(IAAC) Obj: Barnard's Loop (Sh2-276, NGC1927, NGC1909, Sh2-245, LBN835, LBN836) - Inst: 2.75", 3", 5", 18", 24"

Observer: Dave Riddle
Your skills: Advanced (many years)
Date/time of observation: Mid February, 2002
Location of site: North Florida, USA (Lat ~32N, Elev ~0m)
Site classification: Rural
Sky darkness: "Transparent"
Seeing: "Good"
Moon presence: None - moon not in sky
Instrument: 2.75" f/6.8 Pronto, 3" triplet f/7.4 TeleVue Oracle 3
            refractor, 5" f/8 AP refractor, 18" f/5 dob, 24" dob
Magnification: 16x, 35x, 58x, 84x,115x
Filter(s): None. UHC. OIII. H-beta.
Object(s): Barnard's Loop (Sh2-276, NGC1927, NGC1909, Sh2-245, LBN835, LBN836)
Category: Supernova remnant. Emission nebula.
Constellation: Ori, Eri
Data: mag FT 2.05m* (zeta Ori); size 600'x30', 400'x40', 600'x20'
Position: 0548 +0101, 0527 -0358, 0552 -0629
Hi Folks,
How much of Barnard's Loop can be traced visually using small aperture
telescopes? Over the course of four evenings I used five telescopes to survey
the area to determine the extent of the Loop and was quite surprised by what
I saw. I used a 2.8" f/6.8 refractor (the Pronto), a 3" triplet f/7.4 refractor
(an old TeleVue Oracle 3), my 5" f/8 AstroPhysics refractor and my 18" f/5
reflector in rural Florida under "good" skies -- despite the site's elevation
being practically at sea level, a cold front brought dry, transparent air and
good seeing conditions.
The part of the Loop north of the Belt of Orion is a well known "H-beta"
object. My previous attempts to follow the nebula southward with an H-beta
filter revealed nothing and I had pretty much written off the object as
"photographic only." When I spotted a large bright glow _without_ the filter
in the position of the Southern Loop, I initially was puzzled by what I was
seeing and thought it must be one of the weakly illuminated dark nebula that
mottle this area of Orion. A follow up observation made this past Saturday
evening revealed this bright area to be the southern extension of the Loop
and I could trace the entire nebula that curves westward near 53 Orionis
(Kappa) and ends just eastward of Rigel. I found the "curve" west of Kappa
particularly well defined in the Pronto at 16X with an UHC filter. A broad
diffused glow runs north to connect with the part of the Loop that is plotted
in the Uranometria. Westward of the curve is a thin wash of nebulosity that
terminates in the position of the "nonexistent" nebulae NGC 1927 & NGC 1909.
Did Herschel see the western end of the Loop and assign it two NGC numbers?
He suspected the _northern_ end of the Loop during his sweeps and commented
"effected with milky nebulosity" and noted it as "area 27" in his list of
suspected nebulous regions and it appears he saw the extreme western end of
the Loop also. I found "NGC 1909" to been easily visible in all of the small
refractors. My 18" (20 mm Nagler with a broadband filter) revealed a weak
amorphous glow about a degree across oriented NE to SW. I suspected a much
fainter halo in the 5" at 35X. I contend this is the "real" NGC 1909 and it
is at the position Herschel noted (05:25:54 -08:08 {2000.0}). The question
of NGC 1927 is more difficult. I suspected an exceeding faint glow extending
south of the double star GSC 5322:1833 with the 18". Alex Langoussis' 24"
reflector made the object a bit easier -- appearing as a very faint fan
extending about 18 arc minutes southward of the double. The south end of this
nebula appeared very slightly brighter at 84X with an UHC and H-beta filter.
How many amateurs are aware that Barnard's Loop has an optical counterpart
that lies about 28 degrees west of Sh2-276? One theory concerning this
western arc (the optical nebula associated with the "Ori-Eri Bubble") is the
western arc expanded at a greater rate than the eastern arc (the Loop) due
to the absence of interstellar matter -- Barnard's Loop being slowed in
expansion by colliding with interstellar material. The Eridanus arc was
catalogued by Sharpless as Sh2-245 and Lynds noted it as LBN 835 and 836.
I found this arc to be a surprisingly easy target in my 18" at 58X (40 mm
Pentax eyepiece) with an [OIII] filter with a true field a bit over one
degree. This arc appeared as a nicely contrasting "stain" about one degree
long and 15 arc minutes wide running north to south. I didn't take the time
to explore beyond this area although it is possible many more parts of the
Eridanus Bubble are visible -- the dimensions of Sh2-245 are 30 by 600 arc
minutes! The area I observed is LBN 836 at 4 02 +3 48 (2000).
To sum it up, the entire arc of Barnard's Loop is visible in surprisingly
small apertures. While the part north of the Belt of Orion responds to a
H-beta filter, the southern arc can be seen without a filter. Apparently,
the emission characteristics change over the course of the Loop. NGC 1909
certainly is a real object and visible in a 70 mm refractor and this nebula
appears to be associated with the great arc of Barnard's Loop. The Eridanus
Bubble has spots of nebulosity that aren't difficult objects as seen from a
dark site. Although it required a few evenings to observe and map this cosmic
bubble, I received a great deal of satisfaction making the observations.
Dave Riddle
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