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Astronomical Observatory: Cool Images

The Andromeda Galaxy (M31, NGC 224)

Andromeda Galaxy

At a distance of 2.5 million light years (770 kpc), the Andromeda galaxy is the nearest galaxy to our own Milky Way that is similar to it in type and size. As a consequence the angular diameter is enormous (2.8 degrees in our image, about six times the diameter of the full moon). Hence to make an image of the entire galaxy with the limited field of view of our Rehoboth telescope (15 x 22 arcminutes), it was necessary to mosaic together an array of overlapping fields. The image above (and the full resolution version you can access by clicking on it) was made by combining images 98 fields (a 7 by 14 array). Each field was imaged five times in each of three filters (red, green, and blue). Frames were binned 3x3 so that the plate scale of the full resolution image is 1.9 arcseconds/pixel. Hence the dimensions of the full resolution image are 4193 by 5597 pixels or 23 megapixels altogether. The original image is in FITS format and is quite large. The jpeg format version posted here is more compact at 8.8 Mbyte.

North is up and east to the left in our image. The cropped image shown here is 2.33 degrees north-south. (The full resolution, uncropped image that it is linked to is just over 3 degrees.) The 2.8 degree diameter of the galaxy corresponds to a linear diameter of 38 kpc. Higher dynamic range images taken with the Keck telescope show the low density outer portion of the galactic disk extends to a diameter of 67 kpc.

Many features are visible. Most noteworthy, perhaps are the two small elliptical galaxies that orbit M31: M32 (NGC 221) 1.9 degrees south of the core, and M110 (NGC 205) 1.5 degrees northwest of the core.

Imaging Team: The Andromeda project was initiated by Jess Vriesema, who wrote the plans for the 17 nights of observing, did preliminary evaluation of the data (to determine data quality and to plan for retakes of data compromised by poor conditions), and who did the basic calibration (bias, dark, and flat field corrections). Melissa Haegert (Dykhuis) worked with Jess to determine a common set of brightness and offset scales for the various fields. Dan Van Noord did the final color combination, field stitching, and defect eradication.

Right Ascension (J2000) 00h 42m 44.3s (galaxy center)
Declination (J2000) +41d 16' 08" (galaxy center)
Dates observed

2006 September 28, 29; October 11, 12, 19, 26, 27, 28, 30, 31; November 1, 7, 8, 9, 10, 12, and 14.

Filters used BVR (blue, green, and red)
Exposure time

An array of 7 by 14 fields of view were imaged. Each field had at least 5 images in each of three filters. Exposure times for individual images were 60 seconds for R and V and 180s for B.


Wikipedia, Andromeda Galaxy.

Content posted February 2013, by L. Molnar


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