Telescope Reviews: Re: Deep Sky Observing with 70 , 80 and 100mm Bino

Visual magnitude of an extended object would be the magnitude if you could compress all the light of the object to a size of 1 arcminute area. Surface Brightness of an object gives an indication of how spread out the light is and how faint it will really appear.

For an object like M101 that has a visual magnitude of mag 7.7, but an area of diameter 26 arcminutes, that light is spread out over 530 square arcminutes. Hence is has a very low Sb = 14.7. That is averaged. As you get out towards the extremities, it is fainter, in towards the center it is generally brighter. So for instance a galaxy with a bright core might be visible, but it would appear much smaller than its full size because you can see the core but not the extension.

M101 has some brightening towards the core, so the core area actually has a little brighter Sb than 14.7, while the extremities have a fainter Sb than 14.7. Another example is M33 in Tri at Sb 14.0 was easy, this one also has a broad brighter core, so in these cases we generally see just the brighter core area.

Take the example of a DSO listed as visual mag 7. If the object is 10'x10' then it has an area of 100 sq arcmin. The light would be spread over an area 100x greater than the compressed area used to determine the visual magnitude measurement. It would actually appear 100x fainter than the visual magnitude. A light difference of 100x is 5 magnitudes so the Surface brightness of this object would be Sb = 7+5 = Sb mag 12.0.

edz

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