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#1
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So I found this little filter....
I found a green filter at a surplus shop.
It's a sandwich of some kind, gold coated inside one side, silver on the other. A very deep green maybe 2x that of a standard #58. But here's the interesting thing; It almost completely extincts high pressure sodium. It doesn't work on incandescents or mercury, but even when looking at a HPS close by, it's almost invisible. Given that 90% of the light pollution I experience is now due to them, this might make a good nebula filter for a larger scope. It fit in standard filter housing and I'll try it out, if it ever clears up here. -Rich |
#2
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Do you know any very deep green nebulae?
Subtraction is always subtraction. No matter how you look at it. Welding goggles for M42? Anybody? Chris.B |
#3
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#4
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My own night sky is already dark enough thankyou.
My nebul=E6 bright enough for any man. With a modest instrument and a dark adapted eye. * # * *=20 * :^) * Chris.B |
#5
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RichA posted:
You don't seem to understand. Regular nebula filters (OIII, UHC types) reduce the nebular light by substantial portion as well. The only reason you see the nebula "better" is due to the increased contrast between the nebula and the sky background which has been dimmed far more. That's why on small scopes, some nebula filters do more harm than good; They reduce the nebula light too much so the contrast effect becomes almost secondary. Well, I'm afraid that you may not understand things completely either. Emission nebulae have emission lines. Most narrow-band "nebula" filters (Lumicon UHC for example) transmit around 90% of the light which comes from these spectral lines (the OIII doublet at 4959 and 5007 Angstroms, and the H-Beta line at 4861 Angstroms). The "dimming" of these emission lines would be very difficult to detect visually (about 0.11 magnitudes). At the same time, these filters will remove a lot of the skyglow from wavelengths which are not needed, thus improving the contrast. When used *at the proper magnification*, (between 3.7x and 9.9x per inch of aperture) these filters can be successfully used even in some relatively small apertures. Indeed, I have used them in scopes as small as 2 inches on large and bright nebulae. The only thing which small scopes tend to do is have a more limited access to objects which these filters might be able to help, due to their smaller aperture (ie: its not the filter's fault). Thus, this "filters diming things too much on small apertures" is pretty much a myth. I have even used them in my 80mm f/5 refractor to see some fairly *faint* targets like the supernova remnant IC 443 in Gemini (UHC filter) or Barnard's Loop (H-Beta filter). A few weeks ago, I laid my 2" OIII filter in the dewcap of my 8x50 finder to see both sides of the Veil quite nicely, so again, if you use the right power (and understand the limitations of the *aperture*) you can get a lot out of filters even with some smaller telescopes. Clear skies to you. -- David W. Knisely Prairie Astronomy Club: http://www.prairieastronomyclub.org Hyde Memorial Observatory: http://www.hydeobservatory.info/ ********************************************** * Attend the 11th Annual NEBRASKA STAR PARTY * * July 18-23, 2004, Merritt Reservoir * * http://www.NebraskaStarParty.org * ********************************************** |
#6
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On Sun, 12 Dec 2004 01:05:39 -0600, David Knisely
wrote: RichA posted: You don't seem to understand. Regular nebula filters (OIII, UHC types) reduce the nebular light by substantial portion as well. The only reason you see the nebula "better" is due to the increased contrast between the nebula and the sky background which has been dimmed far more. That's why on small scopes, some nebula filters do more harm than good; They reduce the nebula light too much so the contrast effect becomes almost secondary. Well, I'm afraid that you may not understand things completely either. Emission nebulae have emission lines. Most narrow-band "nebula" filters (Lumicon UHC for example) transmit around 90% of the light which comes from these spectral lines (the OIII doublet at 4959 and 5007 Angstroms, and the H-Beta line at 4861 Angstroms). The "dimming" of these emission lines would be very difficult to detect visually (about 0.11 magnitudes). At the same time, these filters will remove a lot of the skyglow from wavelengths which are not needed, thus improving the contrast. When used *at the proper magnification*, (between 3.7x and 9.9x per inch of aperture) these filters can be successfully used even in some relatively small apertures. If there is no perceptable dimming of the nebular light with these filters, then why is it not recommended to use them, especially the narrow band models when imaging? -Rich |
#8
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RichA posted:
If there is no perceptable dimming of the nebular light with these filters, then why is it not recommended to use them, especially the narrow band models when imaging? Because imaging is *not* viewing. The requirements for photography are often *vastly* different than for viewing with the unaided eye. Some color films in particular are not very sensitive to the OIII doublet (it falls in a sort of "dip" in the film's spectral sensitivity curve). Thus, using a nebula filter like the OIII for photography in color would not result in much of a response on the film at all. However, the OIII doublet is near the maximum sensitivity region for the eye, so most people have no trouble in seeing this wavelength. For other photographic purposes the requirements may mean the use of a different filter or set of filters, rather than those which are best suited for visual use. Broadband "light pollution" filters are often used for photography, and it is here that they tend to work the best (even a bit better than visual use). In fact, one of the most useful filters for black and white imaging of emission nebulae is the H-alpha filter, a filter which is almost useless for night visual work. Clear skies to you. -- David W. Knisely Prairie Astronomy Club: http://www.prairieastronomyclub.org Hyde Memorial Observatory: http://www.hydeobservatory.info/ ********************************************** * Attend the 11th Annual NEBRASKA STAR PARTY * * July 18-23, 2004, Merritt Reservoir * * http://www.NebraskaStarParty.org * ********************************************** |
#9
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Len Philpot wrote:
Have you been successful visually on the Witchhead Nebula in any size/power/filter range? I can just get a hint of its glow in my 10x60 binoculars, but that's about it. It is mostly a reflection nebula, so other than broadband filters, most don't seem to bring it out all that well. Clear skies to you. -- David W. Knisely Prairie Astronomy Club: http://www.prairieastronomyclub.org Hyde Memorial Observatory: http://www.hydeobservatory.info/ ********************************************** * Attend the 11th Annual NEBRASKA STAR PARTY * * July 18-23, 2004, Merritt Reservoir * * http://www.NebraskaStarParty.org * ********************************************** |
#10
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On Mon, 13 Dec 2004 01:13:45 -0600, David Knisely
wrote: RichA posted: If there is no perceptable dimming of the nebular light with these filters, then why is it not recommended to use them, especially the narrow band models when imaging? Because imaging is *not* viewing. The requirements for photography are often *vastly* different than for viewing with the unaided eye. Some color films in particular are not very sensitive to the OIII doublet (it falls in a sort of "dip" in the film's spectral sensitivity curve). Thus, using a nebula filter like the OIII for photography in color would not result in much of a response on the film at all. How about the average CCD? Is it's response that much different (aside from infrared sensitivity) to the human eyes? -Rich |
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