Telescope Reviews: IC4592, a HUGE reflection neb in Scorpius, seen!

Here's an object I never expected to ever see visually, as I'd assumed it was a photographic target only. Last year, for some strange reason, I decided to try for it anyway, using a friend's home-brewed 20X100 binocular. This bino uses the objectives from a 22X100 Oberwerk-like bino (but with the Antares brand emblazoned upon) and the body/eyepieces from a Tasco model 124 7X50 having 80 degree AFoV eyepieces. (The original, weak focal reducer lenses, which increased the field of view, were removed.) The TFoV is about 3.5 degrees.

The transparency was a bit better than moderately good, with my SQM reporting 21.2-21.3 mag/arcsec^2.

After studying all the nearby OB associations over the past few years, I was already intimately familiar with the nebulous clouds in Sco OB2, a.k.a. the Upper Scorpius sub-group of the huge Sco-Cen OB association. Sco 2 contains virtually all the luminaries which comprise the Scorpion's Claws and Heart, as well as the rho Oph dust complex and star-forming region.

IC4592 is a *huge* reflection nebula, and of those visible to human eyes, only the Witch Head is of similar size. The illuminating star is nu Sco, located at the north edge of the Claws. The nebula's surface brightness is highest in the vicinity of nu, and along its southwest edge, where the brightness drops off precipitously. Toward the northeast, the brightness falls more gradually. The nebula is elongated in the northwest-southeast direction, and the maximum length is about 3 degrees (!).

In spite of the fairly low maximum elevation of 25 degrees as seen from latitude 45N, I was shocked to see about 1.5 degrees of the nebula's length! But it was not what I'd call an easy observation, by any stretch. I tried to glimpse it in a 10" dob with a 30mm eyepiece, but to no avail. In those conditions it was definitely a binocular object only, meaning both eyes were necessary to differentiate the feeble signal from the sky glow.

Because of this object's large size, it doesn't require more than low power to magnify it to well past the point of detection, even given its very low contrast. Moreover, it has a better chance to be glimpsed when framed by a reasonably generous expanse of surrounding sky. Big 'scopes may fail to reveal anything because the nebula will fill or even spill beyond the available field. Or if anything is seen, perhaps only the "sharp" transition along the SW edge near nu will stand out.

Of course, for those fortunates who live well south of my latitude, a higher elevation should make spotting this cloud even easier if the sky is nice and dark.

Finally, after catching IC4592, I naturally had to see if nearby IC4601 would reveal itself. In spite of the triplet of stars enmeshed within, it was actually a little easier.

Needless to say, I'm anxious to see what the view of this part of the sky will be like when one of those all-too-rare superb nights happens, when the troposphere is clear and dry, and the airglow intensity is low.

Here's a neat, annotated picture of the region. Roll your mouse over the image to see labels. Note that north is to the right.

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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV

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