postscript
1 Introduction...
Abstract...
3 Maps...
to Trace Free--free Emission
Although galactic 
emission is correlated with the galactic
free--free emission, it is not straightforward to estimate the galactic
free--free from maps of the sky in 
. These maps are themselves
contaminated with atmospheric emission! The earth's geocorona emits 
radiation: hydrogen atoms in the earth's exosphere are excited by
solar Lyman 
photons, and about 
of the excited atoms return to
ground state through emission of 
photons (Shih et al. (1985)). The
geocoronal emission varies both diurnally and seasonally with the solar Lyman
flux, and the amplitude of this variation ranges from about 2 to 25
Rayleigh. In principle, the geocoronal and galactic emissions are separable,
since the centre of velocity of the geocoronal line is generally shifted with
respect to the galactic line due to relative velocity of the Earth with respect
to the local standard of rest (LSR), but such separation requires spectral
measurements (e.g. Reynolds (1980), Reynolds (1992)), not simply imaging. The
maps which we use for the analysis below do not have any spectroscopic
information.
Gaustad et al. (1996) and Simonetti et al. (1996) have mapped the 
emission around the North Celestial Pole with resolution of a few
arcminutes, and used these maps to predict the level of free--free
contamination of the Saskatoon results, i.e. on angular scales above a degree.
Gaustad et al. (1996) have generously made their maps publicly available.
These maps are a series of images mosaicked to cover the entire NCP region
north of declination 
. Each image has an effective resolution of
and field of view of about 
. This set of images
ought to provide a convenient data set with which to measure 
,
and therefore, free-free fluctuations on the few-arcminute angular scale.
Several CMB experiments are being conducted in the NCP region
(e.g. Netterfield et al. (1995): Saskatoon, Myers et al. (1993), Leitch et al. (1997): OVRO Ring,
Cheng et al. (1994): MSAM) even though this region is not ideal for CMB measurements.
Firstly, it is at relatively low galactic latitude (the NCP is at
). Moreover, there is enhanced IRAS 100 
emission in
this region as compared to the emission in other regions at similar latitude,
and this emission has been identified as the north polar cirrus cloud by
Gautier et al. (1992). The 100 
emission does not appear to correlate
well with HI. We expect that, in more quiescent regions of the sky, and in
particular at higher galactic latitude, the amplitudes which we calculate for
the free--free emission may represent upper limits.