The Great Orion Nebula, M42, illustrates the difficulty in representing an object with a wide range of brightness. It is bright enough to be seen by the naked eye (the middle "star" in Orion's "sword"), yet there exists a faint wispy network of filamentary nebulosity that requires long exposures to capture. Embedded in the central, bright region is the famous "Trapezium" quartet of stars, that can only be shown by reducing the exposure so that almost none of the nebulosity is present. The following sequence explores this amazing object by zooming in and out, by over-exposing and under-exposing, and by using both film cameras and CCD cameras.
We'll start with an image showing what the naked eye sees (plus a few more stars) as the constellation Orion, on a cold winter's night.
Figure 1. The constellation Orion is often depicted with these connecting lines. Imagine a hunter holding a shield to the right, with his other arm uplifted to the left, and a 3-star belt from which is hanging a glittering sword. The sword is visually seen as 3 stars. The middle of these stars is seen here to be a reddish nebula, called the Orion Nebula, or Meisser 42 (M42). The cold giant, yellow-colored star Betelgeuse marks the upper-left shoulder, and the hot, bright blue star Rigel marks a foot (lower-right). North is up, west is to the right. The sky area is ~36 degrees north/south. This is a low-resolution version (500x750 pixels) of the original (2056x3088 pixels). The lack of stars in the brownish are in the lower-left corner is actually the Milky Way. [2005.12.29, Canon 10D 20mm f/3.5 lens, attached to Celestron CGE-1400 telescope for sidereal tracking, four 30-second exposures, dark frame subtracted, Hereford, AZ, 4650 feet altitude.]
Figure 2. This is a 4x zoom of the previous image,
only the "belt" and "sword" region. Note again that the 3 "sword
stars" are resolved into a pair of stars at the top, M42 in the middle
and a bright star below M42. The reddish nebulosity actually consists
large and small regions, with the smaller and fainter to the
The faintest stars in this image are magnitude 10.3 (i.e., 60 times
than the unaided eye can see on a clear night).
Figure 3. Closer by a factor of 4 and sharper (FWHM
= 30"arc). [Canon 10D digital camera, 200 mm EFL, f/5.6, 10x30s,
crop; Hereford, AZ; 2005.01.25].
Figure 4. Sequence showing effect of increasing
For an object with interesting features at different brightness levels
it's a challenge to show them all in a single image. This set of 4
are the same image, just displayed with different brightness
[Celestron CGE-1400, prime focus (HyperStar), f/1.9, R and G
total 26 seconds, B exposue total 65 seconds; 2003.11.05, Hereford, AZ]
Figure 5. This color image wiith a zoom factor of 2 shows the different coloration of the inner nebulosity. has the same scale but greater contrast (to show the color differences). The brightest nebulosity is dominated by the colors red and green. The "dark finger" seems to be pointing to a group of stars, which are overexposed in this image. [Meade LX200 10-inch SCT, with Meade Pictor 416XTE CCD imager and 616 color filter wheel; 10-second monochrome images taken with RGB filters; Santa Barbara residence; 2001.10.29, 0813Z].
Figure 6. This reduced contrast and brightness version of the previous image shows that the nebulos "dark finger" points to four stars. This group of four stars is called The Trapezium, and is a favorite view when observing M42 visually through a telescope.
Figure 7. Approximately the same scale, and equivalent exposure compared with the previous image, but using a color digital camera (Nikon Coolpix 990, 3.34 Megapixel CCD). The Trapezium group is more distinct, as are the color gradations. [Meade LX200 10-inch Schmidt-Cassegrain telescope, with a Nikon Coolpix 990 digital camera close to a 1/2-inch focal length eyepiece (i.e., eyepiece projection), average of eleven 8-second exposure images, with "dark" frame subtraction, ISO 400, Coolpix focal length 8.2 mm.]
Dimmer version of previous image, allowing the Trapezium group of stars to be easily seen.
Figure 8. This underexposed image clearly separates the
four members of "The Trapezium" and has "lost" the nebulosity which the
Trapezium illuminates. The closest pair of stars in this group of four
is separated by 14 "arc. The faintest of the four is about 2 "arc in
size, indicating fairly good "atmospheric seeing." The brightest of the
four has a visual magnitude of 5.1, which makes it visible to the naked
eye (although the entire group, including nearby stars and nebula, are
what the eye sees). Image size is 13.8 x 9.1 'arc, or 17% the
of the sky represented by the full moon. [Meade LX-200 10-inch
Schmidt-Cassegrain telescope, f/6.3, Meade 416 XTE CCD imager.]
M42 and M43 (upper-left of M42) for 4 brightness settings. FOV = 29.5 x 22.9 'arc. [Canon 10D mounted with focal reducer on Celestron CGE-1400; 15 30-second exposures; 2005.12.29Z]
___________ The pictures below this line are by other, more accomplished astrophotographers _________________
For comparison, advanced amateur Jason Ware took this picture of M42 and M43, apparently using film, with a Meade 8-inch telescope [from Meade General Catalog]. By including color we can see distinctions not apparent in black-and-white images, especially in the lower-left nebulosity, and upper-right. Although the inner region is overexposed, the dark cloud left of the Trapezium is still visible.
This is another widely reproduced photograph by advanced amateur Jason Ware. He used a Meade 6-inch refractor Model 152ED telescope, exposing Fuji HG400 film for a total of 60 minutes.
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This site opened: August 30, 1998. Last Update: December 29, 2005