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Re: (IAAC) Re: Obj: NGC 6781 (Snowglobe, PK41-2.1, HIII-743, PNG41.8-2.9)...



MegaStar gives the magnitude of the central star as 16.7.
Don
-----Original Message-----
From: Lew Gramer <dedalus@alum.mit.edu>
To: Internet Amateur Astronomers Catalog - Discussion
<netastrocatalog-announce@atmob.org>; Amateur Telescope Makers of Boston
<atmob-discuss@atmob.org>
Cc: Lew Gramer (me) <dedalus@alum.mit.edu>
Date: Friday, September 21, 2001 6:04 PM
Subject: (IAAC) Re: Obj: NGC 6781 (Snowglobe, PK41-2.1, HIII-743,
PNG41.8-2.9)...
>
>In response to my observing log, Tal Mentall notes:
>>The SkyAtlas 2000.0 Companion, 2nd Ed. says that central star is 16.8 mag.
>
>
>Thanks for looking that up, Tal! If 16.8 is the correct visual magnitude of
the
>central star, then I'll lay strong odds that what we saw that night was not
the
>central star! On the other hand there seem to be multiple stars within the
ring
>of this little PN... Being of a somewhat complex shape (and the star only
being
>occasionally seen), we might easily have been noting a star that was within
the
>ring, but off-center. And I guess there's the possibility that the
"Companion"
>might actually be quoting a magnitude in some other 'band' than v, without
its
>authors necessarily being aware that their original source was doing so...
?
>
>
>Jeff Bondono, in the database of his wonderful observing planner software,
>'dObjects', gives the central star magnitude that I quoted, "14.95?m". The
>references he mentions for this object's data include: Brian Skiff's Webb
>Society Quarterly Journal article "Precise Positions for NGC/IC Planetary
>Nebulae", and also "SkyCat II" (Sky Catalog 2000.0, volume 2). I'd assume
>that the central star magnitude Jeff gave came from SkyCat.
>
>Doug Snyder, on his excellent Planetary Nebulae web site
http://blackskies.com,
>lists the central star of the "Snowglobe" as *photographic* magnitude 16.3.
It
>is generally not at all clear to me how a 'p' mag can be converted into a
'v'
>mag, (especially when I don't know of any color indices for the star).
So...
>
>
>
>I looked for other reports of internal stars within N6781, and found these:
>
>Steve Coe using a 17" scope, reports at
http://blackskies.com/reports13.htm:
>"A mag 13 star is at the NE edge and once or twice I caught a fainter
glimmer
>of an interior star north of center."
>
>Yann Pothier also in a 17" reports,
http://visualdeepsky.org/logs/msg01780.html:
>"... [There is a] mag 15 star in the ring to the W (and a little S) and
another
>just outside to the ENE..."
>
>R. Bartlett in his StarLore site http://www.starlore.net/ngc6781.htm
reports:
>"You'll need clear dark skies to see this one. Some detail can be seen with
>medium scopes but only more powerful scopes will show the magnitude 16.1*
>central star. NGC 6781 is expanding at a rate of 12 km per second."
>
>Noted deep-sky sketcher Scott Davis's site
http://clifty.com/scott/DSO/6781.html
>contains the following interesting but unattributed information:
>"NGC-6781... MAGNITUDE OF CENTRAL STAR: +15.5"
>
>Finally, avid CCDer Scott Griswold from Astro. Soc. Greater Hartford (along
with
>an unnamed accomplice), notes at
http://ccdguy.homestead.com/planetaries.html:
>"NGC6781: Look at this beauty!  Only 16x45 second images capture this gem
and
>it's central star.  The nebula glows at mag 11.8, and it's central star at
mag
>16.3..well within the reach of a six inch scope [using a CCD]!"
>
>
>Oh, well, it will probably require a more patient look at this amazing
little
>object, with similar skies and a similar instrument, to finally resolve the
>question of the internal star or stars in the "Snowglobe Nebula"! :)
>
>Thanks again for posing an interesting question on this log, Tal.
>
>Clear skies,
>Lew Gramer
>
>
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