by Chris Peterson » Fri Mar 24, 2017 4:28 am
Ann wrote:RGB images show many red nebulas. Of course, without the aid of an Ha filter the camera will only pick up brighter ones.
If you want the greatest sensitivity to Ha, you use a monochrome camera. Next in sensitivity is RGB (just the R channel). Least sensitivity comes from using an Ha filter (although not by a large factor compared with a wideband red filter).
What an Ha filter does is
isolate just the hydrogen, blocking other emission lines and most reflected continuum light. Also valuable, it blocks most light pollution and sky glow. It reduces total signal, but increases S/N (basically, contrast).
To me the Trapezium region looks faintly greenish, for example. In the Trapezium, green OIII could also contribute to the color.
Most people see bright nebulas as greenish. As you say, we are much more sensitive to green light than red. In some cases we are seeing OIII, but with bright nebulas we may also see just the green component of white light. We also easily see Ha emissions, but we see them with our rods, not cones, so they appear gray (as do most extended astronomical sources). There are a few anecdotal reports of observers with exceptional vision seeing a pinkish tinge with a few of the brightest Ha sources.
[quote="Ann"]RGB images show many red nebulas. Of course, without the aid of an Ha filter the camera will only pick up brighter ones.[/quote]
If you want the greatest sensitivity to Ha, you use a monochrome camera. Next in sensitivity is RGB (just the R channel). Least sensitivity comes from using an Ha filter (although not by a large factor compared with a wideband red filter).
What an Ha filter does is [i]isolate [/i]just the hydrogen, blocking other emission lines and most reflected continuum light. Also valuable, it blocks most light pollution and sky glow. It reduces total signal, but increases S/N (basically, contrast).
[quote]To me the Trapezium region looks faintly greenish, for example. In the Trapezium, green OIII could also contribute to the color.[/quote]
Most people see bright nebulas as greenish. As you say, we are much more sensitive to green light than red. In some cases we are seeing OIII, but with bright nebulas we may also see just the green component of white light. We also easily see Ha emissions, but we see them with our rods, not cones, so they appear gray (as do most extended astronomical sources). There are a few anecdotal reports of observers with exceptional vision seeing a pinkish tinge with a few of the brightest Ha sources.