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

Abstract

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 data taken using the CCD imager 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.

Appendix

As an example, Figure A1 represents the PTCs for obtaining the gain values for readout speed of 100 kHz and theoretical gain values of 1, 2, 3, 5 and 10 e$^-$/ADU. Similarly, the gain values are verified for readout speeds of 500 kHz and 1 MHz; those are tabulated in Table 1. The observations log of all the Landolt standard fields used in this study is tabulated in Table A1. The FWHM (in arc-sec; for V-band) values for all these Landolt fields (having an airmass range of $\sim$1–1.5) were also estimated (see Figure A2) using IRAF/imexa. Figure A2 shows the distribution of measured FWHM values in V-band for present observations distributed over 32 nights during a span of 5 years. For nearly 10% of the observed sample, <1 arc-sec FWHM values were observed. The best FWHM obtained for a point source was about 0.43 arc-sec on 17 March 2020 (see a r-band stellar image in Figure A3) along with many faint galaxies detected in the field. We have also shown the contour and radial profile of the stellar image in Figure A3, respectively. The FWHM of the stellar profile is $\sim$2.26 pixels which convert to $\sim$0.43 arc-sec using the plate scale of the CCD (0.19 arc-sec per pixel for ${2} \times{2}$ binning) on the sky.

Figure A1

PTCs of the 4K$\times$4K CCD imager taken at readout speed of 100 kHz and for all available gain values. The measured gain values are ${\sim }0.98\pm 0.05$, $1.95\pm 0.06$, $2.87\pm 0.10$, $4.85\pm 0.12$ and $9.81\pm 0.15$ e$^-/$ADU for theoretical gain values of 1, 2, 3, 5 and 10 e$^-/$ADU, respectively.

Figure A2

FWHM values (in arc-sec) estimated using V-band imaging data of the standard fields (Landolt 1992) discussed in Table A1 using the 4K$\times$4K CCD imager mounted at the axial port of the 3.6m DOT during 2016–2021. For nearly 10% of nights, the FWHM values are <1 arc-sec for V-band.

Table A1 Log of Landolt standard fields (Landolt 1992) used in this study observed in Bessell UBVRI and SDSS ugriz filters from 2016 to 2021.

Figure A3

The field of SLSN 2017egm observed in r-band on 17 March 2020, for an exposure time of 600s, is shown. This image clearly detects many galaxies in the field as faint as $\sim$22 mag along with several faint point sources as expected with this CCD imager. The FWHM of the stellar profile (in red box) was observed as $\sim$0.43 arc-sec. The contour and radial profile of the stellar image are also shown.