Photometric calibration of the Stellar Abundance and Galactic Evolution Survey (SAGES): Nanshan One-meter Wide-field Telescope g, r, and i band imaging data

Abstract

<p indent="0mm">In recent years, there have been numerous wide-field imaging surveys conducted both domestically and internationally, which have had a revolutionary impact on astronomy. Uniform and accurate photometric calibration is very challenging and plays a key role in wide-field imaging surveys. The Nanshan One-meter Wide-field Telescope, with a 1.3° field of view, was used by the Stellar Abundance and Galactic Evolution Survey (SAGES) to image 4254 square degrees of the northern sky in three broadband filters (g, r, and i). In this paper, a total of approximately 2.6 million dwarf stars were constructed as standard stars, with an accuracy of about 0.01–0.02 magnitude for each band, by combining spectroscopic data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 7, photometric data from the corrected Gaia Early Data Release 3, and photometric metallicities obtained by Xu et al. Using the spectroscopy-based stellar color regression method (SCR method) and the photometric-based SCR method (SCR′ method), we performed the relative calibration of the Nanshan One-meter Wide-field Telescope imaging data of the SAGES survey. Based on the corrected Pan-STARRS DR1 (PS1) photometry, the absolute calibration was also performed, and the transform relationship between Nanshan One-meter Wide-field Telescope calibrated magnitudes and the corrected PS1 magnitudes was given. In the photometric calibration process, we analyzed the dependence of the calibration zero points on different images (observation time), different gates of the CCD detector, and different CCD positions. We found that the stellar flat and the relative gain between different gates depend on time. Variations in the telescope’s performance can be classified as either related to sudden changes in pointing or unrelated. By analyzing the spatial distribution of the stellar flat-field, we can verify the orientation and rotation of the horizon for the Nanshan One-meter Wide-field Telescope. The amplitude of gain variation in three channels is approximately 0.5%–0.7% relative to the other channel, with a maximum value of 4%. In addition, significant spatial variations of the stellar flat fitting residual are found in all the g, r, and i filters. For each band, the significant spatial structure is similar on a daily basis but different over longer timescales (e.g., seasonally). To correct the spatial-dependence errors presented by the flat-field correction fitting residuals, we used the fitting residuals from a single day to correct all the observed images taken on that day. Using repeated sources in the adjacent images, we checked and discovered internal consistency of about 1–2 millimagnitude in all the filters. Using the PS1 magnitudes synthesized by Gaia DR<sc>3 BP</sc>/RP spectra by the synthetic photometry method, we found that the photometric calibration uniformity is about 2.4, 2.3, and 0.9 millimagnitude for the g, r, and i bands, respectively, at a spatial resolution of <sc>1.3°.</sc> A detailed comparison between the spectroscopy-based SCR and photometric-based SCR method magnitude offsets was performed, and we achieved an internal consistency precision of about 2 millimagnitude or better with resolutions of <sc>1.3°</sc> for all the filters. And the difference between the spectroscopy-based SCR method and the photometric-based SCR method is mainly from the position-dependent systematic error of the dust reddening map of Schlegel et al. used in SCR′ method. This work has successfully broken through the “1% accuracy bottleneck” of groundbased wide-field photometric survey photometric calibration. Our results can provide method and experience references for the photometric calibration of upcoming survey data.

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