Emission-Free Reference Positions (OFFs)

The position-switching generally adopted by the 1.2 m telescopes requires two emission-free reference positions (OFFs) for each observation, one located above the source in elevation and one below. OFFs were typically chosen a few degrees or more from the Galactic plane and spaced every few degrees in Galactic longitude. OFFs were checked to be CO-free by frequency switching to a noise level that was typically twice as low as that of the survey with which it would be used. The positions of our standard OFFs with respect to the Galactic plane are shown below. An ASCII table giving the Galactic coordinates of the OFFs and the rms noise level to which each was checked to be free of emission can be obtained here

The 4 OFFs marked in red were intentionally chosen within the large, nearby Aquila Rift cloud (v ~ 10 km/s), since no positions totally free of emission could be found sufficiently close to the plane in that region. Spectra obtained using one of these contaminated OFFs are corrected by adding in the Aquila Rift spectral line(s), weighted by the fraction of the time the OFF was used. Low-noise profiles for each of the contaminating lines were obtained by frequency switching. Gaussian fit parameters for the contaminating lines are given below. Note that the positions (24, 4) and (33, 5) contain two lines each.

Gaussian Fits to Contaminating Lines
LII BII Peak Tmb Velocity FWHM
(deg) (deg) (K) (km/s) (km/s)
18 4 0.65 0.94 1.29
23 -4 0.91 10.02 2.23
24 4 3.15 6.51 3.15
" " 0.33 14.10 4.30
33 5 0.45 8.63 6.18
" " 0.24 17.68 2.96

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