In a recent posting [to s.a.a.], I mentioned trying to observe the Horsehead Nebula (dark nebula B33 in silhouette against "bright" (not very!) emission nebula IC434), using my Intes 6-inch f/10 Maksutov at three magnifications, both without LPR filters and with two different filters separately in use. My results were as follows, in which an "X" indicates that I could see B33. Eyepieces were a 55 mm University Optics Plossl for 27x, a 40 mm Meade Kellner for 38x, and a 32 mm Orion Sirius Plossl for 47x. The UltraBlock is a recent unit, the Celestron LPR is one of the old, old, broad-band ones that looks yellow-green by reflection; I bought it in the early 1980s.
| Magnification | |||
|---|---|---|---|
| LPR Filter Type | 27x | 38x | 47x |
| None | detected | detected | detected |
| Orion "UltraBlock" | not detected | not detected | not detected |
| Old Celestron broad-band | detected | detected | detected |
All of the detections were what I would call "difficult"
It occurred to me at the time, that it was odd that I could hold the Horsehead unfiltered in all three eyepieces, but could not see it using the UltraBlock in any of them. I know the field well -- there was no question of not knowing where to look -- and I switched filters and eyepiece combinations many times. Furthermore, I believe that my specific UltraBlock "works" -- that is, it is not a defective filter. I have used it many times to view other objects with obvious effect, and as I was looking for the Horsehead I noticed that nearby NCG 2023, NGC 2024, and IC 435 were clearly visible with the UltraBlock in place.
A possible explanation did not occur to me until I looked up the transmission of the filters: The UltraBlock does not transmit the red hydrogen-alpha line at 656 nanometers wavelength; the old LPR does. My results would be explained if I were seeing IC 434 (against which the HorseHead silhouettes) primarily by this wavelength.
Though plausible, that result goes against the conventional wisdom, that when the human eye looks at nebulae emitting the usual range of hydrogen lines, what counts most is the blue hydrogen-beta line at 486 nanometers. It's not that the red line isn't there, or that you can't see it, it is just that the sensitivity of the human eye is getting pretty low that far into the red.
And I do know that the Horsehead can be seen in a narrow-band filter containing the blue hydrogen line. Ten meters away from my Intes, some folks with a 20-inch Dobson were using a Lumicon H-beta filter to see the Horsehead. I looked, and there it was. They of course had more photons to play with than did my six-inch, and I did not think to ask to borrow their filter to try on my own telescope. Furthermore, I have logged many views of the Horsehead in large Dobsons using Lumicon UHC filters, which have a bandpass similar to the UltraBlock.
So I am wondering, is the Horsehead weird among emission nebulae? Does anyone else have any results that I can compare mine to, particularly involving small telescopes or otherwise difficult detections?
Based on my experience, the Horsehead might be most easily detected in a filter with two narrow bandpasses, one at H-alpha and one at H-beta (but I don't think anyone makes one). A filter with a narrow, highly transmissive bandpass centered on H-alpha might work better than an H-beta filter, and even a broad bandpass red filter, like a Wratten 25, might give good results. I don't have an H-alpha filter, but I do have a Wratten 25, and I will be sure to try it out.