Rich, Thanks, to all, for the comments on this image. I am pretty happy with it. Rich asked: <I'm assuming that if such an image requires 4 hours of <exposure, this nebulae is pretty faint; I don't <suppose this thing can be observed visually (I'm <showing my ignorance here regarding Ha filters as well <-- are these things strictly for photographic use...)? Actually, each exposure was only 20 minutes, but I took lots of them to help smooth out the nebulous areas of the image. If you only shoot a few frames, the noise is pretty bad, but as you add more, things really smooth out. The object is pretty faint, but with a scope the size of yours you may be able to detect it. It would be difficult at best. I really haven't tried to observe it with a large instrument. I don't think Ha filters are useful for visual observing. The emission line that they isolate is at a wavelength of about 650nm, which is outside the range of human night-sensitivity. I am attaching an image borrowed from a website of a prominent filter manufacturer (I cropped off the name of the company). It illustrates the range covered by a typical Ha filter. You can see that it doesn't overlap with human visual range. Here is the text that went along with the image:
From website: <The red lines are the most important lines from artificial light pollution. <The green lines are the most prominent emission lines for nebulas. The grey <curve is the human eye´s night-sensitivity. The blue line is the <transmission curve of the Ha filter.
However, the Hydrogen-Beta line is nearly in the middle of the human range, which explains why Hb filters are so popular for cutting light pollution and targeting emissions. Thanks for the interest. Tyler
