Quote:
Originally Posted by tonybarry
Hi Rob,
The WSAAG crew at Linden Observatory in the Blue Mountains tried one evening / morning in December 2013 to nab IC434 (the Horse) using the Evans 30" dob (0.75m aperture).
It was not visible, despite no moon and a pretty dark site, and we tried for two hours. People had seen the Horse about ten years before at the same site, but the light pollution from Sydney has only gotten worse since then.
The consensus was that it is a doable target if the skies are dark enough. Otherwise, it is not a visual target.
Regards,
Tony Barry
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Hi Tony,
I have observed the Horsehead on dozens of occasions over a large number of years. There are some "tricks" to observing it which I will pass on.
Firstly the technical stuff. I believe and I am open to be corrected, that the correct target designation for the Horsehead Nebula is Barnard 33 (B33) which is the dark nebula made visible as the Horsehead, against the illuminated background of emission nebula IC434.
Now to observing it. First and foremost let me say there are some people who may never see it visually, irrespective of how dark the skies and how big the telescope. The reason for this is that IC434 emits its light in the far red end of the visible spectrum at near infrared wavelengths. Some peoples eyes are not sensitive enough at the red end of the spectrum to actually see the emission nebula IC434. If they can't see IC434 they can't see the Horsehead. This scenario was showcased a few years ago at IISAC, maybe in 2008. Andrew Murrell and I were running sky tours with the 18" and 25" 3RF Obsessions. A couple of people asked me if I could put the Horsehead in the 18" Obsession, which I did. I was not using a filter at this time and it wasn't all that difficult a target for me under pretty good skies (Linden isn't what I would call "pretty good", I would call Linden "fair"). Only about 1/2 the people observing at my scope could see it. One of the people who couldn't see it was a colleague and a very experienced observer. I couldn't figure out how I could see it so easily and he could not see it. I then asked Andrew Murrell to put the Horsehead in the 25" Obsession, which he did. In the 25" Obsession it was a simple target for Andrew and I without a filter and with direct vision. Our colleague and a couple of other people still could not see it in the 25" Obsession. I was somewhat baffled by this so I subsequently spent a lot of time researching why this might be so. What I came up with was different individuals own eye sensitivity to light towards the infrared end of the visible spectrum. I have discussed this subsequently with a number of experienced observers and they have agreed with me on this.
Some tips to enable you to see the Horsehead. I have observed it in scopes down to 10" aperture under pristine skies, although it's a very tough get in a 10" scope and it is a target you would normally chase in scopes > 14" aperture.
1) Dark Transparent Skies. You will struggle to see it anywhere within 100km of Sydney. I would call dark skies anything over 21.3 on a properly calibrated SQM. Places like Ilford, Bucketty, Lostock and some places in the Southern Highlands like Canyonleigh etc and several others, within reasonable proximity of Sydney, will be good enough. Places like Coonabarabran and Mount Kaputar will be better still.
2) Clean optics always help as they increase contrast and reduce light scatter.
3) Use a high light transmission and high contrast eyepiece having an exit pupil between 3mm and 5mm, 4mm is ideal. A 4mm exit pupil gives you the right contrast ratio. In other words in an F5 scope you would use about a 20mm eyepiece. In an F4.5 scope about an 18mm eyepiece and in an F4 scope about a 16mm eyepiece. A couple of mm in focal length either side of this will be fine. What you don't want to do for example is stick a 31mm Nagler or 27mm Panoptic in an F4.5 scope. The large exit pupil will reduce contrast too much. The eyepiece doesn't have to have a huge FOV. A medium FOV eyepiece between 45 and 70 degrees is probably best, but don't be scared to use a 17mm Nagler T4 for example if this is the focal length you need.
4) Try for the Flame or Tank Tracks nebula (NGC 2024) first, before you try for the Horsehead. The Flame Nebula is a little to the east of Alnitak (Zeta Orionis) and is illuminated by it. If the conditions and the telescope are not good enough to see the Flame Nebula, don't waste your time trying for the Horsehead.
If the Flame Nebula was doable then try for the Horsehead. It is located South of Alnitak and you need to push Alnitak just outside the FOV of the eyepiece. Pushing Alnitak out of the FOV greatly increases the contrast within the FOV.
5) A H-Beta filter is the filter of choice but if you do not have one don't despair. A wide bandpass UHC filter helps a lot. You need to use a UHC filter that has a high bandpass in the H-Beta line at 486nm and a bandpass in both OIII lines at 496nm and 501nm. My Astronomiks UHC filter works very well in my 18" Obsession. Not quite as good as my dedicated H-Beta filter, but not far behind it.
I think that just about covers it. Good Luck !!!
Cheers,
John B