Seeing What Isn't There: The joy of observing dark nebulae
I recently received an sms from Tim Cooper and Magda Streicher Tim had picked up a dark nebula in Musca and wondered if it had been recorded before.
My first thought was that he'd picked up the Dark Doodad (SDC 301.0-08.6c, listed in Hartley et al (1986); also see Sky & Telescope 1986 October pp.344-347), a prominent dark nebula near gamma Muscae. In an e-mail he sent me the next day, he wrote:
"I don't know whether it is a dark nebula or just a strip of sky devoid of stars, hence the sms to you. I called it a dark lane when I spotted it on Thursday night (2006.02.23). I started from lambda Muscae hopping to Bennett 47 using the 16x50 binoculars, and the darkness of the lane really popped out and caught my eye. It was long, over a degree, slightly kinked about half way along its length towards the south. It appeared almost black against the background stars.
"Conditions were not as good Friday night, so it was more difficult to make out. Saturday night was entirely cloudy. I think you need really dark skies to see it properly. Interestingly, you can see it quite plainly on Guide if you select the correct parameters.
"It runs from approximately 11h26, -67° 26 to 11h36, -67° 10 and is about 10' wide. It needs further observation and a proper sketch, alas conditions did not permit us to do this over the weekend. I should have sketched it Thursday rather than come back to it later, by when conditions had deteriorated."
I checked several references for his nebula. The Millenium Star Atlas (chart 1003) showed nothing in the vicinity, so I turned to the NASA/DSS image server, from which I extracted a 1° x 1° POSS2 image centred on the reported postion. The image showed a slight under-density of stars, oriented east-west, perhaps 20x5 arcmin across the game was afoot.
Using SIMBAD, only two dark nebulae are located within a 2° radius of the noted position; both are listed in the Feitzinger & Stuewe (1984) catalogue (DCLD 294.0-05.1 and DCLD 296.4-06.8) but their catalogued dimensions don't match Cooper's description. (These and other catalogues can be searched via the CDS.)
In anticipation, I waited for the first clear night. Then, on February 28, a handy power blackout coincided with a break in the somewhat unseasonal clouds. By 21:00 I was only slightly dark adapted but set up and ready to start observing. The ominous presence overhead of bright white clouds (like an exaggerated Milky Way) against a darker-than-usual sky added to the moment.
As always, I started by turning the 11x80 tripod-mounted binoculars to Acrux to adjust the focus, and then swept over to Musca to check up on the visibility of the Doodad. As expected, it was not very apparent, so even though the suburban night sky was darker than usual, the contrast wasn't good a sure sign of thin cloud. When the skies are clear, the Doodad is an obvious black streak from my backyard.
I spent a moment checking out the general area, paying attention to star colours in particular. Mu Muscae was an obvious reward with its orange hue (BV = +1.5). Nearby lambda Muscae seemed to be tinted yellowish, perhaps only in contrast (although I later learnt the BV is +0.16).
Tim's Nebula was readily apparent, directly "above" lamba Mus, about 2° away (west-southwest). I continued examining the area until 21:37 (by 22:15 it was totally clouded over), sketching in the most certain parts of the nebula on Uranometria chart 450 (see figure 1).
From my sketch, the nebula is most noticeable around (RA 11h29m, Dec -67°21') and is 9' broad. Its longer axis measures 1.2° and is oriented ENE-WSW. The eastern and western ends are impossible to pin down as the nebula fades off insensibly; my estimate is conservative and could easily be increased by half a degree on either side. My sketch placed the end-points at (RA 11h35m, Dec -67°10') and (RA 11h24m, Dec -67°24'). The north- and south-facing straight-edges of Tim's Nebula seem quite sharply defined, enhancing the finger-like appearance.
Although Tim's Nebula was a few degrees higher in the sky than the Doodad, the latter is a less-subtle feature. That the Doodad was not prominent shows, as noted earlier, that observing conditions were not optimal, and Tim's Nebula should be a much easier target.
After recording my observations in my log, I searched through my own records of dark nebula observations in the Crux/Musca area. The sketch below (figure 2), made on 15 February 1994 using 11x80 binoculars shows Tim's Nebula. Comparing the two sketches shows good agreement on width of the nebula and how far west it extends. The earlier sketch, however, records a much greater easterward extension, pegging the nebula at around 2.4° in length.
I certainly can't wait for the next clear night! Observing Tim's Nebula was a wonderful exercise. Not only because I haven't been observing as much as I would like to, but also because dark nebulae are exquisite objects.
Dark nebulae are particularly challenging to observe, especially large complex regions such as the one that Tim's Nebula is part of. Although they are highly sensitive to observing conditions and can thus be frustrating when the skies are bad they really "shine" when conditions are good.
Dark nebula are seen not because they are bright (duh) but because they stand out in constrast to the background sky. They come in an amazing range of sizes, with some more suitable for telescopes and others responding better to ultra-wide field telescopes and, particularly, binoculars.
Besides the essential observing techniques of dark adaption and averted vision, dark nebulae respond amazingly well to deliberate image motion. Rocking the binoculars side-to-side gently, tipping it back and forth, and even moving very rapidly over the area of interest helps greatly in bringing the nebulae out.
Beyond technique, a suitable star chart is essential. Ideally, such a chart should cover a large area and show stars down to near the limit of visibility in your binoculars. Use a sharp pencil to outline the features you see, and apply a gentle uniform shading to the overall nebula. Darker areas can be shaded in for emphasis. Having extra sets of charts handy is also a good idea, to avoid clumy erasing of pencil-errors at the eyepiece. Specially-prepared charts you can download for use in this area can be found in the menu column to the right.
As with other deep sky observations, "reducing" your observations immediately (or as soon as possible) after the observing session is important. And, of course, sharing your observations with others is then so much easier and rewarding.
Using a specially prepared star chart makes it much easier to record your dark nebula observations. Such a chart should cover a sufficiently large area of sky and show as many stars as possible and nothing else. All nebulae, galaxies, clusters etc. should be removed so that clutter (and preconception) is eliminated. Most planetarium or starmapping software will allow you to make charts like these on your own (I can heartily recommend the free Carte du Ciel for this). I've found it simpler to use the excellent Millenium Star Atlas as a base chart. The chart is modified to remove all labels etc. making an ideal "skeleton" on which to add my observations.
Just under 2° north-northwest of the darkest part of Tim's Nebula, centered at (11h22m, -66°30') is a suspicious-looking roughly round void, 1° across. One degree south-west of lambda Mus lies 6th mag HD 101162 [11h37m48s -67°37'13'']. Spreading out north- and westward is a fanshaped scattering of stars, magnitudes ranging from 9 to 10.5. The triangular grouping, about 8' across, is quite prominent and takes on a nebulous appearance at a casual glance, appearing to spray outward from the bright primary. At least six stars could be made out under present conditions. After the observing session, I looked up the area in the Digitized Sky Survey take a look for yourself and decide if it's something interesting or not.
nothing more to see. please move along.