A perk of SARA membership that we can use the 20m Skynet dish.
The angular size of the Andromeda Galaxy (M31) depends on how faint an outer region you include:
- Bright visible disk: about 3.2° × 1.0°
- Full extent of the stellar disk: around 4–6° across
- Very faint outer halo: extends to tens of degrees on the sky
For most amateur observations and star charts, M31 is quoted as approximately 3° wide, which is about six times the apparent diameter of the full Moon (the Moon is about 0.5° across).
If you’re using your radio astronomy setup, it’s useful to know that at the distance of M31 (about 2.5 million light-years), an angular width of 3.2° corresponds to a physical diameter of roughly 140,000 light-years, while the extended disk reaches significantly farther. The exact beam-filling factor for your antenna would depend on its beamwidth at 1420 MHz.
For a 20 m dish observing at the hydrogen line frequency of 1420.4058 MHz:
Wavelength: lambda = 0.211 m
Dish diameter: D = 20 m
The half-power beamwidth (HPBW) is approximately:
theta = 1.2 * lambda / D
Substituting the values:
theta = 1.2 * 0.211 / 20 = 0.0127 radians
Converting to degrees:
theta = 0.0127 * (180 / pi) = 0.73 degrees
So the beamwidth is approximately:
0.73 degrees 44 arcminutes
Comparison:
Full Moon 0.5 deg (30′) 20 m dish @ 1420 MHz 0.73 deg (44′) M31 major axis 3.2 deg Sun 0.5 deg (30′)
Since M31 is about 3.2 degrees wide, the 20 m dish beam covers about:
0.73 / 3.2 = 0.23
or roughly 23% of M31’s major axis in a single pointing.
You would need about:
3.2 / 0.73 = 4.4
beamwidths across the galaxy, so approximately 5 pointings along the major axis to cover its visible extent.