Observing Dark Nebulae, Mel Bartels


I’ve enjoyed dark nebulae for many years. With the announcement of the Astronomical League’s Dark Nebulae Program, I decided to embark upon a campaign of formally observing and sketching the dark nebulae. My observations encompass the 35 required AL objects along with 102 of the optional AL objects. I also include sketches of 65 additional dark nebulae not on the AL lists.

My sketches can be found here Astronomical%20Sketches%20of%20Dark%20Nebulae.html

Here are my comments about observing dark nebulae.

Dark nebulae are among the most challenging objects to observe through the eyepiece of a telescope. These opaque clouds of gas and dust are visible by virtue of shadowing more distant stars and glowing gas clouds.

Dark nebulae test sky transparency and darkness, telescope and eyepiece baffling, observer experience and field of view vis--vis aperture.

Increasing aperture increases the perceived contrast between the dark nebulae and the Milky Way glow that they are embedded in by increasing the apparent size of the nebulae in the field of view. However, since approximately 80 of Barnard’s 370 dark nebulae are half degree or larger in size, there is a limit to how much aperture can be employed. A 12 inch F5 scope with a low power wide angle 60 degree apparent field of view eyepiece will see about a degree field. A similar 16 inch will see three-fourths of a degree field. The dark nebulae need to be appreciably smaller than the field in order to be detected. Consequently either the apparent field needs to be increased or the aperture decreased when considering a single scope.

A better choice is a smaller aperture F/4 scope with a low power 80 degree field of view eyepiece which yields a two degree field of view. Best of all is an F3 scope which takes advantage of the ultra-wide 100 degree apparent field of view eyepieces in order to yield the same two degree field of view with the greatest perceived contrast.

Don't forget to try very large binoculars.  Dark nebulae are stunning through such instruments. Even 7x50s used in very dark skies will ferret out the majority of the dark nebulae, though many will be small in size. There is much to recommend true binocular telescopes.

Even more important than aperture+field of view are sky transparency and observer experience.

Traveling to darker skies is equivalent to doubling or tripling the aperture of the scope. There is no substitute for dark skies other than digital imaging. Skies worse than 21.4 on the Sky Quality Meter mean difficult sledding in ferretting out dark nebulae; skies better than 21.7 are an absolute joy: dark nebulae leap out of the sky.

Observer experience dramatically affects results. A beginning observer can see perhaps a quarter of the dark nebulae that an experienced observer will ferret out, given same equipment and sky conditions.

Dark nebulae come with their own set of observing techniques. Dark adaption goes without saying. Blocking all light other than what comes from the eyepiece by using a shroud or cupping the eyepiece makes a difference. Start by star hopping to the correct position then looking for a dark region in the field of view. Dark nebulae seem to take longer to come into conscious view than tiny faint members of galaxy clusters and dim planetaries. Moving the scope about a quarter of a degree then stopping to inspect then back again can make conscious the subtle contrast between darkness and the Milky Way glow. If you cannot see the Milky Way glow, then move the scope a whole field of view away then back again until you the faint glow appears.

Twenty minutes of dark adaptation is sufficient. I don't find that going to extraordinary lengths helps materially in detecting dark nebulae. Set your observing and sketching table at a comfortable height and place it next to the eyepiece facing the sky. Since the dark nebulae are located in the Milky Way and since your night observing program will encompass a set of objects close to each other, the telescope's swing in azimuth will be small. Avoid scopes that require a ladder - they are terribly inefficient. Plan your list of objects ahead of time including starhopping sequences and some idea of the object appearances. Neglecting to plan inevitably results in an ineffective frustrating experience that night.

The most important accessory is a variable brightness red LED flashlight. Being able to set the flashlight's brightness just bright enough to see the star chart and to sketch is crucial. Otherwise, too much time will be wasted waiting for your eyes to return to their dark adapted state. It's fine to make a rough sketch with comments, then draw the final sketch later that night or the next day. Don't wait longer otherwise your visual short term memory will fade. Test your accuracy and understand better the limits of visual sketching by selecting a grouping of two large sized dark nebulae, drawing the pair after centering on one of the dark nebula, then at least a month later, star hop from a different direction and center on other dark nebula. You may discover that what your eye pays the most attention to can alter the sketch in interesting ways.

It’s imperative that sufficient time be given to each field of view. I recommend planning on a minimum of 20 minutes per object when lining up an observing list for the night’s session. Some are hard to find, some are hard to see and some have subtle detail worth extra time. Others are simply indistinct oval dark patches. Don't neglect the beautiful fields of view and nearby objects like globular clusters, open clusters and planetaries.

Test your telescope baffling by looking through the eyepiece while someone shines a flashlight at various angles upon the telescope. You should not see any indirect light fogging the field of view.

Precise star hopping and locating along with studying the objects ahead of time is usually needed to positively identify the dark nebula. There are many subtle variations of darkness and light when aimed at Milky Way regions. While it is beautiful and wonderful to sweep back and forth soaking in all the dark and bright lanes and regions, identification takes precision and time. Clyde Tombaugh extolled the virtues of scanning through the Milky Way regions with a Richest Field Telescope of 5 inches aperture.

Dark nebulae often surprise with subtle detail that takes considerable time and attention to recognize. Take time and let the dark nebulae come to you. You will be delighted with subtle shapes and variations in darkness and lightness. Don’t neglect the variations in the Milky Way’s brightness which can be enchanting.

As you check off dark nebulae, your skill will improve. A few dozen dark nebulae over several months will endow you with a significant improvement in detectability and awareness of detail, perhaps a doubling of ability.

The most difficult dark nebulae only come into view after many minutes of staring. Perhaps it is the combination of clarity of sky along with my concentration; perhaps it is a matter of gradually becoming aware of the eyepiece's view. Invariably, the object disappears after a few moments. I count it as observed when I can see the object three separate times over an observing session. This method is accurate and resilient. From time to time I will record a failed observation only to find out later that the coordinates were in error. Other times I've seen and sketched a shape contrary to what the atlas indicates. Luckily today with digital imaging, online planetarium programs and the internet, occasionally I can find an image of the area. In every instance, the shape I drew matched the digital image in its salient characteristics. Other observers have other methods for determining if what they see is real. The important point is that you use some method or process. Do not concern yourself with what others claim to see on occasion. It is the extensive series of observations over a long period of time that add up to a meaningful result. Variation among observers can be considerable. One amateur could find objects in his favorite 4 inch [10cm] scope that another in his 14 inch [36cm] could not and another in his 20 inch [51cm] could see with difficulty. What matters is what you can observe in your telescope in your observing conditions over a period of time. When using a list of difficult objects that means that some will be visible and some will not. Almost every night there is one object that I fail to see. I reserve it for a better night when transparency is superb and I am rested and ready, perhaps with a larger scope.

An interesting psychological effect is that after intense concentration and onset of eye strain, the dark nebulae and the Milky Way glow can appear to reverse or invert themselves, such that the dark nebula will appear to glow. This effect occurs most often when there is a very faint band of brightness against a band of darkness. Walking away from the eyepiece and returning a minute or two later clears up the situation.

I found a lack of finder charts and descriptions quite limiting and challenging. That’s the purpose of my sketching: to give a sense of location, visibility and detail. I cannot imagine going after dark nebulae without having E. E. Barnard's A Photographic Atlas of Selected Regions of the Milky Way at the observing table. Luckily there is a reprint, published in 2011, available. The online digital scans are of lesser quality and are somewhat less useful at the eyepiece. However, for those using tablets and e-readers at the eyepiece, the PDF format of the online scans is very handy. So either the book or the PDFs on a tablet will work.

The opacity number begs discussion because its meaning becomes confusing as one observer a lot of dark nebulae. Is it absolute or relative? A dark nebula that's barely there projected against a bright region of the Milky Way is difficult to see and might be given a low opacity number. A dark nebula that is very opaque yet projected against a very weak portion of the Milky Way might be given a low opacity number if the number is meant in a relative way, or a high opacity number if it is meant absolutely. Lynds' discussion implies an absolute number when he talks about counting stars that shine through the dark nebula and a relative number when he discusses surface intensity in the general field of view. From the visual observer's viewpoint, a relative number is more meaningful. The opacity number will vary per sky conditions, observer experience and telescope. I find a variation of +-1, sometimes +-2, as sky conditions vary. So it is a quick check as to visibility, but the opacity number cannot substitute for detailed notes.

Since Barnard’s photographic plates were insensitive to red stars, star hopping can be frustrating. I recommend Uranometria in book form or a very detailed online atlas like TriAtlas since these references plot the Barnard dark nebulae, many of them with an approximate outline (plot of an isophote line), though these outlines should not be taken for granted as Gospel truth for what can be seen at the eyepiece.

Finally, though resolution is coarse, online panoramas like the Photopic Survey are useful.

I hope that the subtle beauty of the dark nebulae, the challenge of observing them, the story of Barnard, one of the all-time great observers, along with the turn of the century dispute about their nature will captivate you as much as it does me.

References:

Dark nebula http://abyss.uoregon.edu/~js/glossary/dark_nebula.html

Astronomical League dark nebulae observing program http://www.astroleague.org/al/obsclubs/DarkNebulaeClub

East Valley (Phoenix Arizona) Astronomy Club dark nebulae observing program http://www.eastvalleyastronomy.org/obspgm-barnard.html

Visual observations of dark nebulae by W.S.Franks http://adsabs.harvard.edu/full/1930MNRAS..90..326F

Jeremy Perez's sketches of dark nebulae http://www.perezmedia.net/beltofvenus/archives/sg-dark_nebula.html

A Photographic Atlas of Selected Regions of the Milky Way by Edward Emerson Barnard (Author), Gerald Orin Dobek (Foreword), 2nd edition, published 2011

E. E. Barnard's Catalog of Dark Nebulae, listing: http://www.dvaa.org/AData/Barnard.html

Digital scan of E. E. Barnard's A Photographic Atlas of Selected Regions of the Milky Way http://www.library.gatech.edu/barnard/

Lynd's catalog of dark nebulae http://heasarc.gsfc.nasa.gov/W3Browse/nebula-catalog/ldn.html

Saguaro Astronomy Club's The Best of Barnard's Dark Nebulae http://www.saguaroastro.org/content/Best-of-Barnards-Dark-Nebulae.htm

Atlas of the Universe Dark Nebulae http://www.atlasoftheuniverse.com/darknebs.html

Milky Way Map http://astroshed.com/milkymap.html

Photopic Sky Survey http://media.skysurvey.org/interactive360/index.html

Richest Field Telescopes http://www.bbastrodesigns.com/rft.html

December 2012