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Cosmic Challenge: Jonckheere 320

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Cosmic Challenge:

Jonckheere 320


January 2019


Phil Harrington


This month's suggested aperture range

6- to 9.25-inch (15-23cm) telescopes









Jonckheere 320








On January 22, 1916, while revisiting some of the stars in his Catalog and Measures of Double Stars discovered visually from 1905 to 1916 within 105° of the North Pole and under 5" Separation, French astronomer Robert Jonckheere returned to a vague double in Orion, which he had previously designated as entry number 320. Jonckheere was later to write of this encounter through the 28-inch refractor at the Royal Greenwich Observatory, "I noticed that the object I have catalogued as J 320 is not a double star, but, like J 475, it appears with the larger instrument to be an extremely small bright elongated nebula. As is the case with J 900, this object also appears to be new as a nebula."  (Sidebar: Jonckheere 900 was the featured Cosmic Challenge in March 2017.  And what of Jonckheere 475?  You probably know it better as planetary nebula NGC 6741 in Aquila.)

Above: Winter star map. Credit: Map adapted from Star Watch by Phil Harrington



Above: Finder chart for this month's Cosmic Challenge.

Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window


Cross-listed as PK 190-17.1 in Perek and Kohoutek's comprehensive catalog of planetary nebulae, J 320 is in northern Orion, 7° northwest of Bellatrix [Gamma (γ) Orionis], the Hunter's western shoulder. To get there, first hop 6º due west from Bellatrix to 5th-magnitude 16 Orionis, and then continue another 2º further northwest to a pair of 8th-magnitude suns, SAO 94320 and 94324. You'll find the nebula lying just 15' to their west and just 5' northwest of a 9th-magnitude field star.


That is, you should see it there. That's the rub. J 320 shines at about 12th magnitude, which is bright enough to be seen in an 8-inch telescope trapped under the veil of suburban light pollution. But there are so many stars in the same field that picking out which one is the planetary is a tough job. J 320 only measure 26"x14" across, and indeed is easy to confuse for a close-set double star if viewed at low power, as Jonckheere likely did during his initial discovery. Again, you may need to flash the planetary, if you'll pardon the phrase, by holding a narrowband filter between your eye and eyepiece. Doing so will suppress the field stars, but not the planetary. The culprit will have no choice but to surrender.

Above: J 320 as seen through the author's 8-inch (20cm) reflector.


My notes made through my 8-inch reflector at 56x recall a small, extended object that indeed looked like a pair of close-set stars on the brink of resolution. Switching to 203x, however, quickly dispelled that notion. The planetary's disk, though still fuzzy, was easy to tell apart from a double star. Its central star shines at magnitude 14.4, but evades detection even through my 18-inch, perhaps masked by the nebula's high surface brightness.

Photographs reveal that J 320's ellipticity reflects its lobed structure, which resembles a butterfly in flight. A study conducted with the Hubble Space Telescope's Wide Field Planetary Camera (WFPC2) in 2003 revealed a more complex internal structure than the typical bipolar planetary. Two pairs of bipolar lobes are clearly visible extending from the nebula's core. One is approximately aligned north-south and the other southeast-northwest. In addition, the Hubble images uncover two pairs of faint knots just outside the center of the nebula. As a result of this complex morphology, J 320 has been classified as not just a bipolar planetary, but rather as an example of a much less common breed known as a polypolar planetary. Try to say that three times fast!


Above: Image of J 320 taken with the Hubble Space Telescope.


Finally, be sure to visit the forum thread The joy of Jonckheere 320, a poly-polar planetary nebula created in November 2015 by CN'er iainp.  Viewing J 320 through a 20-inch (51cm) reflector at 546x, his sketch bears an impressive resemblance to the Hubble image.

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 month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase.  Game on!

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.

Phil Harrington's Cosmic Challenge is copyright 2019 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.


  • okiestarman56, John O'Hara, Astrojensen and 2 others like this


Jan 01 2019 07:04 AM

A nice little challenge! I've seen it a few times, but that was some years ago. Last time was with a C8, if I'm right. I'll give it a go again, when the weather cooperates, this time with a 6" achromat.


I'll throw out another challenge: What's the smallest scope you can see it with? 



Clear skies!
Thomas, Denmark

    • PhilH likes this

It is now on my list for January new moon night, More next week on this.

    • PhilH likes this
Jan 03 2019 03:50 AM

Had a go at it yesterday evening with my 6" f/8 Sky-Watcher achromat. Somewhat hazy, rural skies, drifting clouds, NELM around 5.5, seeing mediocre. 


Not visible in the 16x60 finderscope, but immediately visible as a stellar object in the 150mm at 50x (24mm ES68). A very small, slightly elongated disk visible at 179x (6.7mm ES82). Moderate response to blinking with an O-III filter (it's possible the effect is stronger at lower magnifications). 255x shows the disk clearer and it's bright enough to take more magnification. Best view at 343x (3.5mm no-name long eye relief eyepiece). Tried 858x (3.5mm + 2.5x barlow), but that was too much. 343x showed the planetary much like in the sketch in the article, with the exception that one end was slightly brighter than the other. 



Clear skies!

Thomas, Denmark

    • PhilH, CelestronDaddy, Rudi Bjoern and 1 other like this
Rudi Bjoern
Jan 04 2019 03:19 AM

Thanks for the inspiring challenge...

I tried finding Jonckheere 320 with my 5" f/5 newton, but I couldn't find it.

However, I took an image with my 8" f/4 newton, you can see it here:


    • Dave Mitsky and Ohmless like this

I targeted this one earlier this month after reading Phil's column.  I was using the 20" in poor seeing but dark skies, so it wasn't so much of a challenge for the scope other than overcoming the seeing.  Despite the poor seeing I was detecting two sets of lobes/brightening: the more apparent WNW/ESE components, and a more subtle extension not quite perpendicular to that through the same center.   I used 278x primarily and sometimes 357x which was beyond what the seeing handled decently.


There is a tight, faint, small galaxy pair further north, II Zw 29 (PGC 16724/16725) at roughly 16th magnitude each and not quite stellar.  These must be high surface brightness considering their small apparent size and nearly stellar appearance in poor seeing. 

    • PhilH likes this
Dave Mitsky
Jan 11 2019 01:25 AM

I've observed Jonckheere 320 a number of times over the years.  I'll have to take another look at it in the near future.  Here's a report from an observation I made in 1998. 


Observer: Dave Mitsky
Date/time of observation: 1998/12/15 07:18 UT
Location of site: ASH Naylor Observatory, Lewisberry, PA, USA (Lat 40.15 d N, 76.9 d W, Elev 390 m)
Site classification: Exurban
Sky darkness: 5.0 to 5.5 <Limiting magnitude>
Seeing: 6 <1-10 Seeing Scale (10 best)>
Moon presence: None - moon not in sky
Instrument: 17" f/15 classical Cassegrain equatorial mount
Magnification: 118, 144, 202, 259x
Filter(s): O-III
Object(s): Jonckheere 320
Category: Planetary nebula.
Constellation: Orion
Data: mag 11.8  size 7"
Position: RA 05:05.6  DEC 10:42
Description: One of the many deep-sky objects that I observed on this occasion was the tiny planetary nebula Jonckherre 320.  This 7" diameter object was difficult to distinguish from nearby field stars at moderate magnification (118x) without the use of the nebula filter "blinking technique".  J 320 appeared nonstellar (albeit as just a small, circular, colorless, and high surface brightness glow) at higher magnifications, although the use of the O-III filter was a definite plus.  J 320 and J 900, another planetary nebula that I had observed previously, are featured on page 80 of the January 1999 Astronomy.

    • Astrojensen likes this

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