M42 The Great Orion Nebula

This image is the combined data with the color removed and further processed to show the extensive nebulosity of M42.

This is a single unprocessed 40 second blue data frame of the image at the top of this page. In astronomical imaging, the data frames are captured in a FITS format and must be calibrated using: 1. Dark Frames, 2. Flat Field Frames and 3. Bias Frames. Dark frames remove the noise of the CCD chip that is present due to thermal and electrical properties commonly called "noise" The Flat Field frame is used to remove defects in the optical path of the telescope, such as dust on the optics. Bias frames are used to remove the electrical noise from the CCD read-out (When the data is read from the CCD chip to the computer) and other extraneous electrical noise in the camera system. Other defects that must be removed are blooms that occur when the CCD chip pixel capacity is exceeded, which are the long white vertical lines in the frame. To correct a single frame can take anywhere from minutes to hours. In the case of this image, each frame required about 20 to 30 minutes of work. Multiply this by two hundred and seventy five data frames and the many hours required to correct other funnies in the image, over a hundred hours of post processing was required to get to the image at the top of the page. Each FITS Data frame is 6.13 megabits in size for a total of 1.69 Gigabits of collected data.

Single Data Frame after dark/flat/bias subtracted Single Data Frame after de-blooming and stretching Combination of 68 Calibrated Blue Data Frames

Luminance and Ha Data (71 Data Frames) Ha/Red, Green, Blue Data (204 Data Frames) Final Processed Image (275 Data Frames)

There is a technique in astronomical imaging called narrow-band-imaging. This is where filters are placed in front of the imaging camera’s chip to preferentially send data for only certain wavelengths of light through for the chip to record what will ultimately be the final image. This technique is actually what I have been applying in a way with all the color images I’ve been sharing – the camera I am using uses a black and white chip; by placing color filters for Red, Green and Blue in front of the chip, I gather data that will ultimately yield the color images I’ve come up with to date.

With narrow-band imaging, the filters used transmit light of certain scientifically-interesting wavelengths – these would be wavelengths of light transmitted by atoms of elements excited by radiation from nearby sources (usually a star). The Hubble Space Telescope, for instance, uses (among others) filters that transmit the light from ionized sulfur, hydrogen and oxygen. Amateur imagers have access to similar type filters, and have been using such filters to create spectacular false-color images of nebulae. One advantage that this type of imaging has is that it is less dependent on the presence (or absence) of light pollution or the Moon. Bottom line, if the night is clear, a narrow-band imager can image. “Regular” (color) imagers are a little more constrained by such sources (it’s not impossible, just much harder to get a great image with the Moon or light pollution present).

This is essentially a false-color image. As hinted earlier, I shot this through filters tuned to transmit the wavelengths emitted by ionized Sulfur (S-II), Hydrogen (H-alpha) and Oxygen (O-III). I listed the filters In order of increasing wavelength of light transmitted (S-II, H-alpha, then O-III). In the RGB sequence, the S-II data is used to represent the color Red, H-alpha the green, and O-III the blue.

I need to emphasize a point: to the human eye, the S-II color is actually a deep red, H-alpha a deep (but somewhat lighter) shade of red, and O-III a blue-green color (teal, technically). Therefore, these color narrow-band imagers do *not* represent colors we will actually see if we could see these objects and their colors. However, they do show details of the objects that shine in the wavelengths as described above.

The skies of December through around February showcase some of the most spectacular objects visible in the sky. Among them is this nebula, the Great Nebula of Orion. Also designated as M42, this is the 42^nd object cataloged by the French comet hunter Charles Messier. His list is actually a list of objects to *not* look at, as his primary interest was comets. He compiled this list of comet-like objects as a favor to other comet hunters so that when they see these objects, they will know that they are not discovering new comets, but rather looking at “nuisance objects”. It turns out that this list is a list of among the best deep-sky objects out there in the universe.

As hinted by its name, M42 lies in the constellation Orion. This is one of the most recognizable constellations in the sky, visible to the south (at least to Northern Hemisphere dwellers) with its three belt stars all lined up in a (almost) neat row. M42 lies just “below” this belt, and is referred to as Orion’s sword. If you look closely at this “sword”, you might notice that the star in the sword is a bit fuzzy. Guess what? That’s the nebula! (Text by Eric Africa)