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I'm new to amateur astronomy, and I recently acquired my first small telescope. I've just learned a bit about surface brightness after an unsuccessful night of trying to locate a few nebulae that were beyond my scope's abilities, but I'm wondering if it's as cut-and-dry as I've understood so far.

The reason I'm asking is that when I look up the surface brightness of the Great Orion Nebula on Stellarium, it shows up at 13.07, a bit beyond the limiting stellar magnitude of my scope at 11.7. However, I am pretty confident that I was able to see the Orion Nebula with my scope on another occasion. It's a large enough object that I'm pretty sure I observed it, and the features looked correct (but faint and colorless) when compared to high-detail photos.

So to get to the point of my question, are there other things I should keep in mind when determining whether I'll be able to see an object with my scope? I would like to avoid more frustrating nights in the future by not picking impossible targets. I also recognize that as an amateur I might have seen something completely different than the Orion Nebula.

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Although you have not given any information about your telescope, the Orion nebula is easy to see with almost any telescope, even under urban light polluted skies.

Surface brightness is almost always greater (ie fainter) than magnitude - so for example the total magnitude of the Orion nebula is about 4 (from memory) but you say surface brightness of 13. The reason is that the area of the Orion nebula is quite large, at least when measured in arc seconds squared, and the eye/brain has some ability to integrate the light.

This works both ways: there are objects, eg M33 and M101 in my experience, which have a magnitude that would make you think they are easy targets for small telescopes, but because the brightness is spread over a large area, they are difficult to see from a typical urban or suburban location. The important measure in these cases is to compare the surface brightness with that of the background sky.

It's quite a complex topic, and Roger Clark's book "Visual Astronomy of the Deep Sky" has some useful discussion of this, and much more.

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  • $\begingroup$ What units is this value of 13? Is is the magnitude of one square arcsec or square arcmin? $\endgroup$ – uhoh Jun 4 '18 at 8:53
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    $\begingroup$ @Uhoh: it's usually given in mag per square arcsecond $\endgroup$ – Dr Chuck Jun 4 '18 at 9:13
  • $\begingroup$ I've just tried a quick calculation; $13 - (5/2)\log_{10}(60 \times 65) \approx 4$ where $(60 \times 65)$ is the dimensions of the Orion Nebula in arcmin mentioned in Wikipedia, so it might be in fact per square arcminute in this case. $\endgroup$ – uhoh Jun 4 '18 at 9:19
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    $\begingroup$ Yes, you are probably right. Clark's book gives the surface mag as 21.7 mag per arc sec. $\endgroup$ – Dr Chuck Jun 4 '18 at 10:20

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