Object of the Week June 2, 2024 – CN 3-1 & CN 2-1 A (Partial) Tale of the Cannon Catalog

[deepskytraveler]

Cannon (Cn) 3-1, PK 038+12.1, PN G038.2+12.0, VV 171
Constellation: Ophiuchus
Type: Planetary Nebula
RA: 18h 18m 41s
Dec: +10°09’44”
Mag: 13.5(v)
Size: 0.1' x 0.1'

Cannon (Cn) 2-1, PK 356-04.1, PN G356.2-04.4, VV 126
Constellation: Scorpius
Type: Planetary Nebula
RA: 17h 55m 0s
Dec: -34°22’30”
Mag: 12.2(v)
Size: 2”x3”

My original plan for this week’s OOTW was to document one of the many fine planetary nebulae found throughout the constellation Ophiuchus. As I began my research little did I know I would soon find myself going down a rabbit-hole and discovering the Cannon catalog. Here’s my story.

The expanse of sky containing the constellation Ophiuchus is a bountiful hunting ground for every type of deep-sky object. So too is my long-forgotten collection of DEEP SKY magazines from the late 80s and early 90s which I had recently came across. While browsing through the Summer 1988 Volume 6, Number 2 (issue #23) for planetary nebulae candidates in Ophiuchus I found two articles, each by a different author and each focused on a different constellation, however both referencing a planetary nebulae from the Cannon catalog, which I had never heard until now. Multiple Google searches failed to turn up anything on the Cannon objects or the Cannon catalog (with one minor exception). Not surprisingly that one minor exception was a reference by our own Steve Gottlieb in an August 28, 2021 Deep Sky Forum S&T Editors post replying to a member’s query about H-beta targets. Steve’s reply included this short notation on Cn 2-1: “Dims with an OIII filter - best with UHC or H-beta.” It was apparent to me that Steve had at one time observed this planetary nebula.

With the Google searches failing me, on a whim, I searched for these two planetary nebula in SkySafari 7 Pro. Lo and behold not only were these two listed, but so were 5 other Cannon planetary nebulae, which I will list at the end of this article.

Returning to the DEEP SKY magazine, in an article by Jack Marling he wrote the following about Cannon 2-1: “This 12th magnitude planetary on the northeast edge of M7 is readily visible in most backyard telescopes. Photometric measurements yield a true visual magnitude of 12.2 for Cannon 2-1; the object is certainly brighter than its photographic magnitude of 13.8. Cannon 2-1 consists of a small disk surrounding a bright central star.” Jack continues by referencing a June 14, 1985 observation by Pierre Schwaar and his 20-inch telescope at 291x who described it as: “very small, about 2” in diameter. The object has a bright central star and blinks (averted vision shows the nebula, direct vision the star). Cannon 2-1 appears stellar at 138x. I estimate its brightness at 10th magnitude, including the star.”

Returning once again the DEEP SKY magazine, in an article by Phil Harrington, he wrote the following about Cannon 3-1: “About 4° to the northwest of the planetary nebula NGC 6572 in Ophiuchus is another planetary, unlabeled on the Sky Atlas 2000.0. Cannon 3-1 is a 12th magnitude planetary measuring about 6” across, which makes it readily identifiable with 6- to 8-inch instruments at high power. There is little additional information on Cannon 3-1 in any of the popular literature for observers.”

Unfortunately the story stops here, at least temporarily. I trust that the members of Deep Sky Forum will be able to fill in the blanks, so I can tell the rest of story. I had a hunch that Cannon might be Annie Jump Cannon, one of Harvard Computers hired by Edward Pickering in the early 20th century. Her story is fascinating so be sure to check it out. However after digging through many articles about her life and work I’ve been unable to find any evidence that this was her planetary nebulae catalog. Do you know who compiled the Cannon catalog of planetary nebulae?

More than most other OOTW objects we really need your observation reports and sketches of Cn 2-1 and Cn 3-1. As always, give it a go and let us know.

• Cannon 1-1 mag 12.89 PN in the constellation Norma
• Cannon 1-3 mag 11.89 PN in the constellation Scorpius
• Cannon 1-4 mag 12.89 PN in the constellation Ara
• Cannon 1-5 mag 11.89 PN in the constellation Sagittarius
• Cannon 1-6 mag 11.89 PN in the constellation Sagittarius

Author’s note: the magnitude of these 5 Cannon planetary nebulae seem suspect

[obrazell]

You are correct that these nebulae were discovered by Annie Cannon from objective prism surveys with the 24" in Peru. They were published (in the case of Cn 2-1) in a paper by Harlow Shapley in the Harvard Obs bulletins. Cn 3-1 was published by Annie Cannon in another Harvard bulletin in 1926. The others were also published by Cannon. Kent Wallace's book is a good source for this kind of information

[Uwe Glahn]

Google found three more entries Mark

sketch Cn 3-1: 27", 837x, no filter, Seeing II, NELM 7m0+ (Edelweißspitze, Austria)
Cn3-1.jpg
home

sketch CN 1-5: 28", 843x, [OIII], Seeing II-III, NELM 7m5+ (Gamsberg, Namibia)
Cn1-5.jpg
home

The more interesting Cn 1-5 also shows a halo according to Corradi et al. (2003).
I wrote: faint but steadily visible halo with [OIII] and averted vision around very bright core; small, round, ~0,5' diameter halo with better defined N side

[Raul Leon]

Hi, here's my observations of 3 Cannon planetaries : Cannon 3-1 in Ophiuchus ; mag: 13.5 ; size: 6" ; small, round and dim. I used a 4mm Radian at 436x
Scan0570.jpg
Cannon 2-1in Scorpius ; mag:12.2 size:2" ; very small but fairly bright. I used a 4mm Radian at 436x
cannon 2-1.jpg
Cannon 1-5 in Sagittarius; mag:11.9 ; size: 5" ; tiny and evenly lit ; I used a 4.5 Delos at 392x
cannon 1-5.jpg
I used my 14.5 StarStructure f/4.3

[Steve Gottlieb]

For the same three Cannon planetaries, here are my notes using an 8" and a 14.5". Cannon 1-1, 1-3 and 1-4 are between -44° and -49° declination, so may be too far south for many here. I have some comments on other Cannon discoveries, but I'll include them in a separate post.

Cn 3-1
14.5": Easily identified at 140x as a 12th magnitude "star" with a moderate contrast blinking with a UHC filter. Similar view at 182x. Increasing to 352x, though, a very small blue disc, perhaps 4" diameter, was resolved around the central star.

8": Easily visible as a mag 12.5 "star", bracketed by two mag 13-13.5 to the north and two mag 12-13 stars to the south. Adding a NPB filter the planetary has a moderate "pop" and outshines the other stars.

Cn 2-1
14.5": Easily identified at 140x, 6' N of mag 6.1 HD 162817 on the N side of M7, despite a very rich field. Appears as a mag 12.5-13 "star" with a blue-grey color. Excellent contrast gain blinking with a UHC filter. Increasing to 264x and 352x, appears slightly "soft" with the impression of a bright central star and an extremely small (2") halo.

8": Visible unfiltered at 109x as a mag 13 "star" on the NE edge of M6. It was easy to locate just 6' N of mag 6.1 HD 162817. An NPB filter lit it up and Boom! -- it became quite noticeable in the rich star field.

Cn 1-5
14.5": Easily identified immediately at 140x as a 12th magnitude "star" using three equally spaced, and nearly collinear stars 3' to 4' S, including mag 9.2 HD 170101 3' SSW. Superb contrast gain (3 mags) blinking with a UHC filter. At 182x, appeared slightly soft and a very small dull blue disc ~4"-5" was resolved at 226x. Increasing to 352x, the disc seemed 5" diameter and a 14th mag star is close SW [24"]. Located 1° NW of M69.

8": Very easily identified at 95x as a 12th mag star just the north of three brighter mag 8, 9 and 11 stars lined up from east to west. Excellent contrast gain with a NPB filter with the planetary nearly equal in appearance to the brightest star in the string and significantly brighter than the other two. The PN was still virtually stellar at 225x.

[Steve Gottlieb]

To finish my post, Annie Jump Cannon also discovered several compact emission nebulae (even an extra-galactic PN) in the SMC and LMC on objective prism plates taken at Bloemfontein with the 24-inch Bruce telescope (same one used in Arequipa, Peru). These were found while working on a southern extension of the Henry Draper catalogue. An interesting one, N63A in the LMC (within the nebulous cluster NGC 2029), which she found in 1933, turned out later to be one of the first known extragalactic supernova remnants. I viewed it from Australia in a 24" as a small round knot, only ~12"-15" in diameter, and it was faintly visible even at 202x. It was easy to distinguish at 264x and stood out fairly prominently at 429x.

Her last discoveries of planetary nebulae were made at Harvard College Observatory in the late 1930s and early 1940s in collaboration with Margaret W. Mayall. For example, MyCn 26, a 12th-13th mag nearly stellar PN in Ophiuchus. Better known, but much more difficult, is MyCn 18, also known as Etched Hourglass Nebula in the southern constellation Musca.