William Herschel discovered NGC 6946 = H IV-76 = h2084 on 9 Sep 1798 (sweep 1077) and recorded "cF, vL, iF, a sort of BNM. The nebulosity extends 6 or 7'. The N seems to consist of some vS stars; the nebulosity is of the milky kind. It is a pretty object." John Herschel made 3 observations and on 11 Aug 1831 logged "vF; eL; vglbM; r; 5' or 6' dia; irreg fig. A curious object; no doubt a great cluster of eF stars. Requires the eye to be well prepared for seeing it."

Bindon Stoney, LdR's assistant on 6 Sep 1850, described a "New spiral, very fine but faint; 3 branches, of which two terminate in knots, a fourth branch north preceding very doubtful." Two nights later he made an excellent sketch (Plate XXX, fig. 36) that was included in the 1861 publication. On 6 Sep 1855, R.J. Mitchell recorded, "The two following branches unite in one before meeting the nucleus; I certainly see a fourth branch preceding which seems to join the other preceding branch in the same way before reaching the nucleus. Of the four, those which terminate in knots are the brightest."

E.E. Barnard reported (Sidereal Messenger, vol. 5, p286) he first viewed NGC 6946 around 1881 with the 5-inch refractor and marked it as "vvF". But on 28 May 1885 he found it "almost bright, certainly not faint. It is moderate in size, round, vgbM with some small stars grouped around it. Its brightness is a little less than cluster GC 4590 [NGC 6939]."

Ten supernovae (most of any galaxy) have been detected in NGC 6946 as of 2017. The first, SN 1917A, occurred on 19 Jul 1917 when it was discovered by Ritchey and was considered a nova. This supernova, situated 37" W and 105" S of the galaxy's nucleus, peaked at 14.6 mag.

200/250mm - 8" (6/22/81): faint, large, diffuse, brighter core. Situated in a rich star field.

300/350mm - 13.1" (7/27/84): bright arm on the east side highly suspected to branch or split into two arms. Also an arm or brighter region seen on the opposite side of the galaxy pointing west a short way.

13.1" (7/16/82): the central region is elongated and fairly low surface brightness though a spiral arm clearly trails off to the east from the main body creating a non-symmetrical appearance.

400/500mm - 17.5" (8/29/92, White Mountains at 12,000 ft): bright, very large, 6' diameter to main body, elongated 3:2 ~E-W. Three arms are visible. A long bright arm is attached at the north side of the core and trails to the east (Arp's "One Heavy Arm"). This eastern arm splits; a short fainter branch bends south following the core and a long curving bright arm terminates with a very faint, very small HII knot (HK83 #3). On the west side a fainter arm shoots sharply to the north from the core. These outer arms significantly increase the diameter of the main body. The galaxy has a very large brighter middle but the core is just a very small brighter region close SW of the geometric center. A very faint stellar nucleus was seen with direct vision.

17.5" (8/13/88): main spiral arm very prominent and easily seen to split.

17.5" (9/14/85): bright, large, brighter central core. A prominent arm is attached on the NE side of the core and trails to the east. This arm splits - the shorter arm is close to the core and a brighter region or arm to the west.

600/800mm - 24" (5/20/17, 5/22/17 and 6/23/17): Type II Supernova 2017eaw in NGC 6946 was easily identified as a 13th magnitude "star" 2.7' NNW of the nucleus using a photographic finder chart. I was surprised the supernova was only slightly fainter in June, over a month later than first viewed. This is the 10th known supernova in the galaxy.

24" (9/13/12): the bright, long, outer spiral arm on the north side passes very close south of a mag 13.5 star before terminating near the small knot HK83-3 = H69-1. Besides the knot at the end, this arm is a bit clumpy with two slightly brighter regions roughly 1.7' N of center, HK83-213 = H69-11, and 2.4' NE of center, HK83-123/124 = H69-10. The spiral arm extending north on the west side has several mag 14 stars superimposed. A very small brighter nucleus is embedded the very broadly brighter central region.

900/1200mm - 48" (10/23/11): this very bright, showpiece face-on spiral stretches roughly 9'x7' E-W. At 375x, four arms were visible, each containing one or more HII regions. The brightest arm is attached on the west side of the central region and curves counterclockwise to the north and then heads east, passing just south of a mag 13.5 star and spreads out to the NE of the central region. At the eastern tip (4.2' from center) is a bright HII knot of 12" diameter, catalogued as #3 under NGC 6946 in Paul Hodge and Robert Kennicutt's 1983 "Atlas of HII Regions in 125 Galaxies" (HK83-3) and #1 in Hodge's 1969 paper "HII regions in twenty nearby galaxies" (H69-1). A second shorter arm vaguely emerges from the core on the north side, and rotates more sharply around the galaxy on the north side, passing south of the brighter arm, and curving around to the east side. It contains HK83-63/76 = H69-6, an elongated knotty HII complex at its tip, 2.5' due east of center.

On the west side are two additional arms, though the brighter inner arm is better defined. It begins on the south side of the central region and curls sharply to the north on the west side. It passes through a few mag 14 foreground stars and nearly fades out 3.2' NW of center. A dim extension finally ends 3.5' N of center at HK83-285 = H69-20, a very faint, very small knot inside a triangle of stars. An outer arm on the south side rotates towards the west and intersects HK83-503/507 = H69-33, a very bright, round knot of 20" diameter, that rivals the inner core in size. Using a DGM Optics "Galaxy Contrast" filter, the core was dimmed more and the knot appeared nearly as bright. Studies reveal this feature is a circular bubble containing numerous, tightly packed small clusters and a bright supermassive star cluster that resembles a young globular. After this point, the arm becomes more patchy as it spreads to the northwest, but near the end is HK83-527/528, an extremely faint knot just west of a mag 14 star 3.5' NW of center.

Notes by Steve Gottlieb