Refractordude wrote: ↑Sun Jul 11, 2021 1:28 am
Enjoyed the photography images of M16 a few days before the trip to my dark site. So I put M16 on my short list of targets. However, not certain it was M16. I used my cell phone to find it, which is usually accurate. It was a little smaller than a US Dime coin. When I increased the magnification to get some definition the imaged broke down. What does M16 look like to you? I used a 150mm f/8 achro refractor. M16 has a magnitude of 6, and a surface brightness of 14.43. It is 7000 light years away. I need a better understanding of surface brightness vs magnitude. Left click the images. Thanks all
Of the big summer nebulae, M8, M20, M17, and M16, M16 is the faintest and most difficult to see visually. Add a narrowband nebula filter and it's easy to see, but is still not exceptionally bright.
Like many large nebulae, there is an associated star cluster (which is quite easy to see).
Having viewed this object for many decades, I would recommend no less than 60x and no higher than 120x to view it in your 6" scope. It's quite large in extent (a lot bigger than M20), but has a low surface brightness.
Its overall integrated magnitude is about magnitude 6, but the average surface brightness is more like magnitude 13, and the outer parts about a magnitude fainter.
It's visible easily in a 4" scope, but that is with a nebula filter.
In the 12.5", it appears to be almost as wide as the full moon, so not small.
Surface brightness is the brightness of a small portion of the nebula, averaged over the entire object, down to a particular brightness.
Professionals talk about surface brightness in magnitudes per square arc second (a number 8.89 magnitudes different than the magnitudes most of us use).
So if the size of an object is drawn at the line of constant brightness (isophote) of magnitude 25 (16.11 is the number you would use in magnitudes per square arc minute) and, due to light pollution, you can't see to that magnitude, then the object will appear smaller.
Surface brightness is the average brightness per square arc-second down to that magnitude 25 isophote. What that means is that surface brightness is usually understated because we aren't actually seeing the
fainter parts and the average surface brightness of the parts we can see is brighter.
So even though M16 has a low SB, it appears brighter than the figure indicates. And since a good nebula filter adds around 2.5-3 magnitudes of contrast to the image, the difference with a filter can be profound.
Visually in the 12.5" at a dark site, it resembles the eagle on some US coins--seen with wings outstretched and head turned sideways (the star cluster is approximately where the head is).
Not unlike this Japanese tattoo: