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The Blue Flash (NGC 6905)
Planetary Nebula
aka He 2-466, PN G061.4-09.5, PK 061-9.1, ARO 75
RA: 20h22m23.0s Dec: +20°06'16" (Delphinus)
Integrated Visual Magnitude: 12
Angular Diameter: 44"
Mean Surface Brightness: 20 Mag/arc-secē
Distance 4700 ly

Minimum requirements to detect: 4-inch telescope under dark skies


The Blue Flash nebula is a wonderful find; a tiny blue gem amongst a field of scattered diamonds.  Discovered in 1782 by William Herschel, this planetary nebula is accessible in a 6-inch scope, although it is in the larger instruments that it really shines.  According to Steve Gottlieb:
The phrase (Blue Flash) was probably first used by John Mallas, who had a series of articles titled "Visual Atlas of Planetary Nebulae" in Review of Popular Astronomy in the early '60's.  He wrote for NGC 6905: "The Blue Flash." After turning a telescope on this object one will notice the small triangle of stars superimposed upon it.  Glimmering and flashing between the triangle is the little planetary.  Using about 120x on a 4-inch reveals all than can be seen in small apertures.
Susan Delaney wrote of the appearance in a 10-inch,  "At 64x w/Ultrablock, this rich blue nebula has well defined edges, quite large, quite bright, and oblong in shape.  At 158x w/OIII, it lost its nearly uniform bright appearance and now looked unevenly illuminated with the Eastern portion being brightest and the W edges became less defined and mottled."

When I had my first look at NGC 6905 in my 18-inch I was completely unprepared for what I saw.  I had been hunting down tiny, faint planetaries and it was a real joy to come across this showpiece.  At 94x it popped right into view.  It appeared fairly large and very obvious!  I logged it as "very beautiful" even before moving to a more appropriate magnification.  The surface brightness is not high, but it was still quite easily visible.  The appearance was of a smooth, ghostly egg with fuzzy edges.

At 270x I logged, "remarkable!"  Here was a very uniform oval about a faint star.  The edges of the oval appeared sharp, perhaps "ropey", except at the ends, which appeared irregular.

I moved to 430x, which provided the best view.  The east end of the oval was definitely irregular, while the west end appeared to be somewhat "chopped off."  A darker region became apparent near the central star.  It was difficult to get a handle on the shape of this illusive dark inner area.  Sometimes it appeared to be a donut hole, but most of the time it looked like two dark patches on either side of the star, oriented along the long axis of the nebula.

David Knisely calls this planetary "the 'copycat-M97', as it seems to have darker segments on the north and south portions."  He goes on to suggest that the appearance of the dark "holes" may be an optical illusion.

This is definitely a prize planetary!  I very much enjoyed observing it as it provided a wonderful combination of beauty with lots of discernible detail.


The field in an 6-inch f/8 at 50x.  North is down and east is to the right.

Millennium Star Atlas Vol III Chart 1217
Sky Atlas 2000 Chart 9
Uranometria 2000 Vol I Chart 163
Herald-Bobroff Astroatlas B-05 C-21