Are there two ways of determining surface brightness?

[Stonius]

By way of example, take M33/NGC 598 - the pinwheel galaxy in Triangulum.

Cartes Du Ciel has the magnitude at 5.7, whereas Astroplanner has it at 6.2. Okay, not a huge discrepancy, I guess. But the former puts the surface brightness at 14.2, whereas Astroplanner puts it at 23.5! In fact all of Astroplanners objects seem to have very low surface brightnesses in the 20's.

Astroplanner's 23.5 is kind of *almost 4 times the 6.2 visual magnitude, but not quite.

I thought the equations for surface brightness were pretty established and derived from the object's magnitude and size. CDC's mag 14.2 makes sense in that the difficulty of seeing the object is roughly about as hard as a 14.2 magnitude star, but the Astroplanner version at 23.5? I don't even know what that figure means?

Is there a way to convert this number so I can see if it falls below the limiting magnitude of my scope?

Markus

[Sean]

The smaller one is in mag/arcmin2 the other mag/arcsec2. The difference is 8.89.

To convert between the two just add or subtract 8.89.

[Stonius]

Ah, that makes so much more sense. Thanks!

Markus

[Sean]

Skyglow is typically measured in mag/arcsec2 (this is the number displayed on a Unihedron SQM or the iPhone Dark Sky Meter app). To determine whether an object is likely to be visible compare your current sky glow to the surface brightness. An average observer might see an object down to 3 magnitudes fainter than the skyglow. So for M33 you would probably need a sky darkness around 20.5 mag/arcsec2.

For objects like galaxies with bright cores you might still see something even when your sky darkness is worse than above, as quoted numbers assume the object has even brightness.

If you want to approximate you sky darkness you can use this:

http://www.darkskiesawareness.org/img/sky-brightness-nomogram.gif

[Stonius]

Wait, now im confused again. How can you see an object that has a SB 3 magnitudes *fainter than the surrounding skyglow? The other way round I could understand; needing the surface brightness of the object to be at least 3 magnitudes *brighter than the background skyglow in order to have enough contrast for it to be visible, but *fainter? I must be missing something.

Cheers, Markus

[Robh]

Hi Markus,

Read this article by Tony Flanders ...
https://tonyflanders.wordpress.com/surface-brightness/

Regards, Rob

[Stonius]

Ah, that's what I needed to know. A good article - thanks for posting. The critical passage for my understanding was the following;

"In such a case, the object will always appear somewhat brighter than the background, because its own light will be added to the skyglow. But if the combined light is only a few percent brighter than the background, it will not be detectable."

So given that an ideal dark-sky site is around 22 and objects can be detected up to a maximum of 3 magnitudes fainter, does that place a limit on terrestrial visual observing for 25 mag/arcsecond^2, no matter what the size of your telescope?

So any telescope bigger than 19" makes everything brighter, but is no better at separating out low surface brightness objects from the background, since the limiting magnitude of a 19" scope is the same as the lowest possible skyglow at a dark sky site. Is that a correct extrapolation?


Cheers
Markus

[Sean]

A surface brightness of 25 mag/arcsec2 would be very faint however you may be able to detect an object fainter than this under excellent skies. It depends on whether there are any brighter features to give some contrast against the sky. I have never tried looking at something this faint so I will have to give it a go next time the conditions are right :)

Regarding aperture have a look at Mel Bartels page on visual astronomy, especially the section - So why then is aperture the dominant factor?

http://www.bbastrodesigns.com/visual.html

or if you want more detail:

http://www.clarkvision.com/articles/visastro/omva1/index.html

[Robh]

Hi Markus,

The statements made by Tony Flanders are estimations derived from his own personal experience using whatever telescopes he had at the time. And remember, we are specifically talking about surface brightness here. Even at the darkest possible site, the sky will have a minimum inherent skyglow value. Is this exactly 22 mags/sq arcsec as set by a SQM meter?

Whether the limit of 25 mags/sq arcsec for objects applies to telescopes of much larger size is an unknown here. Maybe you could get a bit more but not much more. Also, we are looking at the limits of detection of the human eye with whatever eyepiece we are using in this telescope. And, the sensitivity of one person's eyes can be very different to another persons. I'm not sure that you could give a definitive number as most objects are not evenly bright.

Regards, Rob

[Stonius]

Thanks, Sean. Some great articles there.

So as near as I can tell, the factors involved in discerning an object are;

Surface Brightness - how bright it is across the surface area of the object
Size - Given the same SB, a larger object will be easier to detect than a smaller one
Contrast - How much contrast this level of SB has with the background skyglow
Exit Pupil - Obtaining a brighter image at the eyepiece gives your rods more photons to play with in making contrast assessments

Seemingly it's a balance between, on the one hand, using too little magnification where you get a bright field, but the object in question is too small and therefore doesn't stand out from the background sufficiently for detection, and using too much magnification where the entire image suffers from an exit pupil that doesn't deliver enough photons for your eye to work with.

Markus

[Stonius]

With that in mind. Does anyone tend to use Log Object Contrast (LOC) as a means to estimating the difficulty of targets before heading out into the field?

Markus

[Stonius]

Which is true also. It's quite possible to see just the nucleus and no spiral arms, for example. I'm just trying to find the best method to differentiate between objects I am likely to be able to detect and ones I won't so I can design better observing plans. Selfish, really :-)

-Markus