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Aims: By utilising spectra of early-type stellar probes of known distances inthe same region of the sky, the large and small-scale (pc) structure of theGalactic ISM can be investigated. This paper determines the variation in linestrength of CaII at 3933.661 A, as a function of probe separation for a sampleof stars, including many sightlines in the Magellanic Clouds. Methods:FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic &4 Magellanic open clusters in CaII are used to obtain the velocity, EW, columndensity and line width of IS Galactic Ca for a total of 657 stars, of which 443are Magellanic sightlines. In each cluster there are 43-110 stars observed.Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanicearly-type stars are presented and combined with data from the literature tostudy the Ca column density/parallax relationship. Results: For the fourMagellanic clusters studied with FLAMES, the strength of the Galactic IS CaII KEW over transverse scales from 0.05-9 pc is found to vary by factors of1.8-3.0, corresponding to column density variations of 0.3-0.5 dex in theoptically-thin approximation. Using FLAMES, FEROS and UVES archive spectra, themin and max reduced EW for MW gas is found to lie in the range 35-125 mA &30-160 mA for CaII K and NaI D, respectively. The range is consistent with asimple model of the ISM published by van Loon et al. (2009) consisting ofspherical cloudlets of filling factor 0.3, although other geometries are notruled out. Finally, the derived functional form for parallax and CaII columndensity is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derivedparallax is 25 per cent lower than predicted by Megier et al. (2009) at adistance of 100 pc and 15% lower at a distance of 200 pc, reflectinginhomogeneity in the CaII distribution in the different sightlines studied.

The large- and small-scale CaII K structure of the Milky Way from observations of Galactic and Magellanic sightlines