10 Attachment(s)
Object of the Week January 4, 2015 - Conquering the Horsehead: A study on the Horsehead's visibility
Object: B33, The Horsehead Nebula
Constellation: Orion
Type: Dark Nebula
Coordinates: R.A.:05h 40m 59.0s
Decl.: −02° 27′ 30.0"
Mag: -
Size: 8' x 6'
Most people on the Deep Sky Forum probably know me as a “galaxy” guy. Yet, the object that has mostly influenced my career as a deep sky observer is neither a bright or magnificent galaxy, nor a galaxy cluster full of dim challenges. Nor is it any of the beautiful planetaries, or any of the spectacular gas nebulae. It is though the best known dark nebula, the magnificent Horsehead Nebula (B33) in the constellation of Orion.
Probably all deep sky observers know the beautiful Horsehead Nebula, located less than one degree south of bright Alnitak, Zeta Orionis. This dark nebula, which really does look like its namesake, was discovered in 1888 by Willamina Fleming on a plate taken at Harvard University. Catalogued by E.E. Barnard as the 33rd entry in his famous catalog of dark nebulae, B33 is a large cloud of gas and dust seen in front of the beautiful gaseous nebula IC434 (itself presented as an OOTW, http://www.deepskyforum.com/showthre...–-IC-434). Since IC434 is located approximately 1500 light years away, the Horsehead Nebula is located at a somewhat smaller distance.
My love affair with the Horsehead (and deep sky observing), started after seeing (in 1984, as a high school student) a photo of this nebula in Sir Patrick Moore’s book “The Unfolding Universe”. In page 177 of the book is a beautiful photograph by David Malin of AAT, showing the Zeta Orionis region, and of course the Horsehead figures prominently. Although there was no mention of the Horsehead in the text, as soon as I saw the photo, I knew I wanted to observe this nebula with my own eyes. It took me more than twenty years to achieve this target, but the trip was definitely worthwhile. The road to success was full of obstacles and at times it seemed an impossible task. Yet I now know that this target is well within the capabilities of almost any instrument and any observer! Through this OOTW article I would like to share the morsels of knowledge about the Horsehead visibility that I gathered, and at the same time help to disperse the myth about its “invisibility”.
The photo that started it all, by David Malin, A.A.T.
Attachment 1495
From the moment I started my quest for the Horsehead, I kept encountering a negative attitude about observing this object. Most observing guides mentioned that it was an object that could be barely “glimpsed” at the best of nights under the darkest skies, with at least 8 inches of aperture (and always with a filter). My first attempts with a 4 ½” and a 10” reflector met with total failure. Similarly, my first attempts with a 4” refractor (that I still own – more on this later) were unsuccessful. I kept trying, preparing hand-made charts out of photographs, and aiming for it on the clearest nights. Finally, in a dark, cold night in 2007, along with an observing buddy, we sort of scored: Using a 10-inch dob with a UHC filter and a magnification of 80X with a UHC filtered Plossl eyepiece, at a very dark site, we detected a very faint nebulosity (IC434) with a very slight shadow in the middle. All this was with averted vision. I must admit that I was a bit disappointed, but the flame of the Horsehead observation was rekindled.
In early 2009 I was able to upgrade to a 12-inch dob. With this telescope I had an accidental (ie unplanned) observation of the Horsehead, much better than the previous one. IC434 was detected without a filter, and using a UHC filter the Horsehead fickered with much more certainty in view. Yet this was at a site with significant light pollution! Finally, in November 2009 I managed to observe the Horsehead without a filter for the first time, and also to observe it with a H-β filter for the first time: The Horsehead appeared not as the usual triangular barely detectable notch, but as a dark mass with a slightly offset rounded “head”. Despite this success, I was unable to repeat my Horsehead observation for nearly three years, until I got my current 18” in February 2012. With the 18” and an H-Beta filter, observing the Horsehead became routine. But the turning point in my Horsehead career occurred on December 2013, when I observed (clearly) the Horsehead with my 18” under city skies! Since then I have been able to observe (from dark skies) the Horsehead without a filter even with a 4-inch refractor, and with a H-beta or UHC filter with a 3-inch refractor! So what brought this significant change in the Horsehead’s detectability? Certainly, my eyes did not suddenly become sensitive to the H-beta emission of IC434, nor have I acquired bionic eyes!
Attachment 1496Attachment 1497
One of the most important aspects in changing my capabilities for Horsehead detection was that I discovered that certain eyepieces work much better than others. One eyepiece in particular was nicknamed by me “The Horsehead eyepiece” as it always gave me the best image of the Horsehead independently of the scope used (though a couple more got close). This eyepiece is the Televue Panoptic 19mm. For some reason, the image given by this eyepiece has the most contrast than given by any other eyepiece. Although the Panoptic 19mm and the Ethos 13mm give the same true field of view, the image in the Panoptic is brighter and the details of the Horsehead more defined (especially in the 18”). The 13mm Ethos obviously gives a bigger magnification (and thus a bigger Horsehead in the same field of view) but with or without a filter, the contrast between the dark mass of the Horsehead and the background nebulosity of IC434 is much less. To my surprise, the second best performer was the Televue Ethos 21mm, while the venerable Televue Nagler 31mm also did quite well. The Televue Ethos 13mm had a so – so performance on the Horsehead, as did a set of three old Meade Series 4000 Plossl eyepieces that I own (32, 26, and 15mm). Two other eyepieces failed the Horsehead test: Both the Televue Nagler 7mm and Takahashi LE 7.5mm were poor performers, barely showing the Horsehead in my 18” and not at all in smaller scopes.
I repeatedly tested the visibility of the Horsehead using these eyepieces under my SQM 21.0 skies in my backyard in nights of typically good (not superb) transparency. The visibility tests were repeatable: I got several notes, always with the same rankings. My next move was to compare some characteristics of the eyepieces. I discovered that if the exit pupil was below 2mm, the Horsehead was not detectable even with filters, except with the 18” where the two eyepeieces giving me less than 2mm of exit pupil showed (barely) the Horsehead and only with averted vision. I would have liked to know if the number of elements / group layed a role, but unfortunately I do not know the relevant specifications of Ethos eyepieces. I also wonder how other eyepieces would perform, and especially the Delos series by Televue.
While performing the “eyepiece” test with my 18”, I realized that I could readily detect the Horsehead in front of IC434 even with bright Alnitak in the field of view. This was true even with the small scopes. Yet, almost everything I read either in books and magazines or the ‘net mentioned that “Alnitak should ALWAYS be kept out of the field of view if any chance with the Horsehead is to be expected”. I had no problem at all detecting the Horsehead with no filter in my Takahashi FS102 with Alnitak in the field of view, and the image of the Horsehead with my 18”, Ethos 21mm, and H-beta filter (with Alnitak in the field of view) was – dare I say – spectacular. I have to admit that Alnitak is overpowering as soon as you see it, especially without a filter. Yet, after only a few seconds, eyes do adjust to its presence, IC434 slowly materializes, and a few moments later the dark, spooky silhouette of the Horsehead intrudes in the bright strip of IC434.
The Horsehead as seen with 18", Ethos 21, and H-Beta filter...
Attachment 1498Attachment 1499
...and without filter...
Attachment 1500Attachment 1501
This detectability of the Horsehead under “only” SQM21 skies and with Zeta in the field prompted me to try for the Horsehead with my refractors. It was readily detected with my 6” f5.9 achro, and quite harder in my 4” FS 102. To my surprise, it was finally detected also with my Skywatcher (Synta) ED80 f6 3” refractor! Only a year ago I would not even think of trying, yet now I am seriously waiting for an opening in the weather so that I can try with binoculars! Obviously, as with most targets, the bigger (the aperture) the better. With the smaller scopes the Horsehead was nothing more than a triangular notch, and only with the 6” could I detect a slight extension suggesting the snout. Provided that the exit pupil is above 2mm, the Horsehead as a dark structure can be detected in front of the nebulus stripe of IC434. A complete list of how the Horsehead appears with my four different scopes, a variety of eyepieces, and the use or no use of filters appears at the end.
The Horsehead as seen with my 3" refractor
Attachment 1502Attachment 1503
So, two myths have already been demolished: The Horsehead can be detected in small apertures, AND with Zeta Orionis in the field of view. This brings us to the third demolished myth: The Horsehead needs “special” atmospheric conditions to be detectable. Well, as you may have noted, I mentioned that most of my Horsehead observations took place under relatively dark (SQM 21) skies on good – not spectacular – nights. In addition, I mentioned that in December 2013 I detected the Horsehead from within the edge of a 60,000 people city, but with Orion situated directly above the highest light pollution dome. Yet, with my 18” and averted vision I could definitely see the snout using the H-beta filter (to be fair, without the filter even IC434 was not detectable). This was yet another turning point in my Horsehead observing career: To be able to detect this object from within a city it means it can be detected under conditions far from optimal. So why did it take so long for me to start observing it regularly, and why still most people fail when attempting to observe it under even superb conditions?
One of the main reasons is probably what I mentioned before: The use of eyepieces with a small (or rather too small) exit pupil. But I think there are two other reasons that most observers fail on this quest, and they are both psychological. Perhaps the most important is that the Horsehead has been surrounded with a real halo of mystique, that it is a subject that can be seen only under specific conditions (under superbly dark skies after the passages of cold fronts!), only with moderate to large apertures (10 or more inches), and only with an H-beta filter unless you have superb eyes and you are a Leslie Peltier, a Scotty, an O’Meara. Then and only then you can hope to see the Horsehead. I know, I have been through this myself. Maybe, some years ago, the optics of both telescopes and eyepieces were of an inferior quality, and this has caused many observers until the last decade or so of the 20th century to consider the Horsehead such a difficult (practically impossible) object. Once though you start seeing it, it becomes a much easier object.
This leads us to the second psychological reason for failure: Most observers fail because they don’t really know what they will see. Whether you observe using a detailed start chart, or whether you observe using a photo as your guide, you really need to know what to expect. One common misconception that has been persistent is that the “Horsehead is so small / tiny”. Tiny compared to what? Size is relative. Only a month ago my observing buddy (who had already seen the Horsehead a few times) could not see it in his own 12.5” dob, yet as soon as I glimpsed the Horsehead was there in all its glory. I just told him “It’s there! It’s gona bite you! Just look above and to the right of NGC 2023” – and he went “Oh mine – it’s big – I expected it to be much smaller!”. I personally consider the Horsehead at medium powers quite a big object. It takes a sizeable “bite” of IC434, and if you expect a tiny black spot you won’t see it. The best way to prepare is to create a chart based on the exact position of the Horsehead relative to the surrounding stars, and forgetting about the existence of the snout.
And this brings us to the final part of this study: Once we have found the Horsehead, what can we expect to see? With small apertures (less than 6 inches) and a H-beta filter you should be able to detect a triangular notch in IC434, with the south side vertical to the IC434 streamer, and the top not quite reaching to the western edge of the nebulous stream. Most people will wonder, but IC434 is quite distinct in small scopes. O’Meara in his “Hidden Treasures” book attributes this to the light of IC434 being more concentrated in small scopes. Whatever the reason, surprising as it sounds, IC434 without a filter is definitely easy in small scopes. The extra contrast boost given by the H-beta filter (and to a lesser extent by a UHC filter) helps the dark mass of the Horsehead to appear more easily (although IC 434 is more visible in small scopes, the Horsehead needs the extra contrast to “shine” through). In medium aperture scopes (6-14 inches) things are easier. And, with a filter, it is possible to note a roundish, slightly offset extension on top of the triangular notch. It is hard, but with effort the extension can be detected. With scopes larger than 16” things certainly become much easier. With a H-beta filter the “snout” can be detected with ease, and be held with direct vision, approximating the familiar chesspiece shape seen in photos. Without a filter, even with my 18” and the 19mm Panoptic under excellent conditions, the snout was barely detectable and fuzzier than what was seen with the H-beta. Keep in mind that the southern edge of the “neck”, as well as the eastern edge of the “neck” before the snout, and the top of the head (the “mane”) are quite well defined, while the whole snout and especially the “nostril” area much softer (and fuzzier) than the rest of the Horsehead. To get the best out of your Horsehead observing experience, you still need to follow the basic observing rules for hard objects: Use clean optics, always use your DSA black observing hood, and try for it under dark and clear skies.
A photograph showing the relative brightness of the various parts of the Zeta Orionis complex, taken by the author using a 4" refractor:
Attachment 1504
Needless to say, the Horsehead remains a tough object. But it is an object that in spite of the “cult status” it has obtained as an “almost impossible” object, it can be seen, and enjoyed by everybody who has a decent scope of at least three inches in aperture (but preferably more). The addition of a good UHC filter certainly helps, but the the use of the H-beta filter which boosts the contrast of IC434 with the sky (and dark nebulae!) makes a huge difference.
So, now that you have all the relevant information on the Horsehead,
"Give it a go and let us know!
Good luck and great viewing!"
