Images of the universe: Astrophotography
Bob Fera (www.feraphotography.com)
"Here is a recent image of a beautiful spiral galaxy with the non-artistic name of M74. We see it 'face-on' , and thus are treated to its intricate spiral arms, which are peppered with young bright blue stars and pink star-forming gas clouds.
Last fall I was given the opportunity to try a new telescope built by Planewave Instruments in Torrance, CA, and this was my "first light" image taken with it. The scope is a 17" f/6.8 (focal length = 2939mm) Corrected Dall-Kirkham Cassegrain, which provides a wonderfully well-corrected, flat field across a focal plane big enough to cover the largest CCD chips in use today. The camera I used is an SBIG STL11000M (Santa Barbara Instruments Group of Santa Barbara, CA). It contains a "full frame" (24x36mm) thermoelectrically cooled monochrome CCD chip.
The image is a composite of 13 20-minute exposures through a clear filter, 12 15-minute exposures through a red filter, 11 15-minute exposures through a green filter, and 12 15-minute exposures through a blue filter, for a total exposure time of a bit over 13 hours. It was taken over 4 nights in October 2009 from my home observatory. The raw images were calibrated, aligned and combined in a specialized astronomical imaging program called CCDStack, and final processing and tweaking was done in Adobe Photoshop.
One of the things I like about this image, in addition to the beauty of the main subject, is that the background is filled with tiny, faint smudges that are actually more galaxies way, way off in the distance." Bob Fera
Bob Fera has an unusual hobby. He photographs objects in the night sky that are invisible to the naked eye or even by looking directly through a telescope. An IT manager by day, Fera is an astrophotographer by night, a demanding avocation. Fera shoots what are called "deep sky" objects, very faint celestial bodies and collections of matter. Some of the images he showed at our meeting included galaxies and globular clusters (a cluster of stars that orbits a galactic core), emission nebulae (multicolored clouds of ionized gas or plasma), reflection nebulae (gas clouds reflecting the light of nearby stars) and dark nebulae (gas clouds that block the light of stars and so are only observed indirectly). These photographs included such (at least for some) well-known objects as: the Green nebula, Crescent nebula, Crab nebula and the Orion nebula. The nebula photographs were striking for their rich, deep colors; a characteristic one does not normally associate with the night sky, but which may explain the popularity of nebulae with astrophotographers.
Not surprisingly, shooting these images is possible only with some very sophisticated gear, as well as knowledge of astronomy. The "telescopic lens" Fera shoots with is actually a high-power (2800mm, F8 14" mirror) telescope. The telescope directs light to a dedicated (35mm sensor) digital camera made by Santa Barbara Instruments, specifically designed for astrophotography. The sensor of this camera is monochromatic (lacks RGB pixel filters), so separate images must be taken with three large color filters that are then assembled. Because the light emitted by, or reflected from these objects is so faint, an image must be built up over a long time. This necessitates slowly rotating the mount holding the scope and camera in order to fix the position of the image (compensating for the rotation of the earth), a task that is performed by a separate smaller telescope and software running on a laptop computer. Over all, the "data capture" required to compose a single image takes many hours, often over several evenings, and may easily result in 50 or so files per image. (He takes these images from a specially built room with a sliding roof at his home in Forest Hills near Auburn.)
Assembling a raw image from these files, however, still does not reveal anything visibly recognizable. Sensor (dark frame) and optical noise must first be subtracted. Then, because a typical picture comprises an enormous dynamic range compressed within a small region of the luminance histogram, the histogram must be rescaled in such a way that both highlights and moderate to low light parts of the image are all visible. The resulting picture is a result of image-processing software written specifically for astrophotography by enthusiasts, as well as Fera's skill in composing using ‘curves' and other Photoshop tools.
The results are very impressive, as a visit to Fera's website will show. Although doing astrophotography at Fera's level requires a large investment of time and money, he emphasized that astrophotography with more modest means is possible (he started out with a Canon F1 and less expensive scope), though the objects of interest would, of course, be larger and brighter than those he focuses on.
Speaker invited by Samer Alassaad