An Extinction Map and Color Magnitude Diagram for the Globular

Differential EV −I variations of up to ∼ 0.2 mag on a scale of arcminutes across NGC 3201 are presented in the form of an extinction map. This map, created by calculating average EV −I values for stars in small subregions of the field with respect to a fiducial region, greatly improves the appearance of the CMD of the cluster. We describe how we implemented this technique in detail with our data for NGC 3201. A comparison between our map and that of the same region extracted from the COBE/DIRBE reddening maps published by Schlegel, Finkbeiner, & Davis (1998) (SFD) displays larger-scale similarities between the two maps as well as smaller-scale features which show up in our map but not in the SFD map. Several methods of determining an EV −I zeropoint to add to our differential extinction map are presented. Isochrone fitting proved to be the most successful one, but it produces an average EV −I for the cluster which is smaller than previously published values by ∼ 1.5σ. Finally, our results seem to support the statement by Arce & Goodman (1999) that the SFD maps overestimate the reddening in regions of high extinction. Subject headings: color-magnitude diagrams — dust, extinction — globular clusters: individual (NGC 3201) — methods: data analysis — stars: fundamental parameters