(IAAC) Obj: NGC 6781 (Snowglobe, PK41-2.1, HIII-743, PNG41.8-2.9) - Inst: 17.5" f/4.5 dob
Observer: Lew Gramer; Steve Clougherty
Your skills: Intermediate (some years); Advanced (many years)
Date/time of observation: 2200 Local, 17 Sep 2001
Location of site: ATMoB Clubhouse, Westford MA USA (42oN, 86m elev)
Site classification: Suburban
Sky darkness: 4+ <Bortle Scale (9 worst)>
Seeing: 8 <1-10 Seeing Scale (10 best)>
Weather: Occasional cirrus, moderate to high humidity
Moon presence: None - moon not in sky
Instrument: 17.5" f/4.5 dob
Magnification: 90x, 222x, 285x, 500x
Filter(s): None. UHC.
Object(s): NGC 6781 (Snowglobe, PK41-2.1, HIII-743, PNG41.8-2.9)
Category: Planetary nebula.
Data: Mag 11.8 14.95?m*; size 111"x109"
Position: 191828.3 +063223
First impression of this moderately bright, large PN was
relief: finally a striking object despite mediocre skies!
NGC6781 is quickly found by following a line from Altair,
past mag 4 star Mu Aql, and then about the same distance
farther again (4o). The PN lies in a somewhat empty area,
30' due E of mag 7 blue-white star HD 180504: so if you
find yourself in the midst of a pretty group of 5 or so
mag 5-8 stars, your finder strayed 1o to 2o too far WSW.
Using unfiltered low power and direct vision, what I'll
arrogantly dub "The Snowglobe Nebula"[!] appeared as a
diffuse, slightly irregular disk with a noticeable but
still subtle annularity. And yet each time we upped the
power or applied a UHC, a new impression would emerge...
At 222x sans filter, both observers quickly noticed that
the NNE corner of the ring was nearly twice the width of
neighboring areas, and also brighter and more irregular
or mottled as well. This N "lobe" extended just slightly
outside of the primary ring, but most of its extra girth
was due to its impinging on the dark central area. Using
the UHC on the 9mm, the N lobe was much less noticeable,
and the Snowglobe's annularity was also much more subtle.
At 285x filterless, both observers were suddenly struck
by hard, dark "edges" on both the W and SW periphery of
the main ring. At times (possibly related to the seeing),
the two edges seemed to meet at a definite WSW "corner"!
The W edge was long, and nearly abutted the dark center.
The SW edge was short, and required patience to confirm.
Using the UHC, small pieces of an outer halo, extending
out less than half the width of the main disc, could be
seen just beyond the N and E edges of the main ring. But
these halo arcs were only easily observed when we moved
the main disk just outside the S or W edge of the field.
Finally, at 500x (christening a recently purchased 4mm
Lanthanum) the impression of a hard SW "edge" to the PN
was confirmed in direct vision. That edge, coupled with
its internal complexity but otherwise smooth outer ring,
lead me afterward to pick the strange eponym that I did
for this wonderful little PN - the Snowglobe Nebula. :)
Last of all, with the UHC at this power, in rare times
of steady seeing, tantalizing hints of complex details
began to pop out at us! I saw what I interpreted to be
partial internal rings, possible striations within the
innermost haze, and even some (unconfirmable) filaments
in the main ring nebulosity. Also in this view, arcs of
the halo were a little easier. What a rewarding object!
At most powers, a near-central star was noted with both
averted vision AND direct vision, but only about 50% of
the time in either case! Explaining this seems to point
to a strange dual seeing-contrast effect: apparently if
the star were in a still, crisp column of air, it would
emerge from the faint central haze even to direct vision.
Yet it was faint enough and its contrast marginal enough,
that even slight blurring made it disappear. Have others
observed this with any central star? In any case, using
a UHC at any power always seemed to obliterate the star.
At least two other faint stars seemed to be involved at
times, near the edges of the dark inner core of the PN.
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