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Cosmic Challenge: Galaxies around M13
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Cosmic Challenge: Galaxies around M13July 2016
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If you're
like me, you have probably seen Jupiter, Saturn, the Orion Nebula, and all of
the sky's showpiece objects more times than you can count. And while they are
truly spectacular and well worth revisiting, you may be looking for something
new, something challenging to observe.
That's the premise behind this new monthly e-column here on Cloudynights.
Each month, we will look for objects that, quite honestly, many amateurs
don't even know exist!
Actually, the spark for Cosmic Challenge dates back nearly three decades, to a
column I wrote in Deep Sky magazine entitled "Challenge Objects."
That later evolved into my book, Cosmic Challenge
(Cambridge University Press, 2010), and now as an offshoot, this column
here on Cloudynights.
Of course, what qualifies as "challenging" is difficult to define. An object might be very faint, or very small, or tough to spot for any of a number of other reasons. Some require excellent sky transparency, while others need rock-steady seeing. So much also depends on each person's level of experience, the clarity and darkness of the observing site, and the telescope used.
To help level the playing field, each month will feature one of six instrument categories based on aperture: naked eye, binoculars (less than 70mm), giant binoculars and 3- to 5-inch (7.5-13 cm) telescopes, 6- to 9.25-inch (15-23.5 cm) telescopes, 10- to 14-inch (25-36 cm) telescopes, and 15-inchers (38 cm) and up. This month, for instance, is designed for 10- to 14-inch apertures, as you can see on the column's header. Next month, who knows.
Before we begin, let me give you one word of advice: Patience! Unless you have the time to concentrate on the hunt, to stop and pull back to clear your mind before diving back in, then Cosmic Challenge may prove frustrating. These targets are not suitable for quick viewing sessions. Most can only be seen after patient searching for many minutes -- indeed, sometimes hours -- on end.
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Above: Summer star map from Star Watch by Phil Harrington. |
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Above: Finder chart for this month's Cosmic Challenge. Chart adapted from Cosmic
Challenge by Phil Harrington. |
I’m sure that most of us have seen M13, the Great Hercules Globular Cluster, through many different binoculars, telescopes, and maybe even by eye alone. It’s a real dazzler.
But have you ever look around M13 and examined its surroundings carefully? Within about a degree are six faint intergalactic members of the NGC and IC listings. They make fun challenges through 8- to 12-inch apertures, and even larger if light pollution is a little overwhelming.
The
brightest of the bunch is NGC
William Herschel discovered NGC 6207, but he missed IC 4617. Can you do better? You'll find it centered between NGC 6207 and M13, just touching the western side of a trapezoid of 13th-magnitude stars. This little cigar-shaped spiral is a fairly tough catch with my 18-inch from my suburban backyard, but is clearly visible through my 10-inch under better conditions. Aperture is great, but there is no substitution for dark skies.
If IC 4617 was a little too demanding, NGC 6196, the brightest of a trio of galaxies found about a degree southwest of the globular, should prove easier. Under dark skies, a 10-inch shows this 14th-magnitude target as a small, round glow highlighted by a brighter central core.
A
second, dimmer galactic smudge is nestled 6' to the southeast. The true
identity of this second object is IC 4616, although some references
identify it incorrectly as NGC 6197. The confusion dates back to
German-born astronomer Albert Marth (1828-1897). Marth came to
Next, try your luck with IC 4614, set 3' northwest of NGC 6196. Like IC 4616, IC 4614 is rated at 15th magnitude and takes a special night to be seen through even the largest backyard scopes.
The furthest west of the bunch is NGC 6194. Lying 1.1° west-southwest of M13, this little elliptical galaxy shines at 14th magnitude and spans just 1' across. You'll need at least 150x to see its small, amorphous glow.
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Above: The view of (left to right) NGC 6194, IC 4614, NGC 6196, and IC 4616 through my 18-inch Newtonian. |
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 test. Contact me through my web site or post to this e-column's discussion forum.
Remember, half of the fun is the thrill of the chase. Game on!
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About the Author: Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy. Visit his web site at www.philharrington.net to learn more. |
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Phil Harrington's Cosmic Challenge is copyright 2016 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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- R.bak, John O'Hara, DHEB and 2 others like this
14 Comments
Nicely done Phil!
Next weekend weather permitting I am observing both Friday and Saturday night. There's a couple of objects on your list that I need to visit
Phil,
Here is an image of M13 and NGC 6207 (in the 2 o'clock position) taken with a Zeiss 300/f4 lens stopped down to 49 mm. The exposure was 163 seconds at iso800. No other galaxies are detectable. The limiting magnitude on this image is around 16.
I can not meet your challenge visually as my biggest telescope is 5 inch.
Rudy
http://cloudynights....es/image001.jpg
Nice article, Phil.
These are conveniently located, yet tough galaxies to find. My backyard has around a 5.5-NLM and I've just glimpsed NGC 6207 with a 4" f/5 Genesis refractor. It's a little easier, but still subtle, with a 110mm f/6 Orion EON.
The only time I saw IC-4617 with less than 10" was with my 8" reflector. Not the best tool for the job, but I was under a Bortle Class 3 sky that one night. I could barely detect it with a 7XW in the focuser under those conditions. I need 12"+ to see it most nights from Bortle Class 4/5 skies.
Clear Skies,
Phil
Hi all,
Thanks for the comments. Phil, that's great that you caught IC 4617 in an 8-inch! Good eye, regardless of the skies. You beat me. :-)
And Rudy, nice shot through your 5-inch. I looked closely and though you also caught 4617, but I couldn't quite make it out. But nice capture of 6207 (and M13, of course).
Phil,
One object I suggest is NGC 1297, near NGC 1300, which I don't think you've mentioned in Cosmic Challenge. Below are my notes concerning my observation of it at the 2004 Black Forest Star Party at Cherry Springs State Park. At the time, skies were a solid Bortle 2.
At this year's Black Forest Star Party, in Northcentral Pennsylvania,
I sighted NGC 1297, by accident, while viewing NGC 1300 through my
mid-80s 6" f/8 Astrophysics refractor. NGC 1300 is not "easy" in a 6",
and I wasn't even looking for NGC 1297, but I spotted it with averted
vision in the field of a 20 mm eyepiece giving 62x. Once located, I
could see it with direct vision. Switching to a 13 mm eyepiece
yielded 95x, and darkened the field for a better view, making NGC 1297
steadily visible by direct vision.
The publication, Observing Handbook and Catalogue of Deep-Sky Objects,
stated that this object is not observable in a 150 mm class
instrument. The late Walter Scott Houston wrote that an 8-inch scope
was the smallest with which he successfully viewed this object. I
suspect that the quality of the observing site played the biggest role
in spotting this faint (for a 6") object, as I've seen mag. 6.8 at
this remote site when transparency is perfect, and younger eyes have
spotted 7.0 stars. For me, this particular night yielded mag. 6.5, as
transparency was good but not perfect. I attribute my observation to
the site, as I know I'm no match for the venerable Walter Scott Houston.
Thanks, John. Good suggestions!
NGC 1297 and IC-4617 aren't prominent, but what does help is that they're both small. When the surface brightness of a faint galaxy is roughly comparable to their total integrated magnitude, my approach is simply crank up the power. The object's light won't spread out that much if it's small, but the sky background will progressively darken with higher powers to boost contrast.
Although I prefer observing galaxies with my 14mm and 10mm Delos eyepieces, the 7mm Pentax is the weapon of choice if it's small and marginally visible, particularly with my 8". I've gotten IC-1296, the 14th-magnitude galaxy next to M57, with the 7XW + 8" f/4.9 (164X with the Paracorr) from dark skies. Like IC-4617, IC-1296 is MUCH tougher with the 10mm Delos (115X using the Paracorr) and impossible with the 14mm (82X).
The formula I use for telescopic limiting magnitude (and there are many out there; it's just the one I use) is:
TLM = NLM + 3*log(A/7) + 2*log(P), where
TLM = telescopic limiting magnitude
NLM = naked-eye limiting magnitude
A = telescopic aperture, mm
P = power/magnification
Using this formula, John gained almost 0.4-magnitudes in limiting stellar magnification between going from the 20mm to the 13mm eyepiece. While NGC 1297 isn't stellar, it's still pretty small, so the gain in perception between 62X and 95X makes sense.
Clear Skies,
Phil
I've never seen this particular formula before, I'm used to the simpler formulas that only consider NLM and the aperture of the scope (e.g., 9.8+5logA=TLM (where A is inches).
However, your formula matches more closely my actual experience, as I've consistently beat the old formula with my scopes. I've always attributed this to the ability to find tune the focus of a scope, and I knew magnification, within limits, played a role with point and point like sources, but I've not seen it quantified in any way. Did you find this formula in a reference, or did you derive it through experimentation?
John
John,
This isn't my formula; I found this formula online. Formulas for telescopic limiting magnitude are like plossl eyepieces--everyone has one! The reality is that there's too many variables to consider for a foolproof formula, or at least one that's compact. I use this formula because (1) it agrees quite well with observations I've made over the years, (2) it incorporates the ability of magnification to bolster limiting magnitude to a plausible extent, and (3) it's easy for me to remember.
Simply factoring in aperture is a reasonable approximation at low power, namely because the telescope's exit pupil more closely matches the dilated pupil diameter. It's simply of matter of comparing pupil size to aperture and calculating accordingly. If you set the exit pupil equal to 7 (mag = A/7), the above formula simplifies to TLM = NLM + 5 log (A/7).
Once power is cranked, though, it's a different ballgame. Ohio variable star observer Ernst Mayer (AAVSO president, 1983-1985) was able to see 15th-magnitude stars with a 6" Newtonian.
Another good--albeit extreme--example is Stephen O'Meara's observation of Halley's Comet in January 1985 (13 months prior to perihelion) from Mauna Kea with a 24" (610mm) Cassegrain. Even under perfect conditions and eagle-eyed vision, the best the human eye could do is 8.5-mag. So if we just look at aperture, even Stephen O'Meara shouldn't have gotten past 18.2-mag, though O'Meara's visual estimate for 1P/Halley (quasi-stellar at the time, for all intents and purposes) was 19.7, and he saw a 20.6-mag comparison star! (Clark, April 1994 S&T)
I don't know what power O'Meara was using, but Mauna Kea has EXTREMELY steady skies. If we assume 50X/inch, that's 1,200X for a 24"-scope--certainly doable at that site. If we assume NLM = 8.5 for O'Meara and 1,200X, this formula generates a plausible result:
TLM = 8.5 + 3 * log (610mm / 7mm) + 2 * log (1,200) = 20.5
This formula has some weaknesses in it, like all others. One is that the NLM is simply an additive quantity. That is, if 200X generates a TLM of say, 15.0, under dark NLM = 6.5 skies, it would be only be 13.0 under suburban NLM = 4.5 skies. In reality, once you start cranking power, your sky background gets dark regardless of location. Magnification is somewhat of an equalizer in this regards, though it stands to reason you'd still see significantly fainter stars at high power from dark skies vs. more urban locations.
The second is that the NLM keeps increasing once you get beyond 50X/inch. I don't quite buy that, namely because beyond 50X/inch, you're merely magnifying the star's Airy disk, so the star ceases to appear like a point source.
And, of course, it doesn't work for extended objects, but if we're dealing with DSOs with small angular sizes, it gets you in the ballpark. I'm sure there are other formulas out there that will do the job nicely if they factor in magnification/exit pupil as well as aperture.
Clear Skies,
Phil
Thanks for taking time for a detailed explanation. I also remember Walter Scott Houston claiming 14 magnitude stars with his 4" Clark. When I read it many years ago, I was somewhat skeptical, but not these days with more experience.
Again, I appreciate all of the technical detail. As much as I know the limits of quantitative calculations when it comes to seeing DSOs, I still love to play with the formulas.
John
IC1296 next to M57 is a hard observation for a 12" in dark skies.
IC4617 next to M13 is a tad easier because at least its core has a higher surface brightness.
My personal challenge in the 12.5" has always been to see the 4 galaxies in Burbidge's Chain, right next to NGC247.
It takes high power (say, 300+) and it takes dark skies (NELM 6.5+ or SQM 21.0 and darker)
As for a stellar limiting magnitude, that's a bit easier;
See:
http://www.scopecity...-calculator.cfm
This site is no longer open for business, but still up for information, like this calculator.
To see MCG-4-3-12, mag 17.5 in a 12" would be quite an accomplishment.
I have seen all four and though I can reach several tenths past 17 on stars, I have never reached that deep on DSOs that I know of.
Looking up the magnitude of the four galaxies, I find quite a bit of variation in quoted magnitudes, leading me to believe it is actually brighter than you quote.
What a fantastic idea! Thank you very much Phil, I will give this a crack. Will be harder from Australia as M13 never gets near zenith for us. But please keep the challenges coming!
I only got my first dob 2 years ago and decided the best way to learn the scope and the sky was to set off on a Messier quest. I'm almost there, 100/110. 5 are probably out of the question from Brisbane, and 4 of the remaining 5 will now have to wait for Uma to roll around the sky again before I can seriously attempt them again. With the galactic core high in the night sky for us I'm on a Globular Cluster hunt. There is a list on Wikipedia of 150 in our galaxy. I'm up to 70 of those.
But I always love to hear new observing ideas! I have a young family and work full time, so columns like this are really appreciated!
And not surprising that a great idea has prompted a great thread: I'd never heard of Burbidge's 4! Thanks Don, I might give that a shot with my 14" too.
Cheers all,