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Cosmic Challenge: Abell 70
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Cosmic
Challenge:
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September 2023 |
Phil Harrington |
This month's suggested aperture range:10-inch (25 cm) to 14-inch (36cm) telescopes |
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Target |
Type |
RA |
DEC |
Constellation |
Magnitude |
Size |
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Abell 70 |
Planetary Nebula |
20h 31.6h |
-07° 05.3' |
Aquila |
14.7 |
42" |
Of the more than 80 planetaries listed by George Abell in his 1966 paper "Properties of Some Old Planetary Nebulae," Abell 70 (also known as PK38.1-25.4) is one of the most unique. Actually, the planetary itself is a stereotypical example of a ring nebula, like M57, with a round shell of gas expanding away from the dim progenitor star. But look carefully and there is clearly more here than just that. Photographs show that the ring has a brightening along its northern edge. The allusion to an engagement ring is unmistakable, but that's no diamond. Instead, that odd brighter segment proves to be a distant lens-shaped galaxy that just happens to lie along the same line of sight. Can you spot both through your telescope?
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Above: Evening star map showing the location of this month's Cosmic Challenge.
Credit: Map adapted from Star Watch by Phil Harrington |
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Above: Finder chart for this month's Cosmic Challenge.
Credit: Chart adapted from Cosmic Challenge by Phil Harrington
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If you will be star-hopping, zeroing in on this unlikely odd couple will take some effort, since they lie in the dark, southeastern corner of Aquila. Here's one approach. Begin at the wide double star Alpha (α) Capricorni in neighboring Capricornus and scan about 3° to the northeast. Use chart 6.11 to spot Abell 70 lying to the southeast of the halfway point between two 10th-magnitude stars and just 3' west of an 11th-magnitude sun.
Even though it shines at only magnitude 14.7, Abell 70 is large enough that high magnification is not absolute necessary to identify its disk from among the neighboring stars. To resolve its distinctive annular shape, however, will take a slow, careful examination using averted vision and probably no less than 350x. An O-III filter will also help to accentuate its subtle annularity.
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Above: Abell 70 as portrayed through the author's 10-inch (25cm) telescope. The superimposed galaxy went unseen, but its location is shown as a dotted oval.
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Above: This image by Adam Block was taken using an SBIG STX16803 CCD Camera and Astrodon Gen II filter on the 32-inch (81-cm) Schulman Telescope at Mount Lemmon SkyCenter/University of Arizona. CC BY-SA 3.0 US, via Wikimedia Commons |
Remove the filter to try and catch a glimpse of its galactic companion, cataloged as PMN (Parkes-MIT-NRAO) J2033-0656, behind the ring's northern edge. But don't be surprised if it evades detection. It did through my 10-inch (25cm), which is why it is shown only as a dotted oval in rendering above. Spotting the interloper's dim presence, rated at 16th magnitude, taxes even my 18-inch (45.7cm) reflector under 6th-magnitude skies. Undoubtedly, darker conditions would have made my quest a little easier. But even though the galaxy may come clean, even the darkest, most transparent skies probably will not help to show the planetary's central star that started it all. It shines at only 19th magnitude. A 2011 study entitled A barium central star binary in the Type I diamond ring planetary nebula Abell 70 shows that the central star is actually a binary system, with a hot white dwarf and an unusual G-type subgiant "barium" star.
Abell 70's distance is not well established. Current estimates place it between 13,500-17,500 light years away, but it's getting closer. It is approaching our solar system at a rate of 176,400 miles per hour (283, 900 kilometers per hour). Of course, by the time it gets anywhere this neck of the galaxy, it will have long since dissipated. Right now, the nebula is expanding at a rate of 85,000 miles per hour (38 km per second).
Have a favorite challenge object of your own? I'd love to hear about it, as well as how you did with this month's challenge. Contact me through my website or post to this month's discussion forum.
Until next month, remember that half of the fun is the thrill of the chase. Game on!
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About
the Author: |
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Phil Harrington's Cosmic Challenge is copyright 2023 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder. |
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- John O'Hara, warddl, Strider14 and 2 others like this
12 Comments
Thanks for this challenge, Phil!
I plan to attempt this pair with my 15" under magnitude 6+ skies. I don't have an OII filter (yet) so it will be interesting to see how much detail I can pull.
For the galaxy, it will be tricky, but hopefully I can get a good smoke-free night like we've had this week. Too bad the moon is so bright!
Oohhh another awesome one , thanks phil
I've seen this one a few times in the 12.5" in skies of m.21.3-21.5.
The important thing is excellent transparency of the air and a good O-III dual line filter.
Once the planetary is observed, remove the filter to see the galaxy at the edge. In 12.5', it resembles a brighter knot on the ring.
I can't say I saw much extension to the galaxy, though the galaxy does extend beyond the core slightly.
A 16" can obviously see more:
https://univerzumkep...-nebula-aquila/
so I'm looking forward to my 16" that is coming.
Important:
--Aquila on the meridian
--no moon in the sky
--fully dark adapted eyes
--excellent transparency.
Here is a sketch on a much better night from the same observer and scope:
https://univerzumkep...ebula-aquila-2/
So transparency is critical on this one.
I remember seeing Abell 70 about 15 years ago in my 18" Dob from LBV in the Anza Borrego Desert before I got the mirror recoated. I don't remember what power I used but I remember the OIII filter helped.
I think it's time for a revisit with new coatings on the mirror and my Ethos eyepieces, thanks for the reminder Phil.
I'm making it a point to try this one with my 12.5" Dob. at Cherry Springs State Park, PA this dark moon cycle. Thanks again, Phil, for these posts and your fine book. It's nice to see there are still those of us who do visual observation of the deep-sky. I love the images produced by our imaging brethren, but for me, it will always be visual.
I shall give Abell 70 and it's galactic companion a go with my incoming 14 inch from bortle 1 skies. Thanks for your monthly challenges, I eagerly await them every month!
I shall give Abell 70 and it's galactic companion a go with my incoming 14 inch from bortle 1 skies. Thanks for your monthly challenges, I eagerly await them every month!
Under good transparency, I completed this challenge last night.
Using a 15" f/4.5, I was able to make out the ring shape of Abell 70. I used a UHC filter for the initial detection at 73x. This observation was quite tricky, but averted vision made the planetary more obvious.
Without the UHC, I noticed a very subtle brightening to the northwest. If I didn't know to look for it, I wouldn't have seen it! I'm fairly confident that I saw the core of the galaxy, but I didn't notice any extension. 250x make the detection of this uneven surface brightness more apparent.
This was a tough but fun one! an OII filter is high on my priority list.
I observed this earlier this week in the 12.5". It was more visible with no filter than it was with the O-III or narrowband.
That surprised me. I didn't try a broadband filter, but that could have been the optimum.
The best view was at 166x and 205x, for me.
That's surprising! It's good to hear that my unfiltered observations were similar to your experience, and I wasn't too compromised by lacking the OII.
My best views were 150x and 250x. I figured higher power would help bring the planetary out, but 330x was a little too much.
Note: I believe the PK designation provided is a little off and has been slightly muddled/combined with the PN G designation. It should be PK 38-25.1 while the more up-to-date catalog would be PN G 38.1-25.4 (or PN G038.1-25.4)
I observed this one on 9/12/23 with the 20". The sky was running bright at ~21.0 mpsas. The PN and galaxy were found unfiltered right away at 227x. At 278 and 357x I observed the galaxy as "xF, ave SB, ~4:1 WNW/ESE elongated on N portion of nebula, w//xxF stellar core." Switching to an OIII filter at 227 and 278x the annular structure was only weakly apparent, while the galaxy was greatly dimmed.
Overall, the nebula was of more modest surface brightness than I expected. The PN has a mean surface brightness in the ~22.5 mpsas range from what I can tell, essentially an average galaxy surface brightness.
I expected the annular structure to be more apparent, particularly when filtered, but it seems that much of what is seen of the brightest outer ring in images is redder emissions that the scotopic adapted eye does not pick up well, and the red emissions are lost with the OIII filter anyway.
I observed this last night at the dark site, with (at least for an hour at the beginning of the night) above-average transparency, in a 12" so consider me a legitimate challenger (or just challenged).
From the log: "Ghastly! The faintest circle using averted vision at 190x, v. difficult. Filters only help a little since already so dim." (UHC and OIII)
The filters did help, though each about the same amount I think. With them, the PNe becomes a more well defined circle that is easier to obtain (but not really hold) using averted vision. You gain a tad bit better contrast, but there really isn't a whole lot of photons to spare. Without the filters, it was a very tough and intermittent circular-ish light patch. I don't know if the less clean shape was because of the background galaxy or the sheer difficulty of observing it at all. In neither case was there any visible structure - just an evenly dim patch.