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Thread: Object of the Week – May 29, 2016 – NGC 6210, The Turtle Planetary

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    Member Steve Gottlieb's Avatar
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    Object of the Week – May 29, 2016 – NGC 6210, The Turtle Planetary

    NGC 6210
    Aliases: PK 43+37.1 = PN G043.1+37.7 = Turtle Nebula
    RA: 16h 44m 29.4s
    DEC: +23° 48' 00"
    Class: 2(3b)
    Size: 20" x 13"
    Mag: V ~ 8.8

    Wilhelm Struve, the famous double star observer, discovered NGC 6210 in 1825 while searching for double stars with the 9.6" Fraunhofer refractor at the Dorpat Observatory in Estonia. It was included as #5 in a list of 9 "Nebulae dectae" in an appendix to his main catalogue of double stars published in 1827. He described a "bright planetary nebula, diameter 6", like a star of mag 7". The planetary was earlier catalogued as a star in Lalande's catalogue of 1801 (LL 30510) and is also known as HD 151121 and BD+24 3048.

    Deep Sky historian Wolfgang Steinicke explains why William Herschel missed this bright planetary -- "On 15 May 1784 he swept the region (sweep 217). However he stopped for about 30 minutes for star counts, i.e. the sky passed horizontally through the eyepiece. Unfortunately the strip runs about 1° below the nebula, starting 25 tmin west and ending 6 tmin east of it. Thus the object could not enter his field of view (15'). On 10 April 1785 he inspected the field again (sweep 395), but saw only some Flamsteed stars in the morning twilight."

    John Herschel observed NGC 6210 on 25 May 1830 and recorded "Struve's fifth nebula, very bright, equal to a star of 8 to 9m, 6" diameter and of uniform light, but with the edges boiling and ragged. A fine object like a star out of focus". As far as the challenging central star, Hermann Vogel observed and sketched the central star with the 27" Grubb refractor at the Vienna observatory on 4 Jun 1883. He noted a "Bright planetary nebula of elliptical shape. A delicate star is in the nebula, nearly at the centre. The nebula is surrounded by a faint elliptical shell."

    On 5 May 1850, Lord Rosse (or observing assistant) logged "intense blue centre fading off to some distance all around. Small star nf to which neb nearly extends. Fancied a projection from the bright centre towards np, but I think it was only fancy."

    As far as Lord Rosse's "projection towards np [northwest], James Keeler observed the planetary with the 36-inch Clark refractor. He produced this remarkable sketch (rotated so that north is approximately up) around 1890 showing the two main wings. The sketch is directly from his Lick notebook, and as far as I know was never published.

    NGC 6210 Keeler.jpg

    Before 1918, Heber Curtis photographed the planetary with the Crossley reflector at Lick Observatory and described the planetary as "Central star...surrounded by bright matter in the form of an oblong 8" [seconds of arc] in length. Outside of this is an oblong of somewhat fainter matter 20" X 13", from the ends of which extend two fainter ansae...From the north and south sides of the central oblong extend two longer ansae...making the nebula 43" long...". This sketch was based on the photograph.

    NGC 6210_Curtis.jpg


    So, there are several challenges involved with NGC 6210. The central star, clearly shown on Keeler's sketch, is listed at mag 12.7, but it's buried in the high surface brightness glow and quite a challenging object. My first view as using my 24-inch, using a "reverse O-III filter" that Jimi Lowrey brought along to the Golden State Star Party in northern California.

    24" (7/8/13): Viewed at 375x with an "inverse OIII" or central star filter, which dramatically dimmed this extremely high surface brightness green planetary. Without the filter the central star was not seen at this magnification (the planetary had a strong green color), but with the filter the central star was easily visible

    It was much easier (unfiltered) in Jimi's 48-inch earlier this month during the Texas Star Party week. I noted, "The central star was continuously visible and situated slightly east of center." What's the minimum aperture that will clearly show the central star?

    The very high surface brightness central region is surrounded by a thin fainter outer halo. These notes were made using my 18-inch

    18" (7/22/06): Appears as a beautiful light blue cosmic egg at 225x with an extremely high surface brightness. Appears slightly elongated ~E-W, ~25"x20" with a very faint thin outer envelope. At 325x and 435x the faint outer halo is obvious, increasing the size ~35"x28" with the elongation possibly WSW-ENE. At 807x, the surface brightness was irregular and the faint outer shell fairly prominent. The bright inner disc was oval but not symmetric.

    And what about those remarkable wings or ansae in Keeler's sketch?

    They are shown very well on this HST image from the 1990's

    NGC 6210.jpg

    I took these notes with my 24-inch from remote Likely, California in the the northeast corner of the state

    24" (6/12/15): At 375x and 500x; slightly elongated oval disc E-W or WSW-ENE. The very high surface brightness central region is encased in a small, but obvious lower surface brightness shell. Extremely faint "wings" appear to extend to the north and south, though the northern extension is more definite. These are roughly in the direction of the minor axis.

    Again the ansae were better seen in Jimi's 48-inch (perhaps Jimi or Akarsh will share their observations),

    48" (5/5/15): On close inspection the main body has an irregular outline, is slightly brighter along the north side and bulges out on the southwest end. A thin fainter "strip" or shell with a much lower surface brightness and a pinkish color encases the main body. Two or three broad "jets" or wings (broadest at the base) extend out of the halo on the NNW side and from the SSE end, with the northern wing more obvious. A third wing was sometimes visible on the west side.

    NGC 6210 HST.jpg


    GIVE IT A GO AND LET US KNOW!
    Last edited by Steve Gottlieb; May 30th, 2016 at 03:43 PM.
    Steve
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    Very interesting article, Steve! It shows once again that bright and well-known objects still hold quite a few challenges. My notes from August 28, 2011 when I used my 20" read

    "At 366x oval, almost rectangular. A faint halo is visible around it with two tails, one to the north and one to the south."

    I guess I'll have to try again to inspect the central star and the other details you mentioned.


    Clear skies, Wouter

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    Hi Steve,

    I tried this last night at Fremont Peak (2700’, SQML 21.23, very good to excellent seeing, at meridian) with my 20-inch f5.25:

    “Visible in 80mm finder as a rather bright star. At 333x the inner shell is bright green, slightly oval shape NW-SE, with a diffuse edge. With averted vision, a fainter larger rectangular outer nebula becomes apparent. More time with averted vision brought out very faint extensions to each corner of the rectangle, two tending north and two tending south, which are the Turtle’s “legs.” No head, though.... OIII seemed to enhance these extensions. Without filter, the central star shown through the inner shell very briefly with averted vision as a bright button, just following the geometric center—held about 20% of the time. 533x revealed some additional mottling in the shell but oddly I could not see the CS – must have lost the seeing.”

    --Mark
    Last edited by Mark McCarthy; May 30th, 2016 at 05:11 PM.

  4. #4
    Member Steve Gottlieb's Avatar
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    Very nice observation, Mark. Glad to hear you were able to detect the challenging central star with your 20-inch! I'll probably give NGC 6210 another look tonight (Lake Sonoma), next weekend, or at GSSP next month.
    Steve
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    I am currently looking at this nebula with my 20" at 256x and the central star is quite easily visible with direct vision. Probably due to the fact that nautical twilight has just ended and the contrast of the nebula with the sky background is still pretty low. Back to observing now.

    Edit: at 320x with SQM 21.2 I can see the central star with direct vision but much harder than earlier tonight. With averted vision the central star disappears.


    Clear skies, Wouter
    Last edited by wvreeven; May 30th, 2016 at 10:12 PM.

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    Nice object Steve. I have some observations with 14.5"-27" and also a view through the famous 48" but I don't want to name any names here

    14.5", 82x-404x, CS filter, NELM 6m5+
    with 4mm EP intensive turquoise; with 404x inner shell structure hard but sure to detect; arms only suspected; CS unsure to detect without filter, because of the very bright center; CS easy visible with the CS filter

    16", 515x, NELM 6m5+
    quick and dirty observation; bad seeing, no CS; arms suspected to the N and S

    27", 837x, NELM 7m0+
    highly structured; both arms easy to see with knots at the ends; CS easy, also without CS filter; inner shell structure
    NGC6210.jpg
    Clear Skies, uwe
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  7. #7
    Member Steve Gottlieb's Avatar
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    We should probably mention the CS (central star) filter is sort of a reverse OIII – it kills the OIII lines but mostly transparent at other wavelengths, increasing the visibility of the central star in many planetaries. The first time I looked through one was in the mid-80's, when Jack Marling (Lumicon fame) was field testing a prototype, which he never brought to production.
    Steve
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    Member Daniel_Sp's Avatar
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    Hi,
    I've always had trouble with this PN, I don't know why... This is an observation sketch made witha 12" dobsonian:
    6210_cut.jpg
    power: 412 with [OIII] filter.
    24"-Dobson, f/4.16

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    Member Howard B's Avatar
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    I've never had a convincing sighting of 6210's central star except in Jimi's 48 inch a few weeks ago:

    "A beautiful planetary - bright turquoise with a hint of pink around the bright perimeter. The central star is visible all the time, and there's a bright spot in the outer halo as well as a slightly curved sprite on the other side. 488x, 21.82 SQM."

    Here's my rough eyepiece sketch:

    NGC6210_crop.jpg NGC6210_cropinvert.jpg

    It looks much better if you print it out, and look at it in the dark with a red flashlight! Hopefully I'll have time later this month to finish it properly.
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    Member akarsh's Avatar
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    Dear Steve,

    Wonderful OOTW!

    Attached is my extremely rough sketch* from the same famous 48" on 2016 May 05.
    Turtle.jpg

    The gray regions indicate the ansae. The orange regions indicate brighter parts of the main body of the planetary nebula. I don't know what the leg that's going down in my sketch is supposed to correspond to, but it was quite vague and not distinct. It might just be a faint elongation of the turtle's body or a false positive.

    Clear Skies
    Akarsh

    * What word am I to use for my 'sketch' if Howard's sketch is supposed to be rough? Maybe 'caricature'?
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    Hello
    I observed 6210 last night . It's one of the few DSO's seen easily from my urban backyard. I like to check Uranometria to see what else is close by. Stroll around the neighborhood a little. About 1 degree due west are NGC 6201 and 6203.

    6201 was stellar at low power, 6203 a round smudge. This is probably trivial news to you, but I'm excited to see them at all.
    Regards
    John
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    Administrator/Co-Founder Dragan's Avatar
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    Hi John and welcome to DSF!

    Quote Originally Posted by Jraymond View Post
    Hello
    I observed 6210 last night . It's one of the few DSO's seen easily from my urban backyard. I like to check Uranometria to see what else is close by. Stroll around the neighborhood a little. About 1 degree due west are NGC 6201 and 6203.

    6201 was stellar at low power, 6203 a round smudge. This is probably trivial news to you, but I'm excited to see them at all.
    Regards
    John
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    Member Don Pensack's Avatar
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    My somewhat cryptic notes from 2006, with a 12.5" f/5.75 scope in skies of magnitude 21.6:
    "roundish-oval,v.brite,edge very diffuse & faint,brite ctr ,inner bluish,outer reddish!,can use >300x, hi surf.briteness,cent.* vis"
    My notes show the central star is magnitude 12.5, which should be easy in most scopes.
    I didn't see the extensions that represent the turtle's legs (obviously for a big scope!).
    I did see colors, as Steve reported. I have noted, of late, that my red sensitivity, by comparison, is higher than many observers.
    My notes also indicate that no filter was used for the observation.
    Don Pensack
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