The Magic Horse Head Eyepiece
In many reviews of nebular filters over the years, the type of eyepiece that works best with each filter is not always addressed.
Here is a story. I attended Okie Tex a few years ago at Lake Murray where the skies were excellent. Many large telescopes were set up on the small observing field. The winter sky was beautiful and I was chatting with a 25-inch owner who mentioned he had never seen the horsehead (barnard 33) through his telescope or any telescope despite numerous efforts to do so. I wondered to myself just what combination of eyepiece/filter he had tried. He said he had the H beta filter and used it with his eyepieces. I told him that I would show him the horsehead with the Magic Horsehead eyepiece!
Having some fun with this endeavor, I walked over to my eyepiece case and very ceremonially; I carefully picked up the Magic Horsehead eyepiece, screwed in the Lumicon H beta filter (a very narrow band filter that only passes 8-9 nanometers centered at the spectral line of blue-green hydrogen-beta at 486 nanometers) and gently placed it into the focuser. A few minutes later, having located the field, I asked him to look. After a few quiet minutes, the observer said, Well, I think I see something there. It seemed to be a dark patch but is bigger than I expected. It appears as a thumbprint. Look closer I asked him and see if you can determine if the thumbprint has a extension, and what direction does it extends in? He had difficulty with this request, but then a little while later, said It seems to extend to the left in the eyepiece. Yes, I said, that is the horses head, the neck is the thumbprint portion.
Over the years, I have showed the Horsehead to a number of people who have tried to see it before and never felt that they really saw it. Another time, with a group of people using a 32-inch in a dark location here in Texas, a member of the group also stated he had never seen the Horsehead. This telescope was working at a f 6 focal ratio, and I determined what the magic horsehead eyepiece would be for this telescope. I got the appropriate eyepiece out of my box plopped in the H beta filter, and the horsehead was there in all its glory! What a view, the seeing was so good, that I could see the topknot on top of the horses head! Everyone was amazed and stated the view was the best they had seen of this elusive dark nebula. Except the person who had never seen it before. He saw nothing he said. Try as I might, I could not help him. He just could not make it out. I thought I had never seen it better than that night.
Sometimes even the best combination of eyepiece/filter and dark moonless skies fails to help someone who is not experienced in deep sky observing. This takes time and extreme patience.
But despite this one failure, many people who have never seen the horsehead before have in fact seen it in my telescope with the Magic horsehead eyepiece, and profusely thanked me for showing it to them.
Lumicon who manufactured the first visual astronomy filters, published a small flyer called Getting the most from Lumicon filters a number of years ago. In the data sheet the company explained which filter to use and how performance differs between the filters.
The part that that really is important to this story refers to the exit pupil. Each Lumicon filter has an optimum exit pupil range (magnification). The exit pupil is a measure of magnification independent of telescope aperture. Using a H beta filter /eyepiece , if you use an exit pupil too small, the background sky is too dark. If you use an exit pupil too large, the background sky is too bright and the contrast is not sufficient. Lumicons data sheet says: To determine the optimum eyepiece focal length to use with any filter, simply multiply the optimum exit pupil for a chosen filter by the telescope focal ratio. For example, using a f/6 telescope at a dark site, the H-Beta filter is optimum for 6X (4-7mm exit pupil) = 24 to 40 mm eyepieces. They give exit pupil ranges in this data sheet for all their filters.
To compute the exit pupil is via the focal ratio: exit pupil = eyepiece focal length divided by telescope focal ratio. For example, on an f/8 telescope, a 32mm eyepiece would have an exit pupil of 4mm . If you use a f 10 telescope, and a 32 mm eyepiece the exit pupil is 3.2 mm. Simple math. So what does this have to do with the Magic Horse Head eyepiece? The Hydrogen Beta filter simply works best at an optimum exit pupil range of 4 to 7 mm under dark skies. Near cities, the company states the optimum range is 3 to 7 mm.
My personal experience has shown that the best views of the horsehead I have had, is when I use an eyepiece of 5mm exit pupil under dark skies, near the midrange of Lumicons recommended optimum exit pupil range.
Of course, sky transparency needs to be excellent, and the best indicator is NGC2024 near Zeta Orionis. If this nebula is faint or difficult, then it is not a HorseHead night. But if 2024 is bright and detailed, well, then the Horsehead will be visible.
To try my recommendation, determine an appropriate eyepiece focal length that will give you a 5mm exit pupil. Just multiply your f ratio x 5. For example, my 20 inch f-4 x 5mm gives a eyepiece focal length of 20 mm. My Magic horsehead eyepiece is a Televue plossl of 20 mm focal length. This eyepiece gives excellent transmission of light, (throughput), is well made, and sharp.
Your Magic Horse Head eyepiece, determined by your telescopes focal ratio, should also have a narrow field of view, to keep the star, Zeta Orionis out of the eyepiece. This bright star overwhelms the view. The 50 degree apparent view of plossl eyepieces work well, and the 4 element design allows more transmission of light from IC434 against which the horsehead is projected. IC 434 will appear brighter with the h beta filter using a eyepiece with fewer elements.
Of course, the longer focal ratio telescopes require longer eyepieces to get the optimum 5 mm exit pupil. Dont get crazy, if you dont have an exact 5 mm exit pupil eyepiece, use the one that is closest.
F 4.5 telescopes would use a 22 or 24 mm eyepiece, with f 5 telescopes use the Televue 25mm eyepiece, at f 6 use a 30mm or the 32mm Televue Plossl, at f 8 try the Televue 40 mm and so on.
Have fun with your h beta filter and your Magic HorseHead eyepiece.
Houston Astronomical Society
Ft. Bend Astronomy Club