Seeing through the dust: turning to technology can improve your resolution of globular clusters.
William Herschel thought "the holes in the heavens" represented an absence of stars. American astronomer E. E. Barnard, even with his decades of photographing these "vacuities," was slow to embrace the concept that interstellar matter exists and can block more distant sources of visible light. Today we know that regions of interstellar dust dramatically dim the light of distant stars, and the vast distances to globular clusters, especially those embedded in the galactic disk, allow the dust to do its mischief. As a result, unsuspecting amateur astronomers excitedly focus their scopes on stellar prey only to be greeted by faint cotton balls of light with no stars resolved. Happily, modern technology has provided a solution to this dilemma.
Night vision electronics in the guise of a telescope eyepiece cuts through the dust as if it were not there. I acquired my Collins I 3 image-intensified eyepiece not long after it was reviewed in this publication (S&T: February 1999, p. 63). After many years of looking at the brighter globulars with truly remarkable results, I decided to pursue an observing program of fainter globular clusters that are largely ignored by most amateurs. Here, I describe a sampling of my observations using the I 3 eyepiece firmly seated in the focuser of my 10-inch Ritchey-Chretien Cassegrain reflector. I chose to make my observations "blind." That is, I entered every cluster into my Go-To mount and described the view through the eyepiece. The true field-of-view of my setup is 25', and my site conditions are relatively good. I observe from a site above 7,000 feet in elevation with a naked-eye limiting magnitude of 6.0. Good to excellent transparency is key to resolving these targets.
NGC 6626 (Messier 28), relatively big and bright, is obscured by a little more than one magnitude of dust. That's enough dimming to cause some amateurs to bypass this gem as they head over to magnificent M22.1 find the cluster fully resolved with a diameter of 8'. The nucleus consists of a 1' crowding of five large, closely packed stars surrounded by a heavily populated core of stars extending out to a 5' diameter. As with most globulars, the outliers of the cluster are more thinly distributed, but here the pattern is a bit more irregular than most. The three levels of star density make this a worthy target that I've enjoyed on many nights.
Not far away is the gorgeous cluster NGC 6624. I call it a "starburst" globular in that it has a very intense 1' nucleus with the rest of the stars radiating out in all directions to a total extent of 5'. If you've ever had a well-resolved view of M80 you'll probably recognize this description. NGC 6624 stands out well from the surrounding star field that adds to the attractiveness of the scene.
Our next target is big and beautiful NGC 6553. The total extent is 8' with innumerable pinpoint stars defining the highly circular cluster. A multitude of bright individuals define a 4' core that suggests to my mind the outline of a five-pointed star. Without electronic amplification resolution, the resolution of this cluster would be hopeless as it's dimmed by nearly two magnitudes of dust.
Globular cluster NGC 6304 may take the cake for the most interesting shape it conjures up in my mind. The central portion of its 8' extent features an oval collection of stars containing a straight line of five brighter stars crossing it about Vi of the way down. To me this resembles an acorn in the sky with the line of stars defining the top of the acorn from its main body.
The next cluster has no central concentration at all. NGC 6539 has a 7' extent that can only elicit the classic description of diamond dust on black velvet. Three magnitudes of interstellar dust enshroud this cluster, making it an unresolvable target for all but the largest amateur telescopes. Interestingly, NGC 6539 lies at almost the same distance and is of the same absolute magnitude as M92, a common target for the backyard enthusiast. The fact that it's virtually unknown to many highlights the difficulty that dust introduces to visual observations.
NGC 6287 is a well-resolved 5' cluster. The arrangement of brighter stars on the globular's periphery led me to calling this the "Eiffel Tower" cluster. The top star of the tower is to the southwest, and the heart of the cluster occupies the "observation deck." In contrast to this circular cluster, NGC 6401 forms a distinct arrowhead shape composed of similar magnitude stars in a 2' array. Over two magnitudes of obscuration and a distance approaching 35,000 lightyears make these stars faint but still observable with my equipment.
As far as distance is concerned, NGC 6426 is the hands-down winner in my tabulation. It's a faint, fully resolved loose collection of stars covering a section of sky approximately 4' or 5' across. I observed these clusters from numeric lists with no preconceived notions of what I would see. I was quite surprised to find out later that this cluster was 67,000 light-years away with a magnitude of dust-dimming to boot.
I was even more amazed to see NGC 6256 as a faint but well-resolved cluster of 7' diameter. It's very loose, and my notes at the eyepiece are "class X-XI?" (the Shapley-Sawyer classification class, categorizing this globular as loose and homogenous at the center). This cluster is so obscure that it's neither described in any of my reference books nor listed with any of the professional classifications that I use to compare my own observations.
Our final target, NGC 6749, would be an average cluster for amateurs if not for the whopping 4Vi magnitudes of dimming by interstellar dust. I'm able to see a patch of light approximately 4' in diameter with about half a dozen stars making a faint appearance. These stars are barely brighter than 17th magnitude!
In an article published in 1900, George W. Ritchey wrote, "The visual limit ... of the 40-inch [great refractor at Yerkes] is at about 16.5 to 17 magnitude." He was talking about globular clusters. This comparison provides a measure of aperture increase that I'm achieving with the electronic eyepiece coupled to my 10-inch reflector.
I've been able to make these observations for two simple reasons. A globular star cluster is the ideal target for night vision technology like that originally produced by Collins. The brightest stars of a globular are swollen red giants with peak luminosities in the near-infrared at a wavelength that exactly corresponds to the maximum sensitivity of the eyepiece. This longer wavelength, invisible to the human eye, has a second benefit especially germane to assisted observing of globulars. As wavelength increases, interstellar dust is less able to hinder its travel. This means the electronic eyepiece is seeing more starlight exactly at the sweet spot of its sensitivity. The result is a field of pinpoint stars of varying magnitude with the brightest stars of somewhat larger diameter.
My particular eyepiece is dated and no longer commercially available, but Night Vision Devices sells a wide range of monoculars with specifications that meet or exceed the Collins design. These devices are useful for a wide range of deep-sky objects, not just globular clusters. You may be able to find image-intensified eyepieces on the second-hand market as well. I'm convinced this technology is under-utilized by amateurs and deserves serious consideration, especially at light-polluted observing sites.
I'm amazed by the number of clusters that I've been able to resolve and the cluster diameters that I've recorded. The most satisfying feature of all is the fact that every cluster is different and visually enthralling. I'll never grow tired of viewing them.
ED MIHELICH has resolved 80 globular clusters from mid-northern latitudes.
Caption: DISTANT BRILLIANCE We'll never see the glorious globular cluster NGC 6401 as well as the Hubble Space Telescope can, but with an assist from night-vision technology, we can improve our ground-based observations. NGC 6401 lies 35,000 light-years from Earth in the direction of Ophiuchus and typically looks like a round patch of haze in the telescope eyepiece.
Caption: GLOWING GLOBULAR Dennis di Cicco connected a DSLR camera to his setup to capture this image-intensified view of the globular cluster NGC 6760 through the eyepiece. The eyepiece shows stars some two magnitudes fainter than what he could see through a conventional eyepiece.
Caption: GREEN SCREEN The amplifying element in the author's image-intensified eyepiece is the microchannel plate, which is perforated with millions of tunnels with coated walls. When light strikes the intensifier's photocathode, the emitted electrons enter the perforations, ricochet off the tunnel walls, and create more electrons. The electrons pass through the tunnels, hit a phosphor screen, and create an enhanced image.
Caption: DEEPER LOOK Image-intensified eyepieces are ideal for viewing star clusters. Although your view will be tinted, color photography falsely boosts the screen's green hue in this image.
Caption: WELL RESOLVED The author estimates the resolution of globulars viewed through assistive technology matches that using the red channel of the Digitized Sky Survey with a 15'X 15' field of view.
Dust-obscured Globular Clusters Object Class Mag(v) Size RA NGC 6626 IV 6.8 11' 18h 24.5m NGC 6624 VI 7.9 5.9' 18h 23.7m NGC 6553 XI 8.1 8.1' 18h 09.3m NGC 6304 VI 8.2 8.0' 17h 14.5m NGC 6539 X 9.3 7.9' 18h 04.8m NGC 6287 VII 9.4 5.1' 17h 05.2m NGC 6401 VIII 9.5 5.6' 17h 38.6m NGC 6426 IX 11 4.2' 17h 44.9m NGC 6256 -- 11.3 6.6' 16h 59.5m NGC 6749 -- 12.4 6.3' 19h 05.3m Object Dec Dimming Distance (l-y) NGC 6626 -24[degrees] 52' 1.2 17,900 NGC 6624 -30[degrees] 22' 0.84 25,800 NGC 6553 -25[degrees] 55' 1.9 19,600 NGC 6304 -29[degrees] 28' 1.6 19,200 NGC 6539 -07[degrees] 35' 3 25,400 NGC 6287 -22[degrees] 42' 1.8 30,300 NGC 6401 -23[degrees] 55' 2.2 34,600 NGC 6426 +03[degrees] 10' 1.1 67,100 NGC 6256 -37[degrees] 07' 3.1 27,400 NGC 6749 +01[degrees] 54' 4.5 25,800 Class information is from Strong & Sinnott, Sky Atlas 2000.0 Companion. Angular sizes are from recent catalogs. Visually, an object's size is often smaller than the cataloged value and varies according to the aperture and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0.
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|Title Annotation:||Going Deep|
|Publication:||Sky & Telescope|
|Date:||Jul 1, 2017|
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