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NGC 6441 and the small planetary nebula PK 353.5-04.9

This image is a test of the new QHY 163M camera with short exposure times of 1 second. The globular cluster NGC 6441 was photographed in 2016 with the EOS 6D at the CDK12. The seeing conditions in the 2016 night were rather bad. In comparison to this, the short 1 s exposures could achieve a significant increase in image resolution. The seeing (FWHM) in the summed images of the color channels is 2.2'' (blue), 1.8'' (green) and 1.6'' (red). These are values that we had never measured with longer exposure times at our location before. The luminance channel was artificially generated from the red and green images due to the good seeing values. The finished image is therefore an (RG)RGB image. The Moon, 80 % illuminated, was only 20 degrees away from NGC 6441 at the time of imaging, the limiting magnitude is about 17.3 mag (v).

NGC 6441 (2017) Scale 1:1 Image data: 12.5" Planewave with Reducer ( f = 2.000mm) Canon EOS 6D, 12 x 300 Seconds   NGC 6441 (2018) Scale 1:1 Image data: 12.5" Planewave without Reducer ( f = 2.540mm) QHY 163M, Baader Filter R:G:B 1000:1000:1000 x 1 Seconds

The direct comparison clearly shows: despite the longer focal length of the Planewave astrograph, the resolution of the many short exposures with the QHY 163M camera is significantly higher than the summation of the 12 long exposures. The image processing effort is significantly higher, but for bright objects it is worth the time.

Object description: NGC 6441 was discovered on 13 May 1826 by the Scottish astronomer James Dunlop, who described the object as "a small, well-defined, rather bright nebula with a diameter of about 20 arc seconds". The globular cluster NGC 6441 in the constellation Scorpio is about 38,000 light years away from Earth. It is located in the so-called galactic bulge (bulging part of the Milky Way around the galactic center), "behind" the galactic center. Due to the interstellar extinction, the light of the cluster is clearly reddened (E(B-V) = 0.47 mag) and it appears to us in a yellowish tone. It is one of the most luminous globular clusters of the Milky Way with a mass of about 1.6 million solar masses. Spectroscopic investigations have shown that many stars in the center of the cluster

are "metal-rich", that means an accumulation of heavier elements than helium. NGC 6441 contains a large number of RR lyrae variables, as well as at least 4 milliseconds of pulsars and one of the very rare planetary nebulae in globular clusters. Together with the 3.2 mag bright star G Scorpii, the star cluster forms a pretty pair in the middle of the galactic sea of stars. G Scorpii is an orange K-giant star with 16 times the size of our Sun and is located in a distance of approx. 126 light years from the Sun. The small green nebula directly northwest of G Scorpii is the planetary nebula PK 353.5-04.9. The diameter is less than 10 arcseconds.. The above image shows a size comparison to the full Moon and the location of the planetary nebula PK 353.5-04.9. Click here or on the preview image to load a larger version.

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