. Because of that size re-calculation I mentioned, I was able to see, with an 8" scope, some objects with quoted surface brightness figures as low as 16.1. mag. Obviously, that's below what an 8" scope can see, yet it was a visible object because what was left was the brightest section of the object, which was a lot brighter than that. You can convert brightness per square arc-minute (the most frequently quoted figures) to square arc-seconds using the following formula [sizes to insert are the sizes in arc-minutes]: surface brightness in sq.arc-sec. =mag.per sq.arc-min.+2.512log (2827.4 x max.size x min.size) Example: Can I see a mag.14.1 surface brightness galaxy that's 1.2' x 1.2' ? In square arc-seconds, that SB translates to 23.17. I know my scope can often reach 1.8 magnitudes below my sky brightness. My sky brightness is, say, mag.21.5. Add 1.8, and I can reach 23.3 on that night. The galaxy should be in reach, but very faint, and requiring high power. If that doens't help then I'll quit. I'll quit anyway. On Sat, Apr 16, 2011 at 3:59 PM, Kim Hyatt <kimharch@cut.net> wrote:
Thanks, Jay. I did look at this site and a few others like it but I didn't have my question answered. However, as I've thought about it some more, I've begun to wonder if there is any correlation at all. Perhaps it is more a function of one's own visual acuity. But hey, someone out there has to have a more definitive answer, so I'm still "listening."
Kim
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Jay Eads Sent: Saturday, April 16, 2011 3:29 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Technical question
Kim
Not sure if this will help at all but below the calculator they discuss TLM vs TLM surface brightness and the impact of seeing conditions and magnification on said object. I'm sure those more verse in this will come along, just trying to see if that site has a quick answer for you.
Oh, here is the site:
http://www.stargazing.net/naa/scopemath.htm
Jay
On Sat, Apr 16, 2011 at 11:29 AM, Kim Hyatt <kimharch@cut.net> wrote:
I can't find information that relates a scope's theoretical limiting magnitude to a corresponding theoretical limiting magnitude based on surface brightness. Maybe I'm missing something that is obvious but I can't put my finger on it. Can anyone help?
Kim
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
-- Jay Eads _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1321 / Virus Database: 1500/3577 - Release Date: 04/16/11
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
-- Jay Eads
Dawes Limit= 4.56 arc seconds/objective lens diameter.
Rayleigh Limit= 5.5 arc seconds/ diameter of objective lens but you are probably right the human eye probably varies from person to person, eyepeice used, dark adaption, atmospheric conditions, etc Kim,
Perhaps this will help.
The total integrated magnitude figures listed for objects more extended than stars (DSOs, in other words) are calculated from the sizes to the magnitude 25 per square arc-second isophote sizes (the outline where the object reaches mag.25 per sq.arc-second). In other words, in order to see the size of object they list, and use to calculate brightness, your scope would have to see to magnitude 25 per square arc-second.
Typical dark-sky sky brightnesses are magnitude 21 to 22 per square arc-second. In order to reach magnitude 24, you'll need a large scope at high power.
The result is that the overall brightness (and size) of almost all DSOs are overstated. When the sizes are adjusted down to an actually visible isophote (say, mag.23.5), the overall brightness diminishes on most objects. Ergo, for a normal transportable telescope, the overall magnitude figures are a little brighter than reality.
Surface brightness, on the other hand, goes the other way. If the size of the object is limited to a brighter magnitude, the remainder will have a HIGHER average surface brightness than the calculated one. The fainter sections, which drag the brightness average down, have been cut off because they're not visible. Ergo, the surface brightness figures are a little fainter than what you'll see in reality.
What does this mean? Say you use an 8" telescope at a dark site, which might be expected to reach mag.15 on stars (at high power, where the sky is darker), but only 14 or so on deep-sky objects.
Which figure to use? The SURFACE BRIGHTNESS figures will tell you more about what you will or won't see than the overall magnitude figures. Is that what your trying to figure out Kim?
And remember, the surface brightness figures quoted are dimmer than reality, so you might see fainter "quoted" surface brightness magnitudes than you expect.
The aforementioned M33, with an average surface brightness of 14.1 will actually appear brighter than that. An 8" scope will see several H-II "knotty" regions in the spiral arms and see a lot of galaxy here. M33 has so much low surface brightness area you can't/won't see, the SB figure is pessimistic.
So don't automatically assume you can't see an object if you don't look. Because of that size re-calculation I mentioned, I was able to see, with an 8" scope, some objects with quoted surface brightness figures as low as 16.1. mag. Obviously, that's below what an 8" scope can see, yet it was a visible object because what was left was the brightest section of the object, which was a lot brighter than that.
You can convert brightness per square arc-minute (the most frequently quoted figures) to square arc-seconds using the following formula [sizes to insert are the sizes in arc-minutes]:
surface brightness in sq.arc-sec. =mag.per sq.arc-min.+2.512log (2827.4 x max.size x min.size)
Example: Can I see a mag.14.1 surface brightness galaxy that's 1.2' x 1.2' ? In square arc-seconds, that SB translates to 23.17. I know my scope can often reach 1.8 magnitudes below my sky brightness. My sky brightness is, say, mag.21.5. Add 1.8, and I can reach 23.3 on that night. The galaxy should be in reach, but very faint, and requiring high power.
If that doens't help then I'll quit. I'll quit anyway.
On Sat, Apr 16, 2011 at 3:59 PM, Kim Hyatt <kimharch@cut.net> wrote:
Thanks, Jay. I did look at this site and a few others like it but I didn't have my question answered. However, as I've thought about it some more, I've begun to wonder if there is any correlation at all. Perhaps it is more a function of one's own visual acuity. But hey, someone out there has to have a more definitive answer, so I'm still "listening."
Kim
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Jay Eads Sent: Saturday, April 16, 2011 3:29 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Technical question
Kim
Not sure if this will help at all but below the calculator they discuss TLM vs TLM surface brightness and the impact of seeing conditions and magnification on said object. I'm sure those more verse in this will come along, just trying to see if that site has a quick answer for you.
Oh, here is the site:
http://www.stargazing.net/naa/scopemath.htm
Jay
On Sat, Apr 16, 2011 at 11:29 AM, Kim Hyatt <kimharch@cut.net> wrote:
I can't find information that relates a scope's theoretical limiting magnitude to a corresponding theoretical limiting magnitude based on surface brightness. Maybe I'm missing something that is obvious but I can't put my finger on it. Can anyone help?
Kim
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
-- Jay Eads _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1321 / Virus Database: 1500/3577 - Release Date: 04/16/11
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
-- Jay Eads _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
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erikhansen＠thebluezone.net -
Jay Eads -
Kim Hyatt