Andromeda Galaxy (M31)

The Andromeda Galaxy is the largest spiral galaxy in the local group and is located about 2.5 million light-years from Earth. Cataloged as Messier 31 (M31), this deep sky object is observable in the northern hemisphere for much of the year, lying in the constellation Andromeda, next to Cassiopeia. Many scientists worldwide predict that Messier 31 will collide with the Milky Way in approximately 4.5 billion years and merge to create a massive elliptical or lenticular galaxy. Obviously, we won’t have to worry about this happening for a while, but it is fascinating to think of this scenario playing out over time.  

Location & Best Time to See it

As far as galaxies go, Andromeda is one of the easiest ones to find in the night sky. It is located in the constellation of the same name, but the Andromeda constellation is only easily recognizable to some observers. The easiest way to find M31 is to triangulate the position using the bright stars Alpheratz in Pegasus and Mirach in Andromeda. Another way to find it is through the two constellations on each side of Messier 31, Pegasus and Cassiopeia. These two constellations are effortless to find, even under moderately light-polluted skies. The photo below from Stellarium is a good representation of where to look to find the Andromeda Galaxy.

From mid-northern latitudes, M31 is visible for at least part of every night of the year. However, it is best to view it at its peak in the sky close to the zenith to minimize the effects of turbulence in the air and localized light pollution. So the best time to photograph the Andromeda Galaxy from mid-northern latitudes is during the winter and fall from August to December. 

Photographing the Andromeda Galaxy

Messier 31 is a sought-after target for amateur astrophotography enthusiasts due to its massive size, bright magnitude, and magnificent spiral structure. As well as being the closest central galaxy to the Milky Way, M31 is one of the brightest objects in the Messier Catalog. Since the Andromeda Galaxy is massive and luminous, just a short 30-second exposure is enough to capture the intense glow of the core and surrounding inner dust lanes. It's even possible to observe this galaxy with the naked eye using binoculars, even under moderately light-polluted skies. 

As is the case with all deep-sky astrophotography targets, it is best to capture several long exposure images and stack the data together to increase the amount of signal (light) collected overall for a healthy signal-to-noise ratio. We do this by adjusting our exposure times for each of our light frames (30 seconds - 3 minute exposure shots should be good), capturing the four main calibration frames (light, dark, flat, and bias), and finally stacking these shots together through DeepSkyStacker. A good tutorial on how to use the software is here.

Here is how to take the four main calibration frames:

Light Frames - These are your signal frames; the instructions above are how to take these.

Dark Frames - These frames' purpose is to increase your photo's signal-noise ratio. Keep the same settings you had for your light frames and cover your lens/telescope with the lid. 

Bias Frames - The purpose of these frames is to reduce the noise in your photo. Keep the same settings on your camera for your light frames, change the exposure time to the fastest it can possibly be, and cover your lens/telescope with the lid. 

Flat Frames - The purpose of these frames are to eliminate vignetting/light falloff and other artifacts in your image due to dust, dirt, or smudges on the sensor or telescope in your photo. These are tricky. In simple terms, cover your lens/telescope with a white shirt and point it at a constant light source, like the morning sky. Then experiment with the exposure times until the histogram looks like the one below. This is shown below for monochrome and color cameras.

I used my school's telescope - an Orion ED 80mm f/7.5 Apochromatic Refractor - and my unmodified Canon Rebel EOS T3i (600D) DSLR camera to capture this photo of Messier 31 below.

Andromeda Galaxy Details:

• Messier: 31

• NGC: 224

• Object Type: Barred Spiral Galaxy

• Constellation: Andromeda

• Distance: 2.537 million light-years

• Apparent Magnitude: +3.44

• Apparent Size: 178 x 63 Arc Minutes

Resources:

Messier 16 - The Andromeda Galaxy (NASA)

StarCharts - Andromeda Galaxy

Gallery of Andromeda Galaxy