The ionizing cluster of 30 Doradus - I. Internal reddening from NTT photometry and multi-object spectroscopy

UBV profile fitting photometry is presented for 1469 stars within 90" of th e center of the ionizing cluster of 30 Doradus (NGC 2070). A value of 0.82 +/- 0.03 is found for the extinction parameter S = E(U-B)/E(B-V), constant over almost the whole area covered by this research, with some evidence for local variations. Two reddening components can be identified: a smooth and moderate one that increases towards R136, and a clumpy one varying widely across the face of the cluster. The total-to-selective extinction parameter , R-V = A(V)/E(B - V), is found to be in the range of 3.0 to 3.7, consisten t with previous determinations. A new visualization tool, the colour-magnit ude stereogram, is introduced and used to argue that: (1) the observations imply mass segregation in a dust filled cluster; (2) that there has been co ntinuous dust formation near the cluster core up to the present; and (3) th at an insidious systematic error has plagued previous determinations of IMF slopes. A method to obtain an unbiased estimate of the slope is discussed.