I’ll be interested to read his take on the 3nm. I don’t have a 5nm yet, but have had 2” versions of 3nm and 7nm for about 5 years now. I also have an Astronomik 12nm. I’m in Bortle 7 skies and usually the 3nm is my choice, particularly if I decide to view nebulae under anywhere from half to full moon present. There is definitely more rolloff towards edges, but I’m not usually going below F/3.2 unless I am using a camera lens and putting outside the objective. I have one 300mm F/2.8 lens (112mm objective) that is modified with a 2” telescope back so I can still use 2” nosepiece with 2” filter at the back, but it is straight through viewing because of limited back focus, so it doesn’t come up much unless I’m planning on lower targets.
In each application though, there is still substantial center area that benefits from increased contrast. My inspiration to try a 3nm was the original article that pushed me towards NV Astronomy and this excerpt here:
”We have concentrated on a very tiny portion of the visual spectrum which, interestingly, gives us two great advantages, first, the tiny portion of spectrum which is Hydrogen-Alpha carries the vast majority of detail information in the luminance portion of an astrophotograph. Most of your expert astro-photogropher's use narrow Hydrogen-Alpha filters to bring out the detail in their nebula pictures. Look at almost any nebula shot by Russell Corman, Robert Gendler, or Ken Crawford, just to mention a few-they all use narrow Hydrogen-Alpha filters to capture the luminance detail in the image. The wispy, cloudy detail in virtually all nebula are principally in the Hydrogen-Alpha region. Putting this filter in front of a 25 dB gain block gives you amazing detail, as well as allowing you to see portions of the nebula you've never seen before. Then there is the improved signal-to-noise factor that you benefit from simply by reducing the bandwidth. Signal-to-noise is inversely proportional to bandwidth. It takes a much larger signal to give you the same signal-to-noise ratio in a wideband reception situation as it does in a narrowband situation. It would require the slide-rule-types to figure out exactly what that improvement equates to, but-there are several dB to be gained simply by narrowing the band pass of light. There is also a small price to pay in that your intensifier tube is starting out with a lower level of protons to work with, and is thus working closer to the noise floor than it was before. One more factor comes into play here, and this one is one that you would not ordinarily think of. The I3 has an automatic gain control, or as the tekies call it, an AGC. What this means is that when a strong signal enters the band pass of the I3, such as a bright star, the gain automatically throttles down to keep it from being over driven. What this does is reduces the amount of gain available for all of the information within the eyepiece, therefore, the much lower level information is also reduced by the same amount. Lower gain, less wanted information. The Hydrogen-Alpha filter has an enormous roll to play here, if you recall, the people among us that like to look at the sun use a very narrow Hydrogen-Alpha filter system to observe the sun directly, in most cases a small fraction of a nm in width, sometimes as little as 0.05 nm or less. The sun puts out almost no energy in the Hydrogen-Alpha region of the spectrum. That is why these very narrow Hydrogen-Alpha filters are used for solar observation allowing you to use fairly large aperture telescopes to look directly at the sun. which viewed without the filter would damage your eye and even damage the scope. What does this have to do with the system? I'll tell you. First off, when you look at a nebula through the system, the stars (suns, if you will) are attenuated by as much as 20 dB or more and thus the gain of the I3 is ramped up, compensating for the lack of bright stars in the field to the point that the nebulosity comes booming through. If you have ever looked at the black and white Hydrogen-Alpha photographs on the web or in magazines, you will notice something else you may not have paid attention to before. The number and size of the stars in the image are dramatically reduced. The reason should now be obvious, they are not there as bright or in as large a number because they have been attenuated by the Hydrogen-Alpha filter. This unique combination of light gain, filtering and manipulating of the AGC all working together to give you an image that simply cannot be replicated in any other way, with the exception of long exposure photography.”
Excerpt from Cloudy Nights article - Collins I3 “The System”, by Lynden - May 10th 2007