[Lvas] June Observer's Challenge "Supplemental"

[roger ivester]

All,

 

I was looking for the latest Deep-Sky Wonders article by Walter Scott Houston concerning the finest globular cluster of the northern sky, M-13 and the illusive propeller. 

 

August 1992 Sky & Telescope, Deep-Sky Wonders, by Walter Scott Houston, page 225. 

 

Recently, an obscure feature of M-13 has become a visual quest for amateurs. During the middle of the last century, observers using Lord Rosse's large reflector in Ireland noted three dark lanes radiating propeller-like from a common point southeast of the cluster's center. These lanes were well enough known that the Rev. T.W. Webb mentioned their visibility with a 9-inch reflector in his classic, late-19th-century observing guide Celestial Objects for Common Telescopes. 

 

I have the two volume set by the Rev. T.W. Webb. Volume Two: The Stars, written 1859, and revised by Rev. T.E. Espin in 1917. The Dover edition, first published in 1962, is a revised and enlarged republication of the sixth edition of the work written by the Rev. T.W. Webb.   

 

I have always found this most historical book to be invaluable as a referene source. This was one of the most used and coveted deep-sky reference books for over 100 years. 

 

Celestial Objects for Common Telescopes: Volume two: The Stars, page 143.

 

h. describes "hairy-looking curvilinear branches" well seen with 8-inch "With" mirror; E. of Rosse, who noticed this spiral tendency, detected also three dark "lanes" or rifts in the interior, beautifully seen by Buffham with 9-inch. "With" mirror. I have also perceived them.

 
Our "Observers Challenge" for June will be M-13. We will be looking for the illusive "propeller" within M-13. Three dark lanes radiating from the central region. See my sketch of Lord Rosse's sketch of about 1850 from Ireland.
 
WSH brought presented this challenge in "Deep-Sky Wonder's" in Sky & Telescope July 1953. 
 
In 1980 John Bortle saw the lanes with his 12.5-inch reflector at 176x. Dennis di Cicco at Stellafane in 1981 was surprised by how easily the lanes were seen with the 12-inch f/17 Porter turret telescope at about 180x. However, even knowing their orientation and appearance, he was unable to see them at 95x with a 12-inch that was set up nearby. Sighting the lanes seems to depend upon a careful balance of aperture and magnification. 
Both Bortle and Dennis di Cicco commented on the importance of magnification. Most observers note that they appear best at a magnification of about 200x. Information from "Deep-Sky Wonders" by Walter Scott Houston, selections and commentary by Stephen James O'Meara; Sky Publishing Corporation Cambridge, Massachusetts 1999.   
  
Roger Ivester
   

 
 

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