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Astrophotography by Bob Franke

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NGC 6946 - The Fireworks Galaxy

Click the image for full size view. (3000 x 2000 - 1.10 MB)

Click here for a highly stretched view, showing foreground galactic clouds


Click the image for a full size view. (3000 x 2000 - 1.10 MB)



Modern technology has provided the means to identify many objects in this relatively close galaxy. The red symbols are from the SIMBAD data and the green from the NASA/IPAC Extragalactic Database (NED).

Roll your mouse cursor over the image to turn the symbols on and off.

The red arrows indicate the apparent motion direction and speed for the foreground stars.

Red Circles = star clusters
Red Squares = super nova remnants
Red Triangles = HI (21cm) sources
Red X's = X-ray sources
Green Squares = HII regions
Green X's = X-ray sources
Green Triangles = radio sources



12.5" RCOS @  ~f/9 (2880 mm fl) 0.64 arcsec / pixel.  The Zoomify image scale is 0.64 to 2.56 arcsec / pixel.


Paramount ME


SBIG STL-11000 w/ internal filter wheel, AstroDon Filters

Acquisition Data

8/10/2010 to 8/20/2010 Chino Valley, AZ... with CCDAutoPilot3 & CCDSoft.  AOL guided


Lum (no filter)  240 min (16 x 15 min, bin 1x1)

Ha                 480 min (16 x 30 min, bin 1x1)

RGB               270 min (  6 x 15 min each, bin 2x2)


  • CCDSoft, CCDStack, Photoshop CS w/ the Fits Liberator plugin, Noel Carboni's actions and Russell Croman's GradientXTerminator.

  • eXcalibrator for (b-v), (v-r) color calibration, using 15 stars from the NOMAD1 database.  The lower image is corrected to compensate for galactic extinction.

  • PixFix32 (pre-beta) to repair hot/cold pixels and column defects.

  • CCDStack to calibrate, register, normalize, data reject, combine the sub exposures and LRGB color.

  • PhotoShop for LLRGB plus Ha combine &  on-linear stretching.  The Ha data was added to the LLRGB's red channel, using the lighten option for opacity.


North is ~ to the top. The image is rotated 40 clockwise.

NGC 6946 is located just 10 million light-years away, behind a veil of foreground stars, in the constellation of Cepheus. During the 20th century, at least six supernovae, the death explosions of massive stars, were discovered in NGC 6946. The most recent nova was 2004et. This high frequency of supernovae is the reason for the Fireworks Galaxy nickname.

Because NGC 6946 is at low galactic latitude, we view it through a lot of dust and various nebulae. This causes the light to become reddish, just as the Sun is red at sunset. This effect is called Galactic Extinction. The top image was calibrated with foreground stars and shows the galaxy with the effect of the extinction.

The bottom image was color corrected for galactic extinction and shows NGC 6946 with its more intrinsic color. The color correction factors, from the NASA/IPAC Extragalactic Database (NED), produce an image that was obviously too blue. Assuming the Nasa data is correct, sometimes it is not, then the upper image is not red enough. Because of this discrepancy, the lower image was corrected by balancing the histograms for each color channel, with the other two. This generally works because the sum of the color, in a nearby spiral galaxy, is basically white.