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