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Surface Brightness

Jeffrey Herrera

August 11, 2017 14:23

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It is an unfortunate fact that many viewing locations skies are becoming less "dark", which is why I think this next request is increasingly crucial. I'm personally in Brooklyn (by the beaches where it's a bit darker) using an 18" Teeter f/3.7 and of course it's still challenging for DSOs beyond the popular object lists. So, it would be extremely helpful to add Surface Brightness (SBr) to the 'Object Info' screen but also to be able to Sort/Filter by that information. One of the formulas I have seen (from one of my favorite astronomy books "The Urban Astronomer's Guide") is as follows:

SBr = m +5 x log(d) - 5 x log(70)         where     SBr =Surface brightness, m=Visual magnitude, d=object size in Arcseconds

The popular example used is M101 -listed w/ a visual mag of +7.9 However, for most people viewing from local sites, a challenging SBr of 14.2.

So, it would also be really nice to sort the observing lists and searches by SBr (or SBr equivalent -whatever works for the programmers best). This way a person can begin to arrange a "Challenge DSO" list for themselves. When you combine this, w/ other requests (like for Filters, SQM, etc) observers can more quickly learn to stretch the perceived limitations of their scope & location -and improve their observation techniques to be able to bag those faint fuzzies.   

Thanks so much!

FYI - I'm adding (and have already added) a bunch of requests only b/c I want to be able to drop my supplementary apps and save time at the scope. There's no disrespect intended - I love the program and in addition to SkySafari Pro, I also own Starry Night, SkyFi3 -and even SkyBT (which I rigged to use successfully w/ my Android phone back in the day).  So basically, I'm your best customer ;-)

 

 

 

 

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5 comments

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  Bill Tschumy August 11, 2017 14:51

  Jeffery,

  We have thought about this many times.  I'm somewhat skeptical of relying on that formula for the surface brightness calculation.  It works reasonably well when the object has a uniform brightness over its entire area.  It works much less well when the object has a bright core and an extended dimmer area.  The core can easily be seen although the formula might say the overall surface brightness is to low.

  Another example is that our sizes for globular clusters are close to the true object's size even though the outer halo is generally not visible though the eyepiece.  The formula would give it a much lower SB than is justified and might cause people to neglect an object that is definitely visible.

  I wish I had a good solution for this.

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  Jeffrey Herrera August 11, 2017 16:24

  Hi. I do respect the difficulty as I agree with your examples, and understand these types of formulas work best for diffuse objects. It's definitely not a definitive measure but then again the problem is that neither is visual magnitude for many Nebulae, Galaxies -and even Globulars from my smaller scope. However, IMHO, seeing the two next to each other in 'Object Info' , which is already next to "Size", would be very helpful because the mind will quickly create a ballpark expectation over time (no matter what formula is used for Surface Brightness). A label like "Suggested Surface Brightness" might also tip the user this is not rock solid -though in a 'Pro' app, users already know better than to Not try something based on a single field description. The astro apps I've seen use a measure like this do vary from each other [I guess differing formulas] -but one gets used to the internal consistency of each apps' SB attempt -so I still find it useful when combined with visual magnitude. I'd even settle for just Nebulae and Galaxies having it. I understand if you decide not to, but I'd love for SkySafari to put their best SB formula out there and go for it - people will use it. Thanks

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  Bill Tschumy August 11, 2017 16:28

  Jeffrey,

  We can consider it and see how it goes.  I would love to have good SB estimates.

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  Rapettif May 11, 2019 22:52

  I also think that the surface brightness would be a good addition. Many charts list it, and i'm sure there are some "official" SB values somewhere.

  The magnitude for extended objects is much more misleading, because it's the measure of all the light from the object, which means that if all the area of the object were concentrated in a point, like a star, it would look the same than a star of that same magnitude. So, big objects can have what you could think are bright magnitudes, but really be impossible to see in your current sky, while a star of the same magnitude could be very bright.

  I tend to think that magnitude works best for stars, open clusters, and very small objects, while surface brightness is better for globular clusters, nebulae, galaxies, etc.

  Of course, nothing is "the definitive measure of brightness", because the brightness of extended objects varies over their surface, and it seems to be a little bit subjective where their boundaries lie. Big globular clusters, like Omega Centauri and 47 Tucanae, have very bright cores, which are visible from the city, something that their surface brigthness (as normally calculated) won't tell, because their size is very big, and their limits defined to very faint borders.
  
  For the amateur astronomer, maybe some measure like "brightest part of the object" could make sense. For example, taken from the object's image, make an histogram with brightness in the X axis, and "pixels" on the Y axis. But instead of using pixels, the best would be to use a normalized surface area, for example, little 1x1 arcminute squares. Of course, would be the same than pixels if the image is resized so that the FOV per pixel is 1 arcminute square. Or whatever surface works best, maybe 1 arcminute is too much, i don't know.
  So, with this kind of measure, you could search for objects with more than "n" arcmin squares of their surface over "s" surface brightness. Makes sense? Call it the revolutionary "Surface Brightness Histogram", should sell very well, if it works like i think :)

  Of course, the images should be taken with something that has a similar response than the eye in the night. Images with highly sensitive h-alpha cameras, or something like that, wouldn't give an accurate result for our eyes. There are many public image catalogs, probably some of them could work, i suppose.

  Regards

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  Bill Tschumy May 13, 2019 15:04

  Unfortunately, surface brightness is not generally specified in most catalogs.  

  It is possible we could get it for the most common catalogs (Messier and NGC) but then there is the question of how to merge it in with our data.  We might also be able to roughly calculate it by using the width and height of the object and the apparent brightness, but this is somewhat problematic.

  We have certainly thought about this before but don't have a reliable way to get this information for most objects.