The Horsehead Nebula Finder Kit
by Jeffrey L. Polston (January 20, 1999)
In the world of astronomy, there are thousands upon thousands of objects in the heavens for us to explore. Some range from very bright naked eye magnitudes, while some lay at the extreme edge of detection with a large aperture telescope. Some are big, covering several degrees of sky, while others are tiny, measured in small arc seconds. Out of these objects, we all have favorites. Some people like open clusters while others prefer the globulars. Some people like galaxies while others go more for the nebula that dot our nightly skies. Even among our favorite objects, we still revere some with a special uniqueness. We all have our personal reasons for chasing our various astronomical quarry.
This article was written to help as a guide toward an object that is special to me. It's an object of obvious beauty. It has a very unique characteristic. You can show a picture of it to anyone, whether they be amateur astronomers or not, and they will at least get part of it's name correct. They might not know what they are looking at, but they will definitely tell you what it looks like. Unlike M17, which some people call the Swan nebula, while others call it the Omega nebula, this object is not open to interpretation. What is this object? I'm talking about the Horsehead nebula.
The Horsehead nebula is a dark nebula located in the constellation of Orion. As the name implies, this dark nebula, silhouetted by the glowing emission nebula behind it, looks like a horse's head. It reminds me of a knight chess piece. Of all the objects in the sky, this is one that looks exactly like it is named. It is located about 30 arc minutes south of the bright, 2nd magnitude star Alnitak. Alnitak is the left most belt star of Orion. "That's easy", you say, "I can point my telescope at Alnitak without even looking". Yes that's true. While you can probably easily point your telescope directly at the Horsehead nebula, you'll find it very difficult to actually see the Horsehead nebula. The Horsehead nebula is very faint and takes excellent skies to be seen. Even then, you need a trained eye to be able to pick it out of the black background of space. Remember, it is a dark nebula.
I've been searching for the Horsehead nebula every since I heard of it. When I saw the beautiful picture of that dark nebula nestled in amongst the glowing red of some emission nebula, I just had to see it with my own eyes. At the time I didn't know what dark nebula or emission nebula were. I just wanted to see this awesome object that looks exactly like its namesake. It's right beside of Alnitak. "Easy", I say. I pointed my 60mm refractor at the star. What did I see? Nothing! Well, I did see Alnitak. Month after month I tried. Year after year I would glance towards the heavens at night, trying to spot this elusive object. My observing skills and my instruments advanced. I used my 8" f/4.5 Newtonian. Nothing. I used my 8" f/10 Schmidt-Cassegrain. Nothing. I used my 10" f/4.5 Newtonian. Nothing. As my knowledge of astronomical objects grew, I began to realize why I was failing at seeing the Horsehead nebula, despite my valiant efforts. I knew I was looking right at it but still not seeing it. I read of success reports from people who purchased a special $100 filter just for finding the Horsehead nebula. With large "light buckets" and these super filters, the sightings seemed quite routine to them. But I didn't want it to be that easy. Besides not having a large telescopic "light bucket" nor the desire to spend $100 hard earned dollars on a one object filter, I was driven to see it with my current arsenal of telescopes. I would allow myself to use a narrow band filter since it's a great observing aid to have in your astronomical toolbox anyway.
So, now that we know what the Horsehead is and basically where it is located, how do we go about actually finding it and most importantly, confirming that we can actually see it? Well, that's the purpose of this "kit". With the instructions, directions, hints, and charts of this kit, hopefully the elusive Horsehead nebula will finally be "roped" and observed.
I've heard of people spotting the Horsehead nebula with telescopes in the 6" size range. Seems like I've even heard of people seeing it in smaller instruments. I've also heard of people not seeing it in huge telescopes with mirrors in the 20" range. The usual minimum size that I've seen most people recommend is an 8" telescope. Of course, the larger the telescope the better. Don't be disappointed by negative sightings with bigger instruments. Seeing conditions and experience have a lot to do with viewing the Horsehead nebula.
Do you know the image orientation of your telescope? It can vary depending on the type of telescope you have and the accessories you use. First there is the normal image, sometimes referred to as a corrected or erect image view. This is the view you get when you use your own eyes to view something. You will also get this type of image if you use an erect image prism on a refractor or Schmidt-Cassegrain telescope. The prisms are sometimes referred to as amici prisms.
A second type of view is a common astronomical telescope image. This is what you get when you use a refractor or Schmidt-Cassegrain without any kind of diagonal. The image is inverted, or upside down. This is also the view through a Newtonian reflecting telescope.
The last type of view is the mirror image. This is what you get when you use a diagonal with a refractor or Schmidt-Cassegrain telescope. The image is erect, but is flipped left to right.
Finder Charts and Images
I have created a few charts and images of the area around the Horsehead nebula. Use the one that matches your telescope configuration. Figure 1 is an actual image of the Horsehead nebula. Figure 2 is a chart of the Horsehead nebula with a normal view. Figure 3 is a chart of the Horsehead nebula with a mirror view. Both charts of Figure 2 and Figure 3 have the guide stars labeled. Figure 4 and Figure 5 are negative images of Figure 1. Figure 4 is a normal image while Figure 5 is a mirror image. Some people like to use actual images at the telescope. Use which ever image or chart that you like the best.
As I have mentioned previously, there is a special filter that one could use to see objects such as the Horsehead nebula. It's called a hydrogen-beta filter. They sell for about $100. Since I don't want to spend that kind of money on a filter that will only be useful on a handful of objects, the filter I use is a Lumicon UHC, which is a narrow band filter. This is a good, general purpose filter and may help you in your search.
Well, the most obvious starting point is the leftmost belt star of Orion, Alnitak. Alnitak is a 1.7 magnitude blue-white star. At 817 light-years distance, it blazes in the eyepiece. In fact, it's so bright that it can hinder your efforts to see the Horsehead nebula. Sometimes glare from Alnitak will bleed over into your field of view, making it difficult to see the faint nebulosity in the region. Even so, it's a great jumping off point because it is easy to find and it lies so near to the Horsehead nebula. Another reason it is a good staring point is because right next to Alnitak is a nebula that will help you gauge the conditions of the skies and whether or not an attempt to see the Horsehead nebula is even worth your time. So, to begin your quest, center Alnitak in a low power eyepiece. While you're there, take in the dazzling beauty of this spectacular star.
Located 18minutes north east of Alnitak is a truly spectacular nebula. It goes by a few names. I've heard it called the Tank Tracks. I call it the Flame Nebula. To me, with its dark divisions, it looks just like a burning bush. Because of its proximity to bright Alnitak, and its visibility being dependent on the sky conditions, this nebula is easy to overlook. I had been looking at Alnitak for years and never knew the nebula was there. When a fellow observer showed it to me during a telescope purchase, I was amazed that such a beautiful sight had eluded me. Again, seeing the Flame Nebula is very dependent on your sky conditions. If you have too much light pollution or haze in the air, it can disappear. For this reason, I consider the Flame nebula the "litmus test" for the Horsehead nebula. If the Flame nebula does not "leap out" at you, you need not consider trying to see the Horsehead nebula. And of course, I'm talking about trained eyes here. A new comer to astronomy may not see the Flame nebula because of their inexperience in looking at such faint objects, even on perfect nights. So for me, I find Alnitak, then nudge the telescope over in the direction of the Flame Nebula. If I can easily see the Flame nebula and its dark lanes, I know it's a good night for hunting the Horsehead nebula. If I have to search for the Flame nebula and use a lot of averted vision to even detect it, I go no further.
Averted vision? This is the process of not looking directly at a faint object in order to see it better. Sound silly? Well because of the optic nerve, the center of your eye is not very sensitive, especially in low light situations. A well known astronomer's trick is to look to the side of an object and detect it with their peripheral vision. Another trick is to nudge the telescope slightly. Our eyes can detect moving objects easier.
Hitting the trail
Believe it or not, the trail from Alnitak to the Horsehead nebula is not very long. The Horsehead nebula is only about 27 minutes south and slightly east of Alnitak. That's less than the diameter of the full moon. At this point I add my UHC narrow band filter to the optical mix. I start moving the telescope in the southerly direction until I come across the 7.5 magnitude star, SAO132451 (refer to proper chart for your telescope). This star is about 22 minutes from Alnitak. To verify my position and bearings, I then identify two similar magnitude stars nearby. One is SAO 132464, which is a 7.8 magnitude star about 9 minutes to the east and slightly north of SAO 132451. This star has some associated nebulosity and should look a little fuzzy to you. The second star, which is an important one, is SAO 132438, a 7.6 magnitude star that is about 13' southwest of SAO 132451. I identify SAO 132438 by the fact that there is a tiny 10 magnitude star about 2 minutes to the west of it. I hope all these SAO numbers didn't confuse you. Just take a look at the proper diagram to make sure you know which ones I'm referring to.
The main two stars to be concerned with are the first one of SAO 132451, and the last one of SAO 132438. Here's why. If you form a mental image of an equilateral triangle out of these two stars and an imaginary point to the southwest, the Horsehead nebula will be contained within that imaginary corner to the southwest. So, if you have this in your eyepiece, the Horsehead nebula is in your eyepiece! I've been at this point many times and still have not seen the Horsehead nebula. Once I have positively identified this area, I pump up my power a little bit. A little extra power will help to improve contrast on faint objects. I use around 125x to about 180x. I basically try to frame my imaginary triangle as best as I can. Also, by obtaining a more narrow field of view, you help to exclude the star Alnitak and its glare.
Refining the search
You should start nudging your telescope slightly and use all the averted imagination, uh I mean averted vision you can muster. If you really concentrate and it really is a good night, you will probably start to pick up the glowing nebulosity that runs from Alnitak to down in this region and contains the dark nebula known as the Horsehead nebula. This is as far as I have gotten. One night with my 10" f/4.5 telescope, I was able to see the band of nebulosity. Another guide might be three tiny, faint stars that "frame" the Horse's head. The stars are about 12.8 magnitude. GSC 4771:1184, GSC 4771:1041, and GSC 4771:1037 seem to curve around the top of the Horse's head.
Good luck in your search. A frequent comment I've heard is that the Horsehead nebula is bigger than the observer imagined it would be. Another comment is that it is hard to actually see the shape of the horse's head, but rather you notice a notch in the surrounding glow of nebulosity. The knight chess piece, the horse's head, or even a notch, I'll settle for anything I can get. Hopefully this article will help you and I finally see this beautiful, elusive object known as the Horsehead nebula.
Update: Since writing this article, I've actually seen the Horsehead nebula a few times through larger telescopes, ranging from 18" to 24" in size.
Charts and Diagrams
Figure 1. Full color image of Flame and Horsehead nebula (normal view), courtesy of Anglo-Australian Observatory.
Figure 2. Finder chart of Horsehead region. This is a normal view.
Figure 3. Finder chart of Horsehead region. This is a mirror view.
Figure 4. Negative image of Flame and Horsehead nebula (normal view). Original color image by courtesy of Anglo-Australian Observatory.
Figure 5. Negative image of Flame and Horsehead nebula (mirror view). Original color image by courtesy of Anglo-Australian Observatory.
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