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{ Practical astronomy | Astronomy | Exernal galaxies }

External galaxies

spiral galaxy
M31 Andromeda galaxy, on 2012-01-01.

The image shows the famous Andromeda galaxy, the nearest "full-size" galaxy outside our own Milky Way. (The southern sky has the two Magellanic Clouds, which are physically smaller but closer companions of the Milky Way.) The galaxy has a bright central bulge of stars, and a larger circular disc of stars, gas and dust. The disc is inclined to our line of sight and appears elliptical. Note the dark lanes of dust and gas that obstruct the otherwise smooth distribution of starlight from the disc. There are two small companion galaxies. NGC205 is somewhat top-right from M31. Less obvious, and difficult to distinguish from a star, is M32 a similar distance below M31.

Physical parameters:

The exposure is limited by the brightness of the sky due to light pollution. The individual frames are close to being dominated by sodium light reflected and scattered by the Earth's atmosphere. Direct light from a nearby streetlight and from a neighbour's security light are screened by placing a cylindrical shield toward the front of the lens, or better, by using separate screens on tripods to cast shadows on the lens aperture.

The short exposure of the frames limits how faint an object can be discerned over the noise. By aligning many frames on the stars seen in them and then stacking the frames into a single image, the noise is reduced relative to the signal from the sky. This is because the noise is random and changes from frame to frame, ultimately averaging out to nothing. The signal is the same in each frame and remains unaffected by averaging many frames.

The noise level is mostly due to taking the frame and not so much due to the length of the exposure. Taking 100 frames reduces the noise by a factor 10. But if we could take a single frame with 100 times longer exposure, we would have 100 times the signal with only slightly more noise. That would reduce the noise by a factor of 50 or more.

The exposure of the individual frames is limited by several factors: the sky background will at some point overexpose the frame; tracking inaccuracies will eventually smear out the image; ultimately, the exposure time might exceed the battery life or the length of the period of darkness. Light pollution poses an additional problem, because the background brightness it introduces into the image, at very high contrast, is not smooth and even enough to be removed properly. Using a dark-sky location or a light pollution filter to suppress sodium and mercury spectral lines would improve matters.

Image parameters:

Attribution-Noncommercial-No Derivative Works 3.0 Unported
Content available under Creative Commons licence
Attribution-Noncommercial-No Derivative Works 3.0 Unported

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