We have completely mapped the Galactic globular cluster NGC 1851 with large-field , ground-based VI CCD photometry and pre-repair HST /WFPC1 data for the central region .

The photometric data set has allowed a V vs . ( V - I ) colour–magnitude diagram for \sim 20500 stars to be constructed .
From the apparent luminosity of the horizontal branch ( HB ) we derive a true distance modulus ( m - M ) _ { 0 } = 15.44 \pm 0.20 .

An accurate inspection of the cluster ’ s bright and blue objects confirms the presence of seven “ supra-HB ” stars , six of which are identified as evolved descendants from HB progenitors .

The HB morphology is found to be clearly bimodal , showing both a red clump and a blue tail , which are not compatible with standard evolutionary models .
Synthetic Hertzsprung–Russell ( HR ) diagrams demonstrate that the problem could be solved by assuming a bimodal efficiency of the mass loss along the red giant branch ( RGB ) .
With the aid of Kolmogorov–Smirnov statistics we find evidence that the radial distribution of the blue HB stars is different from that of the red HB and subgiant branch ( SGB ) stars .

We give the first measurement of the mean absolute I magnitude for 22 known RR Lyr variables ( < M _ { I } ( { RR } ) > = 0.12 \pm 0.20 mag at a metallicity [ Fe/H ] = –1.28 ) .
The mean absolute V magnitude is < M _ { V } ( RR ) > = 0.58 \pm 0.20 mag , and we confirm that these stars are brighter than those of the zero-age HB ( ZAHB ) .
Moreover , we found seven new RR Lyr candidates ( six ab type and one c type ) .
With these additional variables the ratio of the two types is now N _ { c } / N _ { ab } = 0.38 .

From a sample of 25 globular clusters a new calibration for \Delta V _ { bump } ^ { HB } as a function of cluster metallicity is derived .
NGC 1851 follows this general trend fairly well .
From a comparison with the theoretical models , we also find some evidence for an age–metallicity relation among globular clusters .

We identify 13 blue straggler stars , which do not show any sign of variability .
The blue stragglers are less concentrated than the subgiant branch stars with similar magnitudes for r > 80 arcsec .

Finally , a radial dependence of the luminosity function , a sign of mass segregation , is found .
Transforming the luminosity function into a mass function ( MF ) and correcting for mass segregation by means of multi-mass King–Michie models , we find a global MF exponent x _ { 0 } = 0.2 \pm 0.3 .