Calibrating on Planets

For a planet we have to account for the loss of signal due to the coupling to the beam, because all planets used for calibration are extended relative to the JCMT beam. For our Uranus data the flux density S$_{\rm beam}$ is therefore the total flux density, S$_{\rm tot}$ divided by the coupling of the planet to the beam, given by:

$$K = \frac{x^2}{1 - e^{-x^2}}$$ (6)

where x is

$$x = \frac{W}{1.2 \times \theta_A}.$$ (7)

The FCF, in Jy/beam/V is therefore

$$FCF(Jy/{\rm beam}/V) = S_{\rm beam}/V_{\rm peak}$$ (8)

For 850 $\mu$m we find K = 1.021 for $\theta_{A}$ = 14.5'', which gives S$_{beam}$ = 66.5 Jy/beam. The peak signal that we found for our high S/N Uranus map, V$_{\rm peak}$ = 0.2477 V, or an FCF = 268.5 Jy/beam/V. This FCF applies to a jiggle maps with a 120'' chop throw. If we do the same for our jiggle maps of Uranus with a 60'' chop throw, we derive FCF = 245.2 Jy/beam/V, i.e. a map with a 60'' chop throw is $\sim$ 10% more efficient than one with a 120'' chop throw. Even though Jenness et al. ([14]) found no difference in FCF as a function of chop throw when calibrating in Jy/aperture we find that the difference is now smaller than compared to when calibrating in Jy/beam but still noticeable. For a 40'' aperture the difference is 6%.