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In the present work, recent characterization results of the 4K$\times$4K CCDImager (a first light instrument of the 3.6m Devasthal Optical Telescope; DOT)and photometric calibrations are discussed, along with measurements of theextinction coefficients and sky brightness values at the location of the 3.6mDOT site based on the imaging data taken between 2016 and 2021. For the4K$\times$4K CCD Imager, all given combinations of gains (1, 2, 3, 5, and 10e$^-$/ADU) and readout noise values for the three readout speeds (100 kHz, 500kHz, and 1 MHz) are verified using the sky flats and bias frames taken duringearly 2021; measured values resemble well with the theoretical ones. Usingcolor-color and color-magnitude transformation equations, color coefficients($\alpha$) and zero-points ($\beta$) are determined to constrain and examinetheir long-term consistencies and any possible evolution based on $UBVRI$observations of several Landolt standard fields observed during 2016-2021. Ourpresent analysis exhibits consistency among estimated $\alpha$ values withinthe 1$\sigma$ and does not show any noticeable trend with time. We also foundthat the photometric errors and limiting magnitudes computed using the CCDImager data follow the simulated ones published earlier. The average extinctioncoefficients, their seasonal variations, and zenith night-sky brightness valuesfor the moon-less nights for all ten Bessell and SDSS filters are alsoestimated and found comparable to those reported for other good astronomicalsites.

Photometric calibrations and characterization of the 4K$\times$4K CCD Imager, the first-light axial port instrument for the 3.6m DOT