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42. NGC 3556 (Messier 108) and NGC 3587 (Messier 97, Owl Nebula)
Notes: This image benefits from applying the zoom feature of your browser or keyboard. While the objects are small, the image illustrates one of the advantages of wide field astrophotography for novice astronomers. One can go in search of a deep sky object, such as NGC 3556 (center object), as we did, and find unexpected objects in the image, such as the Owl Nebula, NGC 3587 (upper left). Fortuitous finds come along frequently.
NGC 3556 is a barred spiral galaxy, and most amateur images illustrate its flatness. Since an 80mm lens gathers less light than the lenses many other astrophotographers are using, much of the disk is invisible to us despite a total exposure time of 10.5 hours. Consequently, our image doesn't look very flat and it doesn't show the dust clouds in the spirals as clearly. But it does seem to show more internal structure than some other images, such as those on the Wikipedia page. The structure shows up better in this crop.
There are several stars from the Milky Way in front of it, including the bright star in the center. The Wikipedia page describes NGC 3556 as "an isolated member of the Ursa Major Cluster of galaxies in the Virgo supercluster." The galaxy is spitting off X-rays from many sources.
If our telescope happened to be placed on a planet orbiting a star in NGC 3556, it would be too distant to see and image the object on the upper left, NGC 3587, which emerged as a supernova in our own Galaxy about 6000 years ago, a time when agriculture and domestication of animals was just emerging in Eurasia. NGC 3587 illustrates how viewers' biases may influence what is seen. Since the common name is the Owl Nebula, one tends to focus on the eyes. Our attention is diverted from another feature, the fan-shaped dispersions from center on the diagonal axis orthogonal to the axis of the dark spots. To understand deep sky objects, it may be better to avoid using common names. Binary structure like that of NGC 3587 is common in planetary nebulae. Perhaps it is predetermined in the shape of the dying star or the embryonic stage of the supernova explosion. The two large spots may be the open ends of a barrel-shaped cavity like that of NGC 7293. We made a higher contrast bitmap and cropped to the nebula to highlight these structures.
The Calar Alto observatory writes: As other planetary nebulae, the shine of M 97 comes mainly from the emissions from ionized hydrogen and oxygen atoms, what gives these objects their dominant reddish and greenish hues. But this object has, also, a considerable portion of light from the [central] white dwarf that has been reflected by nebular particles. This mechanism makes its color bluer, and distinguishes this from other planetaries with a smaller reflection component (www.caha.es/the-owl-nebula.html).
Technical Notes: All subexposures were taken when NGC 3556 was past the meridian, so that the scope was pointing slightly westward at the beginning of the session. Avoiding the tipover makes it much easier to align and combine. This is the first time we have made an effort to match up parallel series of darks and lights by temperature. This seemed to work well, as we see very little noise in the image. After attending a talk given by David Weixelman at a meeting of the Nevada County Astronomers, we reduced the eastward counter weighting hoping to improve the roundness of the stars.
Our first processed image had very eggy stars. This surprised us, because we had discarded about 25 percent of our subexposures to eliminate the worst. We suspected that we had made a bad choice of reference stars for the stacking and aligning step. We had chosen stars near the margins, and they may have had edge distortion, making it difficult for Nebulosity to calculate the centers. The second time we chose stars within the frame given by the 50 percent zoom and we tried to pick stars that were small and round. The result was much better. The CGEM mount has a sticky deck axis, making it impossible to balance in DEC, but it seems to be performing reasonably well. Zoom in/out with ⌘↑(+/-) (Mac), ctrl (+/-) (PC).