OBSERVER’S CHALLENGE* – SEPTEMBER, 2022
by Glenn Chaple
NGC 6751 Planetary Nebula in Aquila (Magnitude 11.9; Size 26”)
For the third consecutive month, the Observer’s Challenge features a planetary nebula. Having explored NGC 6210 (July) and NGC 6772 (August), we turn to NGC 6751. Nick-named the “Glowing Eye Nebula” or the “Puffball Nebula,” NGC 6751 is located a little over one degree directly south of the 3rd magnitude star lambda (λ) Aquilae at the 2000.0 coordinates RA 19h05m55.6s, Dec -5o59’32.9”. It shares the same low-power field with the ruddy-hued carbon star V Aquilae, which lies one-half degree to its northwest (refer to the accompanying Finder Charts).
As was the case with NGC 6210, NGC 6751 was another William Herschel “miss.” It was discovered on July 20, 1863 by the German astronomer Albert Marth, who spotted it with a 48-inch reflecting telescope. Despite the large aperture of this instrument, NGC 6751 can be picked up with a 6-inch scope – even smaller under dark-sky conditions. Visual observers will see the bright, inner part of NGC 6751, which, at 26 arc-seconds in diameter, is approximately equal in apparent size to Saturn’s disk. Imagers might be able to capture a faint outer halo that spans twice that diameter.
I tackled NGC 6751 on the evening of August 24, 2022, using a 10-inch f/5 reflecting telescope. It took on the appearance of a 12th magnitude star at 40X. A wide-field eyepiece at 80X revealed its non-stellar nature. At 208X, NGC 6751was small, dim, and roundish. I was unable to glimpse the 14th magnitude central star. Switching to a 6-inch f/8 reflector, I was still able to pick out NGC 6751 from a rich Milky Way field. An O-III filter held between my eye and the eyepiece dimmed or eliminated the surrounding field stars, while NGC 6751 maintained its brightness.
A with many planetaries, the distance to NGC 6751 is uncertain. A NASA website cites a distance of 6500 light-years and a total diameter of 0.8 light-years.
NGC6751 Finder Chart A
NGC 6751 Finder Chart B
Created using the AAVSO’s Variable Star Plotter (VSP). Numbers are stellar magnitudes, decimals omitted. The magnitude 3.4 star is lambda (λ) Aquilae, the magnitude 4.0 star is 12 Aquilae, and the unmarked star near lower center is V Aquilae. Stars plotted to 11th magnit ude. North is up in this 1½ by 1½ degree field.
NGC 6751 Image
Mario Motta, MD (ATMoB) “NGC6751 was a challenge, visually small sphere, with obvious central star, easy to see. Imaging was difficult due to dynamic image range. I took this with Ha, O3, S2 NB filters, but also lum filter to include the bright central star. I noticed faint outer nebulosity in Ha and O3 images, but getting that to show without blowing out the central portion and losing the star was a challenge, A little bit of a compromise to get very faint outer nebulosity and inner bright sphere and star. Taken with my 32 inch scope, and ZWO ASI6200 camera and above filters.”
NGC 6751 Sketch
Sketch by Glenn Chaple (ATMoB)