We present the UV photometry of the globular cluster NGC 1851 using images acquired with the Ultra-violet Imaging Telescope ( UVIT ) onboard the ASTROSAT satellite .
PSF-fitting photometric data derived from images in two far-UV ( FUV ) filters and one near-UV ( NUV ) filter are used to construct color-magnitude diagrams ( CMD ) , in combination with HST and ground-based optical photometry .
In the FUV , we detect only the bluest part of the cluster horizontal branch ( HB ) ; in the NUV , we detect the full extent of the HB , including the red HB , blue HB and a small number of RR Lyrae stars .
UV variability was detected in 18 RR Lyrae stars , and 3 new variables were also detected in the central region .
The UV/optical CMDs are then compared with isochrones of different age and metallicity ( generated using Padova and BaSTI models ) and synthetic HB ( using helium enhanced Y ^ { 2 } models ) .
We are able to identify two populations among the HB stars , which are found to have either an age range of 10-12 Gyr , or a range in Y _ { ini } of 0.23 - 0.28 , for a metallicity of [ Fe/H ] = - 1.2 to - 1.3 .
These estimations from the UV CMDs are consistent with those from optical studies .
The almost complete sample of the HB stars tend to show a marginal difference in spatial/azimuthal distribution among the blue and red HB stars .
This study thus show cases the capability of UVIT , with its excellent resolution and large field of view , to study the hot stellar population in Galactic globular clusters .