Before observing each target source, we chose one calibration source (e.g., 3C147, 3C295, 3C358, 3C401, 3C383 or NGC7027) near the target source to observe it for pointing correction of the telescope. We took one and two scans for calibration and target sources, respectively, with scan lengths of three minutes each. C- and Ku-band receiver features were observed with left and right circular polarization and were subsequently averaged. The continuum at both C- and Ku-bands was successfully detected toward 32 of the 38 sources. For the other 6 sources, the continuum at C-band was detected, while no Ku-band signal was obtained. 4 sources remained undetected, while 2 sources (Mol12, J205703.98) were not observed at Ku-band. Table \[Table1\] presents obtained continuum temperatures. The typical rms value of the continuum at Ku-band for the 4 undetected targets was $\sim$ 0.1 K ($\sim$ 0.15 Jy, see below), which is also listed in Table \[Table1\]. The two unobserved targets are marked by “-”. Nevertheless, this does not affect our main results (see details in Sect. \[subsec:correct\]).

The noise diode signal inserted into the receiver system may differ slightly between the C-band frequencies of H$_2$CO and H$_2$$^{13}$CO. Thus we performed separate flux calibration observations at 4.829 