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High-resolution Hubble Space Telescope (\textit{HST}) optical observationshave been used to perform the deepest photometric study of the poorly studiedGalactic globular cluster NGC 6284. The deep colour-magnitude diagram (CMD)that we obtained reaches 6 magnitudes below the main sequence turn-off. Weprovide the first determination of the gravitational centre ($C_{\rm grav}$)and density profile of the system from resolved stars. $C_{\rm grav}$ issignificantly offset (by $1.5-3''$) from the values in the literature. Thedensity profile shows the presence of a steep central cusp, unambiguouslyindicating that the cluster experienced the core-collapse phase. Updated valuesof the structural parameters and relaxation times of the system are provided.We also constructed the first high-resolution reddening map in the clusterdirection, which allowed us to correct the evolutionary sequences in the CMDfor the effects of differential reddening. Isochrone fitting to the correctedCMD provided us with new estimates of the cluster age, average colour excess,metallicity, and distance. We find an absolute age of $13.3 \pm 0.4$ Gyr, anaverage colour excess $E(B-V) = 0.32 \pm 0.01$, a metallicity [Fe/H]$= -1.36\pm 0.01$, and a true distance modulus $(m-M)_0 = 15.61 \pm 0.04$ that sets thecluster distance at $13.2 \pm 0.2$ kpc from the Sun. The superb quality of theCMD allowed a clear-cut identification of the Red Giant Branch (RGB) bump,which is clearly distinguishable along the narrow RGB. The absolute magnitudeof this feature turns out to be $\sim 0.2$ mag fainter than previousidentification.

A Photometric in-depth look at the core-collapsed globular cluster NGC 6284