Here are some anomalous reports of mine, done in extremely favorable conditions at a moderately high altitude mountain top site (3,400 ft) overlooking the Pacific ocean, in northern California. I say "anomalous" because of two things that set the circumstances off from those of many other normal deep sky observers: (1) I've specialized in this object for over 20 years, researched it, published an entire website about it, and by now have probaby seen it between 50 and 60 times; and (2) I have used it regularly for specific tests: of telescopes, filters, eyepieces, and binoculars. I dare say that you can't just INTEND to get it in small aperture telescopes, and go forth and achieve it. No; it is elusive, and shows up when the object is close to the meridian, when sky transparency is best, and when the Milky Way is very clearly viewable in the immediate region. I try it several times during the appropriate seasons, and find that if I obey these rules, there is about a 75% chance that I can see it in SOME telescope, if not every instrument in my collection. But it has shown up in the smallest apertures only occasionally...
1. Evidence of seeing it in 8x42 binoculars equipped with Lumicon h-beta filters (drawing & description) and table of other observations with apertures ranging from 60 to 200 mm: found in this article, part of my Horsehead Project website.
2. Recent test involving 50 mm, 80 mm, and 114 mm aperture instruments: this entry in my observing astro-blog;
3. My last observation, not a very good one, in my 120 mm f/5 Orion refractor, mentioned here in my blog's third test report article about the instrument.
My most satisfying views with my own telescopes have been in a 17.5 inch Dob, and in the last year or so with my C-11 and 10" Dobsonian: H-beta filtered views that were almost as clearly defined as a very good monochrome photo. One of my two or three all-time-best views was in a 7" AP StarFire refractor (with H-beta filter); and I've seen it in other telescopes numerous times with a UHC/UltraBlock filter, and with NO filter at least once or twice.
These views were all at an observing site that is about 10-12 miles (as the crow flies) from downtown San Jose, California. But not just ANY out of town site will do: this particular location is high in the mountains, and my observations were aided by local ground fog that came in below me, cutting off much light pollution. I have rarely been able to see it there with NO fog or low clouds to suppress the lights of San Jose, Morgan Hill, and Gilroy (though on one or two supremely clear nights, I've done it.)
Theoretically it is made visible by dark skies; but I don't find that the ONLY requirement. An observing site I've used that is very dark (7-7.5 NELM) has sometimes failed to yield the Horsehead, due to its low elevation of only 1400 feet. Going up another 2 to 3 thousand feet above sea level really helps!
Even years ago, before light pollution was severe in the region of the Santa Clara valley, Lick Observatory astronomer Gene Harland told me he was unable to see it in the fairly large aperture finders, and the guiding eyepieces, of scopes he used to photograph it. And in the 1980s, I was unable to get it in the old 22" Tauchmann telescope at Lick Observatory, using a UHC-type filter.
Aperture is also NOT the determining factor either. For all practical purposes, the quality of views I've had in the 17, 11, and 7 inch scopes were nearly comparable (except for image scale.)
I was once scolded by somebody that my claims of having seen it in small apertures would just "frustrate" people who couldn't do it themselves. Well: so what? The point is to know what the human eye is CAPABLE of detecting, given enough opportunities to test every potentially satisfactory set of conditions.
Steve Waldee
San Jose, CA.
Horsehead Project Website:
http://home.earthlink.net/~astro-app/horsehead/index.html