Cassiopeia and the Perseus arm, l = 120°

Rotation curves come from the inner Galaxy. Spiral structure is easier to see from out here.

Why it is interesting. At l = 120° the line of sight leaves the solar circle immediately and heads outward, so there is no tangent point. Under circular rotation, velocity on this sightline only gets more negative with distance. The flat-curve model runs from 0 down to about −110 km/s at R = 20 kpc. Because velocity maps cleanly to distance here, gas that is bunched up in a spiral arm appears as a separate bump in the spectrum rather than blending into everything else.

What you’ll see. Usually two components: local gas near zero, and a second, weaker peak at a few tens of km/s negative from the Perseus arm. Seeing that second peak with a small antenna is a good result and a fair test of your baseline. If the second peak moves when you change your bandpass correction, it was the bandpass.

Practicalities. At +63° declination this pointing is circumpolar from anywhere north of about 27° N, so it is available any time of night, any time of year. That makes it a good repeatable reference for comparing receiver changes. Cassiopeia A, one of the brightest continuum sources in the sky, lies about 8° away near l ≈ 112°, b ≈ −2°. A wide beam may catch some of it, which shows up as total power rather than line.