Meade Image Processing Manual de usuario descargar pdf (Pagina 29)

Galaxies are the main building blocks of the Universe. They are congregates of hundreds

of billion stars; the whole universe is built of billions of these galaxies. However, in the

night sky, they are all very faint. It is almost impossible to detect the spiral arms of any

galaxy by visual observation. All we can see in a telescope is, at best, the faint glow of the

central region (called the bulge).

This galaxy is quite spectacular in long exposure images, because of perspective: we see it

almost exactly face-on. Galaxies are of three main morphological types: spirals, ellipticals,

and irregulars. This one is a spectacular example of an Sc-type, with widely open spiral

arms. It contains an unusual amount of interstellar matter in its spiral arms, with many huge

molecular clouds, which are sites of present-day star formation. These fluoresce in a few

spectral lines, Hα foremost. In order to make these HII regions stick out, we took 1.5 ours

worth of Hα exposures, and (with Photoshop) mixed the Hα images into the red channel.

This procedure gave the stars a somewhat unreal color, but it greatly enhanced the

HII regions.

Notice that the starlight that makes up the spiral arms is blue in color. The spiral arms are

the youngest, short-lived parts of a galaxy; their light is appropriately due to short-lived

stars – massive, blue, and bright – which never have the chance to leave the spiral arms,

living such short lives. This particular galaxy has an unusually small percentage of non-

spiral-arm disk stars, which would be yellow on average, filling up the disk between the

arms. The yellow glow is visible on our image only close to the center (in the bulge). This is

an indication that this galaxy is undergoing a particularly vigorous phase of star formation

in the spiral arms, because the older stars are missing.

The limiting magnitude on this image is about 19

. Stars are large on the image, their

diameter is about 5 as. We do not know the reason for this large star size (short exposures

gave about 2.5 as), but we suspect that the culprit is dome seeing, which can be improved

by fixing the thermal balance of the dome.

The main source of noise on the images is due to the fluctuation of the background, due to

streetlights. While some small improvement could be achieved by longer exposures, and a

somewhat more careful mixing-in of the Hα images, it seems impossible to make much

headway without relocating to a dark site.

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Meade Image Processing Manual de usuario Pagina 29