 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.
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