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7/20/2010

David Knisely's Useful Targets for the H-Beta Filter List

I received via PM over at CloudyNights.com, permission from David to post this list as long as I acknowledge and give him credit for it which I hope I am doing here. If you have a H-Beta filter (I do! See my March 12th, 2010 report where I saw the Horsehead Nebula in Orion with it) note that some of the objects are viewable now, while in July and August with the H-Beta up in Cygnus. So give them a try, I will when I am back to DSO's and leaving the Lunar landscape and double stars alone!


Useful Targets For The H-Beta Filter by David Knisely

While the H-Beta is probably one of the less-used nebula filters, the commonly expressed idea that it works only on a handful of objects is not necessarily true. Here is a list of some of the more prominent objects that the H-Beta may be at least somewhat useful on. Some may require larger apertures, but a few have been seen from a dark sky site by just holding the filter up to the unaided eye and looking at the sky. Some of these will also be helped by a narrow-band filter like the Lumicon UHC.

1. IC 434 (HORSEHEAD NEBULA)
2. NGC 1499 (CALIFORNIA NEBULA, naked eye and RFT)
3. M43 (part of the Great Orion Nebula)
4. IC 5146 (COCOON NEBULA in Cygnus)
5. M20 (TRIFID NEBULA, main section)
6. NGC 2327 (diffuse nebula in Monoceros)
7. IC 405 (the FLAMING STAR NEBULA in Auriga)
8. IC 417 (diffuse Nebula in Auriga)
9. IC 1283 (diffuse Nebula in Sagittarius)
10. IC 1318 GAMMA CYGNI NEBULA (diffuse nebula in Cygnus)
11. IC 2177: (Diffuse Nebula, Monoceros)
12. IC 5076 (diffuse nebula, Cygnus)
13. PK64+5.1 "CAMPBELL'S HYDROGEN STAR" Cygnus (PNG 64.7+5.0)
14. Sh2-157a (small round nebula inside larger Sh2-157, Cassiopeia)
15. Sh2-235 (diffuse nebula in Auriga).
16. Sh2-276 "BARNARD'S LOOP" (diffuse nebula in Orion, naked eye)
17. IC 2162 (diffuse nebula in northern Orion)
18 Sh2-254 (diffuse nebula in northern Orion near IC 2162)
19. Sh2-256-7 (diffuse nebula in northern Orion near IC 2162)
20. vdB93 (Gum-1) (diffuse nebula in Monoceros near IC 2177)
21. Lambda Orionis nebular complex (very large, naked-eye)

In addition, a number of the brighter nebulae like NGC 7000 or M42 will respond to H-Beta use for revealing certain specific detail, although other filters may provide a somewhat better view overall.

2 comments:

  1. Anonymous8/01/2012

    Mr. Knisely's very useful list may be augmented by one compiled in 1999 and posted by Steve Gottlieb. I have located it in a Wayback Machine archive, here:
    http://tinyurl.com/bwzueau

    Now, an important caveat:
    David Knisely says that "a few have been seen from a dark sky site by just holding the filter up to the unaided eye and looking at the sky." Since the 1980s, when I acquired my first Lumicon filters, I became aware of the extremely fussy nature of these interference filters. They are prone to reflecting ambient light even from large angles at the side; and if the filter is tilted off-axis even the slightest amount, the PASSBAND WILL SHIFT somewhat, changing their response and efficiency!

    The "received wisdom" often given -- i. e., to hold the filter up to the eye -- is, I fear, entirely wrong, and misleading: causing failure of the observation in most cases. I am not alone in feeling this way; I have found many comments about it on the Net, such as the explanation by Mike Barr in a CN review of nebular filters for binoculars; he states:

    "Another big drawback...is reflections. If you own a narrowband nebula filter, you know the surface of the filter looks like a mirror. When you put two mirror surfaces over ... eyepieces, the effect of any ambient light and skyglow that sneaks past your face is greatly intensified. It's like viewing through a low-contrast fog. The only way I could get a good view was to cup my hands tight around my eyes and the eyepieces, excluding all stray light. I strongly recommend getting a dark observing hood if you plan on buying these filters."

    This reflection problem may be especially severe if you have to wear spectacles for your so-called 'naked eye' filter tests.

    I have also had this discussion with the very experienced deep sky observer Jaakko Saloranta in Finland, and close friends who have observed with me here in the SF bay area. Our conclusion: DO NOT SIMPLY HOLD A BARE SCREW-ON FILTER UP TO THE EYE!

    If you have any intention of trying to discern the VERY faint Lambda Orionis nebula or Barnard's Loop, for instance, it's essential to get ambient light AWAY from the filter surface sides, and also to prevent it from impinging on your own retina, from the periphery outside the filter.

    I used to work with Jack Marling years ago, and he advised and assisted me in developing an old, obsolete pice of software (called "Lumiview") for calculating proper exit pupils for his nebular filters. We had many discussions of their best intended use, and he even found that the filters were so sensitive that they gave different passband widths in scopes of varying FOCAL RATIO, due to changes in the angle of the light cone out of the focuser! So, tilting the filter is a no-no.

    I have a solution, a "filter mask" made by getting a cheap hardware store plastic eye protector mask, cutting a hole that is just about the size of the threaded part of the filter, and then spray painting the inside flat black. If the filter does not fit in tightly, you may have to apply a couple of strips of tape during each use. It's also better to emply a 2" filter for direct visual naked eye tests, due to the eye's intrinsic wide field.

    This mask at least keeps the filter approximately on-axis, as well as preventing most light interference from the sides. It also improves one's attention to detail by eliminating distracting unfiltered light.

    I have dealt with this topic in detail in my own article on "Using Eyepiece & Photographic Nebular Filters" which may be found here:
    http://freescruz.com/~4cygni/horsehead/NebulaFilters.htm

    Best,
    Steve Waldee, amateur astronomer, San Jose

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  2. Anonymous8/01/2012

    Incidentally, WRT Mr. Knisely discussing hand-holding the H-beta filter for naked-eye observations:

    Apparently, as I've JUST discovered, even he doesn't really do it this way. I am an avid reader of his posts, commentaries, reviews, and observations (many of which are extremely informative!) and I found that, in an old CN post from October 25, 2002, he says:

    "I was surprised one crystal-clear night (Febr. 13th of this year) when viewing Barnard's Loop using the H-Beta mounted on a very short rubber ring light baffle made from an old rubber eyepiece eyeguard to keep side light out of my eye and off of the back of the filter..."

    So, David: it would be very nice if you could add this helpful recommendation to your various filter articles, object lists, & reviews. The eyeguard he mentions, if I am not mistaken, is one that is a big, floppy, rather soft rubber 'thingy' that scrunches down and fits tightly against the eye. One could insert a 1.25" filter into the end that slides over the top of the ocular. This however still will NOT solve the problem of 'tilting the filter off-axis'.

    Steve W.

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