\[$^{18}$O\]/\[$^{17}$O\] = 3.65, Penzias [@p81]; see Kramer et al. [@kra99] for a discussion of the validity of these techniques), we can put in general upper limits on the optical depth of (1–0) of order unity. At the (-5,-5) and (-25,-5) offsets, there is however evidence that (1–0) is optically thick and in fact we derive (1–0) optical depths of 1.0 and 1.5 respectively at these two offsets (errors 50% ). In these positions we used the (1–0) data and, assuming $T_{\rm ex}$ = 10 K (consistent with the observed (2–1)/(1–0) line ratio) and \[$^{18}$O\]/\[$^{17}$O\] = 3.65, we found an average  column density of $\sim$ 2$\times$10$^{15}$ cm$^{-2}$ in the central 25.

It is also interesting that we find that (1–0) and dust emission maps have similar sizes, whereas  maps are somewhat smaller. Fitting a 2D gaussian to the  and  maps in the same fashion as for the dust emission, we find the parameters given in Table \[gauss2d\] for the angular sizes of and  in L1521F. We deduce from these data a linear size for L1521F seen in (1-0) of roughly 14000 AU and dimensions in (2-1) of 13000 $\times $ 6000 AU. It is interesting to note the smaller sizes derived 