[Friends_of_SSASTROS] Notes From The Field - September Observer's Challenge

[Mike McCabe]

 Mike, thanks for sharing your image! This object was a tough confirm in the eyepiece through a small telescope, and certainly worthy of the 'challenge' title.
Mike M.

    On Sunday, September 17, 2023 at 10:10:21 AM EDT, Michael Collver <michael.collver at gmail.com> wrote:  
 
 Below is an EAA image I captured of NGC6891 last month. (for me the biggest difference between it and the “starlike” EP view was the more apparent Bluish color.)Sinc. MC 





On Sep 16, 2023, at 12:57 PM, Mike McCabe <cartech2000 at yahoo.com> wrote:

Discovered on September 22nd of 1884 by Englishastronomer Ralph Copeland, NGC 6891 is a small planetary nebula located in theconstellation Delphinus. Like all planetary nebula, this object is the remnantsof a Sun-like star in its dying phases. Once a medium sized star consumesenough of its hydrogen and internal pressure wins the battle against gravity, thestar sheds its outer layers leaving a hot white dwarf star at the center whichthen ionizes the cast off gasses and causes them to glow.
NGC 6891 is located approximately 12,000 light years from us,and at that distance adopts the appearance of about 17” of an arc in diameter. Whilethis may sound similar to the size that Saturn appears to us (less the rings),in truth is appears much smaller from a visual perspective depending on thesize of the instrument used to view it. To experience seeing the full breadthof the object, one needs to use astrophotography to resolve the faint outershells of the structure. 
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Although located in Delphinus, I found the star hop fromAltair to be a fairly straightforward process and perhaps the easier route iffor no other reason than Altair was readily visible in my magnitude 4ish sky.On the evening that I chose to chase the challenge, the sky was compromised byboth smoke and passing high thin clouds making the parallelogram in Lyraincomplete to the naked eye, even while lying at the zenith. The two starsnearest the target at magnitudes 7.6 and 7.8 were difficult at best in the 50mmfinderscope. As I often do for challenge targets, a finder chart was producedvia Stellarium and printed on a single 8.5”x11” sheet of paper for use at thetelescope.

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In November of 2021 the Hubble Space Telescope produced themost detailed image to date of NGC 6891. Data gleaned from that image allowedastronomers to calculate the ages of the various shells surrounding the remnantwhite dwarf, and found that they range in age from 28,000 years old to 4,800years old. I took the known distance of 12,000 light years and average apparentdiameter of 17” and input those specs into vcalc.com to come up with an actualsize of about one light year in diameter.

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A large personal telescope used under reasonably dark skieswill give you your best chance of resolving NGC 6891 even remotely close towhat it really looks like. In the image above, CN’er Butch Haneuf shared asketch that he made of the planetary nebula as seen through a 24” Newtonianreflector operating at 762x. The small size of the target makes highmagnification necessary to glean detail.

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Another noted astronomical sketcher, Jeremy Perez, producedthe drawing above back in July of 2008 using an 8” Newtonian reflectoroperating at 240x. From the expanded notes on his website it was notable to seethe sky quality that he was working under; NELM 6!

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My own experience from my backyard working under a Bortle 7sky and using a 5” F/8 refractor operating at 226x was one that showed me lessdetail than any of the above examples. In fact the PN was difficult to discernat lower powers and at the higher power the only things in the view were a 9thmagnitude star, two 12th magnitude stars and the PN itself. Whilethe PN is listed at magnitude 10.5, like many PN’s it exhibits a much highersurface brightness of magnitude 7.75. On occasion throughout the observation adull blue/grey tint was noted in condensed nebulosity. 
Clear Skies!
Mike McCabe
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