Finally Lassoed the Horsehead!
Posted 12 January 2013 - 02:15 PM
The night was very transparent with poor seeing, as Clear Sky Chart had predicted. I swung the Dob over to Alnitak, Zeta Orionis. Attaching a spacing ring onto my 2" twist-lock adapter, I screwed a Lumicon H-Beta filter onto the end and slipped it all into the focuser. This setup would allow me to try several 1.25" eyepieces on B33 without having to remove the filter - and without hitting the filter with the end of an eyepiece.
The eyepieces I used were a UO Abbe Ortho 25, BGO 18, Brandon 24, Sterling Plossl 25.1 and XW20, in that order. I tried to select eyepieces that were somewhere around 24mm focal length - to give me about a 5mm exit pupil for my f4.8 Dob - as suggested by Barabara Wilson in this thread:
The Magic Horse Head Eyepiece
The following Megastar chart of the area posted by RolandosCY in another thread helped me tremenously in finding the Horsehead.
Bound and determined : Horsehead thru a ten..BUT..
This was the first night I recall ever seeing IC 434. Common sense would suggest that if you cannot see IC 434, you cannot see the Horsehead. So I always look for IC 434 first. If I don't see it, I don't bother looking for B33. That night IC 434 was fairly easy to see, though not as obvious as NGC 2024 northeast of Alnitak.
Many previous nights I could see NGC 2024 easily, but had no sign of IC 434. Now IC 434 looked like a faint wedge beginning west of Alnitak and extending southeast to a point. The area immediately to the east of IC 434 was clearly darker than that to the west. I used the Megastar chart to locate the Horsehead, situating it on the east edge of IC 434, south of an arc of four stars.
I also kept in mind that the apparent size of the Horsehead is about the same as the dark central area of NGC 2024. I glanced up at 2024 to get a check on the size of its central dark nebula before hunting the Horsehead. Of course, when actually trying to catch the Horsehead, I was careful to keep Alnitak and NGC 2024 out of the field of view.
The first eyepiece in the focuser was my UO Abbe Ortho 25. This was also the first eyepiece that ever showed me the Horsehead. I threw my DarkSky Apparel hood over my head to enhance my dark adaptation. I could just barely make out a slightly darker notch where the Horsehead was supposed to be. Bingo! Next I put in my BGO 18. As expected for an eyepiece with somewhat better coatings - fully multicoated instead of just multicoated - the Horsehead was somewhat easier to tease out from IC 434. (The higher magnification and narrower exit pupil may have helped a bit, too.)
Then I tried my Brandon 24mm. Mine has the amber coatings, which are supposed to be an improvement upon the older, bluish coatings. As expected given the Brandon's reputation, the Horsehead appeared more distinct than in either the UO Abbe Ortho 25 or the BGO 18. But the improvement over the BGO was subtle.
Next in goes my humble and inexpensive Sterling Plossl 25.1mm. Surprise! Best so far in show! The Horsehead was much more prominent than in the Brandon 24. Not a subtle improvement, but an obvious one. The large dark nebula immediately east of IC 434 was darker than in any of the other eyepieces. The outline of IC 434 was easier to tease out. And the Horsehead itself was a much more distinct notch in IC 434. At times, for an instant now and then, I could see a fairly clear outline of the Horsehead and even catch a glimpse of the snout. Very impressive, especially for an eyepiece which only cost me about $45 new!
Finally I tried my new acquisition, an XW 20mm. It has a 70 degree AFOV, so I had to be extra careful to keep Alnitak out of the field. This forced me to view the Horsehead rather close to the edge of the field, which probably had an negative effect on the view. But in any case, the XW 20 showed the Horsehead well. However, I would give it second place to the Sterling Plossl. Maybe if I'd used an XW closer to the magical 24mm, it would have performed better. But you gotta run the horses you've got.
I regret that I did not think to try my Televue Plossl 25mm on the Horsehead. That eyepiece never left the stable except to prime my Paracorr when going after the H400 objects. I didn't use the Paracorr either in viewing the Horsehead.
Here is a rundown of the eyepieces I used, showing their focal lengths, coatings, AFOV, and the TFOV, magnification and exit pupil in my 10" f/4.8
Dob, as well as my overall grade of their performance on the Horsehead Nebula (B33).
MC = Multi-Coated
FC = Fully Coated
FMC = Fully Multi-Coated
EP = Exit Pupil in mm
AFOV = Apparent Field of View in degrees
TFOV = True Field of View in arcminutes
One consequence of this comparo for me has been that I've taken the UO Abbe Ortho 25mm out of my deep sky eyepiece case. I have a pair of these. I'll keep them for binoviewing planets and the Moon, but not for viewing DSO. And I've decided to keep all my Sterling Plossls - 12.5 to 25mm - in the case for deep sky observing.
Posted 12 January 2013 - 04:29 PM
Posted 12 January 2013 - 07:11 PM
Posted 12 January 2013 - 09:07 PM
Posted 12 January 2013 - 10:07 PM
Very well told. I enjoyed the workings-out of what worked best and why along with the surprise results.
Posted 13 January 2013 - 01:06 AM
Did you go back to the earlier eyepieces again, so as to verify it wasn't mostly a case
of improved conditions or improved dark adaption?
I was wondering when someone would ask this question ... but I was pretty sure it was going to be you to ask it!
Well, the answer is, no, I didn't. My primary purpose that night was to bag H400 objects. I was able to locate and observe 25 more of them. Hunting for the Horsehead, and then comparing how it looked in different eyepieces, was a sideshow for trick ponies, not an act for the center ring. Besides, 25 mph gusts and temps in the mid-20's do not encourage exhaustive scientific controls. My observations are what they are.
However, I did spend nearly an hour on the Horsehead and the five eyepieces. For me, that is an enormous investment of time on any object that is not a planet or the Moon. I didn't start observing the Horsehead until about 10:45 pm, so my eyes had been dark adapting for over four hours. Also, over the course of my observations of the Horsehead, the sky was consistently transparent.
And for what it's worth, at the beginning of each observation with each new eyepiece, I would get my bearings by going back to Zeta Orionis (my Dob does not track). Even through an O-III filter, I'm sure this reset my eyes to an adequate base-line level of dark adaptation. Then, before I started viewing the Horsehead again with each eyepiece, I put the Dark Apparel hood back over my head to help my eyes dark adapt better and to shield them from ambient light.
I'm confident my report gives a good representation of the relative ability of these five eyepieces to present an image of the Horsehead Nebula. I've already made changes in my deep sky line-up of eyepieces based on the results.
Posted 26 August 2013 - 09:14 AM
I've made this mistake before when talking about the Horsehead. (I notice that other observers make the same mistake.) Maybe it's because I use the O-III filter much more often than the H-Beta? Probably.