Selecting Optimal Stellar Calibration Fields for the CSST Imaging Survey

TL;DR

The paper addresses the challenge of selecting stable, high-quality calibration fields for CSST's decade-long imaging survey. It combines a two-stage framework—initial spatial feasibility based on orbital visibility with extinction constraints, followed by instrument-specific screening using Gaia densities, Tycho-2 bright-star avoidance, and Planck/SFD extinction maps—to identify optimal regions. The methodology yields six globular clusters (M13, M92, NGC 104, NGC 362, NGC 1261, NGC 1851) that satisfy all calibration requirements, with quantified metrics on visibility, stellar density, and extinction, and notes minor localized extinction anomalies that can be mitigated in planning. The work provides a practical, generalizable framework for calibrating wide-field UV/optical space surveys and directly supports CSST's goal of achieving precise photometric and astrometric stability, while outlining future steps to finalize calibration-pointing strategies.

Abstract

The Chinese Space Station Survey Telescope (CSST) will perform a decade-long high-precision wide-field imaging survey that relies on rigorous on-orbit calibration. This necessitates stable celestial benchmark fields to maintain photometric and astrometric consistency throughout the mission lifetime. We establish comprehensive selection criteria including observational visibility, stellar number density, bright-star contamination, and interstellar dust extinction. Using the CSST Observation Strategy Analysis Tool (COSAT) and all-sky dust maps from Planck and SFD, we constrain eligible regions to the ranges of ecliptic latitude $ |β| > 50^\circ$ and galactic latitude $|b| > 15^\circ$. From an initial sample of 29 candidate clusters meeting these spatial constraints, six globular clusters (M13, M92, NGC 104, NGC 362, NGC 1261, and NGC 1851) are identified as optimal calibration fields, fulfilling all the critical criteria. These selected clusters are recommended as optimal calibration field candidates for CSST's on-orbit calibration program, and are fundamental to achieving unprecedented photometric precision in CSST's space-based survey.

Figures (7)

• Figure 1: All-sky distribution of visibility parameters ($D_{\rm obs}$, $H_{\rm obs}$, $\sigma_{\rm frac}$) for the MSC and MCI in ecliptic coordinates from 2027 to 2038, respectively. The red and cyan lines represent the Equatorial and Galactic Planes, respectively. The magenta points represent the celestial poles and green lines represent declination at $\rm \delta = \pm \ang{48}$. Right insets show binned averages of each parameter versus ecliptic latitude. Black dots represent parameters at individual HEALPix regions; red broken lines and error bars represent binned averages ($\Delta \beta= \ang{10}$) with $1\sigma$ uncertainties.
• Figure 2: The distribution of $E(B-V)$ as a function of Galactic Latitude ($b$). The black points represent the $E(B-V)$ values measured at the systematic grid sampling points. The red solid line denotes the median $E(B-V)$ value at each galactic latitude. The green dashed line corresponds to the extinction threshold $E(B-V) = 0.15$. The cyan dashed lines mark the intersections between the red median line and the green threshold line. These intersections are located at $b = \pm \ang{15}$ for the SFD map and $b = \pm \ang{16}$ for the Planck map, respectively.
• Figure 3: All-sky distribution of candidate star clusters (green = open clusters, red = globular clusters) superimposed on the Planck$E(B-V)$ dust extinction map with Mollweide projection in ecliptic coordinates. Dashed yellow lines indicate the ecliptic latitude constraint $\lvert \beta \rvert =\ang{50}$, while dotted black curves show the galactic latitude constraint $\lvert b \rvert =\ang{15}$. The six recommended globular clusters are highlighted with enlarged red circles.
• Figure 4: Distribution of the candidate star clusters overlaid on the preliminary survey regions of the CSST. The legend of this figure is consistent with that of Figure \ref{['fig:all_sky']}.
• Figure 5: The number of observable days in each month for the six selected candidate star clusters in a four-year period. Each cell represents the number of observable days in a given month for one cluster and their brightness are related to the number of observational days (see the $\rm N_{obs}$ in the colorbar) in each month. Blue color for MSC and red color for MCI, respectively.