The great globular cluster, M13 is located high overhead for observers in the northern hemisphere during the summer months, allowing for excellent viewing.  When observing this cluster back in 1977 with my 4 1/4-inch f/10 reflector, the most I could see was a moderately bright ball of unresolved stars.    

While observing M13 with the small reflector, I was unaware of the three dark lanes cutting into the SE edge of the cluster, known as the propeller.  I had never heard of the propeller at that time, but this scope was far too small to see this very faint feature.  However, thirty two years later in May of 2009 using a much larger telescope, I was finally able to see the three dark-lanes.     

A 10-inch telescope “might be considered the minimum aperture” required to see this most unique feature.  If you have plans to observe the “elusive propeller” a magnification of around 200x seems to be the optimum.  

The propeller challenge seemingly became popular due to Walter Scott Houston, columnist and writer of “Deep-Sky Wonders” suggesting that observers look for it.  

Houston first wrote about the propeller in the July 1953 edition of “Sky & Telescope magazine.”  He brought it up several more times through the years, however,  it has been only in recent times that the propeller has gained much attention in the amateur astronomy community. 

Lord Rosse mentioned three dark rifts in the 1850’s, and T.W. Webb in “Celestial Objects for Common Telescopes” noted that the lanes were seen by Buffham, using a 9-inch reflector. 

John Bortle saw the lanes in 1980 using a 12.5-inch reflector, and Dennis di Cicco saw them easily with the 12-inch f/17 Porter telescope during Stellafane in 1981.  (Source: “Deep-Sky Wonder’s” By Walter Scott Houston, selections and commentary by Stephan James O’Meara.  Sky Publishing Corporation, Cambridge Massachusetts)  

In May of 2009, I was able to observe the propeller with both 10 and 12-inch Newtonian’s.  My first sighting of the propeller came when using a friends 12-inch from the southern rim of the South Mountains in North Carolina.  I would rate this site as very good with a NELM of 6.0 (at the zenith) and maybe even better on an excellent night.  

The propeller was fairly easy to see with the 12-inch f/5 reflector from this location.  However, seeing the propeller from my suburban backyard and 10-inch f/4.5 Newtonian, was possibly seem only using my imagination.  🙂

The following sketch was made using Steve Davis’ Meade 12-inch f/5 reflector at 190x, just off Moore Mountain Road in Rutherford, County.  

The sketch was made with a No. 2 pencil and a blank 5 x 8 note card.  The colors were inverted using a scanner.