Horse Head & Exit Pupil

[scarp15]

There has recently been a number of excellent informative accounts and reports concerning observing the Horse Head Nebula.  To summarise, key requirements are;

Dark / very dark sky

Transparent, steady sky condition

Observe when Orion has reached its highest point

10" -12" plus aperture

A good quality H-beta filter

Knowledge (and expectation) as to where to look and how to observe

Low to medium power eyepiece.

I read an account, that to assist observing this challenging and faint subject would also benefit from a large exit pupil i.e. around 7mm?

I would like to ask those who have successfully observed this subject on their selection of  eyepiece and exit pupil.

[scarp15]

I think that I would like to elaborate a little on this topic.

What are your thoughts / opinions concerning (if there are any) particular advantages in observing / detecting faint DSO's such as emission nebula and faint galaxies with a large exit pupil  / low power eyepiece and does this off-set the disadvantages of pupil dilation, kidney beaning around the edges?

This is assuming that you are observing from a dark location.

My former 35mm panoptic was comfortable to use and clearly detected faint galaxies and nebula such as the Crescent, Veil from a dark location. The view I received was a little grey though The exit pupil was 7.6mm. I now use a 26mm nagler eyepiece, which is both dynamic and stark in contrast compared to the panoptic with an exit pupil of 5.6mm.

I miss the subtly of detecting certain objects that the panoptic was capable of, even though I appreciate that the nagler is excellent, and I do wonder if the lighter view through the panoptic has more reach in terms of detecting some very subtle and faint objects.

Sorry if this should have been posted in the eyepiece section.  

[Luke]

At the SGL star party last year, Sarah, Mark and I saw the HH with a 19 Panoptic in a 400P F4.38 dob, giving an exit pupil of 4.3 I believe. I couldn't really see it with a 24mm Panoptic, exit pupil 5.5. As far as I remember, Mark *could* see it with the 24...

I'm not much of a barn owl, an exit pupil of around 7 washes out the view for me, even at star parties.

[scarp15]

Interesting thanks for that Luke. Someone once advised using an eyepiece at 150X mag on the Horse Head. It would appear that regarding this particular object, it is necessary to explore using various low and medium powers, but perhaps keep the the eye relief reasonable. Hope to get a crack at it one day.

[jetstream]

Scarp,I have not seen the HH,but see the Flame neb,with no filter every time out in my dark sites.A couple of times I have seen the long edge of IC 434,no filter.On these with no filter the 17mm Ethos with an exit pupil of 3.5mm works best for me.i have tried the Ultrablock with the 21mm Ethos 5.2 mm exit pupil and the 17 Ethos with no luck-actually made the neb disappear.The next step then is the Hb filter,which is said to work best with larger exit pupils of 4mm-7mm http://www.lumicon.com/store/pg/15-LUMICON-Nebula-Filters.aspx,if no go in the 21E,17E or 18mm BCO, then its time to try a 25mm good ortho of some sort,to keep Alnitak's glow out of the FOV completely.I'm hoping the 18mmBCO,with its great contrast and light control will do it.You may benefit from a 25mm ortho too,if the 26mm Nagler & Hb filter doesn't pull it into sight.I really think you have a good chance with your scope from dark skies

[John]

I think the 18mm Baader Classic Ortho is a good contender for this task too. I've found it an excellent deep sky eyepiece, even a touch better than the Baader Genuine Ortho. The narrow field of the ortho design does help keep the brighter stars around the HH area out of the FoV.

Another contender would be the Tele Vue 20mm plossl which shows just about the highest % light transmission of any eyepiece of any brand in tests that I've seen. 

I've yet to have a really dark and transparent night here since I've had my Lumicon H-Beta filter but I'm still hopeful 

[acey]

I observed IC 434 close to the meridian in  February 2010 with a 12" f4.9 dob, Lumicon UHC filter, and 20mm TeleVue plossl, exit pupil 4.1mm, at a dark site (SQ 21.5 approx). I couldn't hold the whole nebula in view, but was only able to see parts of it in glimpses, hence couldn't hold the whole horsehead outline in view, but was able to get glimpses of the nebula edges and the notch. On a previous occasion I had sketched parts of the nebula as they came into view, and found that I built up a picture of the horsehead outline, so during subsequent viewing I had a better idea of what to look for.

In my opinion, exit pupil is not a particularly significant factor. The most important are sky darkness, transparency, and viewing the object when it is at the meridian, which apply to all deep-sky viewing if you want to see things at their best. For the Horsehead, a filter (UHC or H beta) seems to be pretty much obligatory too. Seeing (air steadiness) is not important for viewing faint fuzzies, but knowing exactly where to look is important, and knowing what to expect certainly helps.

There are lots of opinions about optimum exit pupil. I would say that in general you should just start with low magnification and work up until nothing more is gained. There is laboratory evidence that visual acuity at low light levels is best for a pupil size of about 3mm, and a study of binocular exit pupils by one individual led to 3mm being proposed as optimal for deep sky viewing. But with telescopes we usually have a range of exit pupils at our disposal, so just try everything.

I would class 7mm as very large, except for young people (no older than their twenties). By middle age, the eye pupil probably won't be dilating beyond 5 or 6mm. I use a 6.5mm exit pupil (TV 32mm plossl) as a finding eyepiece, purely because of its conveniently large true field, then switch to a 8-24mm zoom (exit pupil 4.9 to 1.6mm), and do most of my DSO viewing at the 8mm end (1.6mm exit pupil). If the object shows fine detail then I go to a 4mm eyepiece (0.8mm exit pupil). So in general, for my purposes (faint DSOs at a dark site) I would advocate an exit pupil below 5mm.

The potential benefit of a large exit pupil (approximating the eye pupil itself) is that there is the least loss of surface brightness: if the telescope could have perfect transmittance (zero light loss) then the image surface brightness will be equal to the object surface brightness when the exit pupil and eye pupil diameters are equal. With finite light loss, the image still has maximum surface brightness at this lowest useable power. But the real challenge of the horsehead is trying to see a black shape against an almost black background, and you need magnification for that.

Incidentally, I use my Lumicon UHC with all my scopes (apertures 80 to 305mm) and at all magnifications. I hear people say that filters are only appropriate to certain apertures, f-ratios or magnifications, but in my experience the UHC works with anything.

[acey]

A footnote on the idea of 3mm being the "optimal" exit pupil for DSO observing: this was the exit pupil used by William Herschel, whose nebula sweeps were made using an 18.75" reflector at magnification x157. It was with this set-up that Herschel discovered IC 434, though he made no mention of the notch or horsehead shape (the dark nebula B33), which was first noticed photographically in the late 19th century.

[scarp15]

Thanks for the insightful and the educational contributions, concerning the implications of exit pupil and suggestions for potential eyepieces.  

Perhaps the final requirement, when hunting for faint nebula (which to a varying extent applies to most D.S.O's) is that it is essential to be fully dark adapted.

Cheers