Chinese Plate-Digitizing Project

• Chinese Plate-Digitizing Project is a national initiative that digitizes and calibrates nearly 30,000 historical astronomical plates from 11 telescopes spanning 1901 to 1999.
• It utilizes a custom high-precision digitizer along with Gaia-based astrometry and rigorous photometric calibration to convert fragile archival data into science-ready products.
• The project supports time-domain astronomy by providing WCS-calibrated FITS images and extensive catalogs for studies of variable stars, proper motions, and historical light curves.

Searching arXiv for the core papers on the Chinese Plate-Digitizing Project and closely related plate-archive digitization work. {"query":"Chinese Plate-Digitizing Project astronomical photographic plates astrometric measurement photometric calibration arXiv", "max_results": 10} The Chinese Plate-Digitizing Project is a national program to preserve, digitize, calibrate, and disseminate China’s historical astronomical photographic plates. Led organizationally by the Shanghai Astronomical Observatory, it consolidated plate holdings from five observatories into the Sheshan Plate Archive, digitized nearly a century of observations, and produced astrometric and photometric data products tied to Gaia-based reference systems and the Johnson–Kron–Cousins photometric system. In its current published form, the project encompasses about 30,000 plates from 11 telescopes spanning 1901–1999; 29,314 plates were scanned, 15,696 single-exposure plates were processed astrometrically, and 15,680 single-exposure plates were successfully calibrated photometrically (Shang et al., 2024, Mingyang et al., 14 Jul 2025).

1. Historical formation and institutional scope

Chinese astronomical photography began in 1901 with the installation at Sheshan of a 40 cm double-tube refractor that continued producing plates until 1995. Over the twentieth century, Chinese observatories accumulated approximately 30,000 astronomical photographic plates, and the Sheshan Plate Archive now holds 30,750 nighttime plates, including some with uncertain provenance. The project formalized a national rescue and digitization effort in 2012, after which Shanghai Astronomical Observatory organized the transportation of plates taken at night from stations across the country to Sheshan for centralized preservation (Shang et al., 2024).

The archive integrates material from 11 telescopes at five observatories: SHAO, NAOC, PMO, YNAO, and QDO. The observational span is nearly a century, from 1901 to 1999, and the holdings cover solar-system objects, variable stars, clusters, nebulae, and extragalactic targets. The pointing distribution is strongly concentrated along the ecliptic, reflecting extensive solar-system observing programs, but the archive also includes equatorial and other targeted fields (Shang et al., 2024).

The project is not limited to image rescue. Its stated goals combine physical preservation, centralized archival management, digitization, astrometric reduction, photometric calibration, and public release through the National Astronomical Data Center. This suggests that the project should be understood as a full data-infrastructure program rather than as a scanning campaign alone (Shang et al., 2024, Mingyang et al., 14 Jul 2025).

2. Archive composition, physical condition, and preservation regime

The archived plates are heterogeneous in telescope scale, field of view, and plate size. Long-focus systems such as the SHAO 1.56 m reflector and the YNAO 1 m reflector produced small-field, higher-precision material, whereas instruments such as the NAOC 60/90 cm Schmidt and short-focus patrol refractors produced wide-field survey plates. The collection therefore combines material suited to precise astrometry with material optimized for large-area patrol and survey work (Shang et al., 2024).

Before scanning, the project carried out a national inventory and a four-level condition grading. Grade 1 plates have intact emulsion and no mold; Grade 2 plates have minor defects affecting less than 25% of the area; Grade 3 plates have moderate defects affecting less than 50% of the area; Grade 4 plates have severe damage and were excluded from scanning. The archive contains 11,475 Grade 1 plates, 9,835 Grade 2 plates, 8,004 Grade 3 plates, and 1,436 Grade 4 plates (Shang et al., 2024).

Physical preparation followed explicit cleaning procedures. The emulsion side was cleaned with an air blower, and the glass side was wiped with cotton cloth plus distilled water and then dried with a lint-free cloth. Digitization was performed under controlled environmental conditions of $20^\circ \mathrm{C} \pm 0.5^\circ \mathrm{C}$ and $50\% \pm 5\%$ humidity. This preservation regime directly links conservation and digitization: physical stabilization and accurate scanning were treated as a single workflow (Shang et al., 2024).

A common misconception is that historical plate projects can defer preservation once imaging begins. The Chinese project indicates the opposite. Plates were centralized, graded, cleaned, and scanned within a controlled archive environment, and the project retained the distinction between archive management and scientific calibration rather than collapsing them into a single imaging task. A plausible implication is that long-term scientific reuse depends as much on physical stewardship as on digital processing (Shang et al., 2024).

3. Digitization system and astrometric reduction

Digitization was carried out with a custom high-precision plate digitizer developed by SHAO and Nishimura Optical. The system comprises a linear array camera, dual-sided telecentric lens, two-dimensional motion platform, and LED light source, all mounted on a granite platform. It operates in line-scanning mode, covers up to $300 \,\mathrm{mm} \times 300 \,\mathrm{mm}$, achieves positional precision better than $1\,\mu\mathrm{m}$, scans each plate in no more than 10 minutes, and outputs 16-bit FITS images (Shang et al., 2024).

From 30,750 archived plates, digitized images were acquired for 29,314. These were then classified into 18,226 single-exposure plates, 4,632 multiple-exposure plates, 364 grating plates, 5,778 near-Earth moving-object plates, and 314 test plates. The published astrometric pipeline in the first large release focuses on the single-exposure subset (Shang et al., 2024).

Astrometric processing begins with source extraction using SExtractor, with XWIN_IMAGE, YWIN_IMAGE, and MAG_AUTO retained among the core outputs. An initial blind solution is obtained with Astrometry.net, and Gaia DR2 provides the astrometric reference catalog. For plates contaminated by scratches, mold spots, or dust, the project introduced a scikit-learn SVM classifier trained on SExtractor-derived features to separate stellar from non-stellar detections; this recovered 2,184 additional successful initial solutions from 4,714 problematic single-exposure plates (Shang et al., 2024).

The plate solution is expressed through polynomial plate models with a magnitude-equation term. In generalized form,

$$\begin{aligned} \xi &= a_0 + a_1 x + a_2 y + \cdots + p_{1\xi} m + p_{2\xi} m^2,\ \eta &= b_0 + b_1 x + b_2 y + \cdots + p_{1\eta} m + p_{2\eta} m^2, \end{aligned}$$

where $(x,y)$ are measured plate coordinates, $(\xi,\eta)$ are standard tangential coordinates, and $m$ is the Gaia DR2 $g$ magnitude. Telescope No. 01 adopted a 2nd-order model, while the other calibrated telescopes adopted 3rd-order models, because higher orders produced negligible improvement over 3rd order in the model-selection tests (Shang et al., 2024).

The resulting astrometric performance depends strongly on focal length. For long-focus systems such as the SHAO 40 cm double-tube refractor, SHAO 1.56 m reflector, and YNAO 1 m reflector, typical plate accuracy is approximately $0.1''$ to $50\% \pm 5\%$0. For medium- and short-focus telescopes, the distribution ranges from $50\% \pm 5\%$1 to $50\% \pm 5\%$2. The final plate solutions are written to FITS headers in TAN-SIP form, so the digitized images function as WCS-calibrated astronomical data rather than as unregistered scans (Shang et al., 2024).

4. Photometric calibration and catalog construction

The second major stage calibrates 15,696 conventional single-exposure plates to the Johnson–Kron–Cousins $50\% \pm 5\%$3 system. Of these, 15,680, or 99.9%, were successfully calibrated; 16 plates had too few standard stars. The calibration uses SExtractor MAG_AUTO as the instrumental magnitude and ties each plate to the closest JKC band selected empirically on a plate-by-plate basis (Mingyang et al., 14 Jul 2025).

The standard-star basis is the BEST database, which provides synthetic photometry derived from corrected Gaia DR3 BP/RP spectra. For each plate, the target band is chosen by fitting a second-order polynomial between standard magnitude and instrumental magnitude for each candidate JKC band and then examining the residual slope as a function of $50\% \pm 5\%$4; the flattest color residual defines the closest band. This procedure is necessary because metadata are incomplete: about 18% of plates have no emulsion type recorded and about 54% have no filter information recorded (Mingyang et al., 14 Jul 2025).

The adopted photometric model is

$50\% \pm 5\%$5

where $50\% \pm 5\%$6 is a 4th-order magnitude term, $50\% \pm 5\%$7 is a 2nd-order color term, and $50\% \pm 5\%$8 is a position-dependent flat-field term decomposed into a 3rd-order large-scale polynomial plus a local small-scale numerical correction. The workflow also includes magnitude-based rejection of false stars, robust weighting to compensate for non-uniform standard-star density in magnitude and color space, and iterative refinement of the photometric solution (Mingyang et al., 14 Jul 2025).

The achieved external photometric precision is summarized below.

Observatory	Typical precision	Best precision
SHAO	0.15 mag	0.07 mag
NAOC	0.23 mag	0.08 mag
PMO	0.17 mag	0.06 mag
YNAO	0.11 mag	0.05 mag
QDO	0.19 mag	0.11 mag

The limiting magnitude, defined as the magnitude of the faintest 5% of calibrated stars on a plate, ranges from 13.4–17.7 mag for SHAO, 13.7–18.5 mag for NAOC, 12.7–18.3 mag for PMO, 12.2–17.5 mag for YNAO, and 13.6–18.0 mag for QDO (Mingyang et al., 14 Jul 2025).

The released photometric catalogs comprise 33,282,558 unique sources and 229,271,500 individual observations. The earlier astrometric catalog contains 43,321,336 unique Gaia DR2-matched sources and 290,417,004 individual detections. Variable candidates are identified by comparing calibrated magnitudes with BEST standards as a function of plate precision: there are about 4.0 million detections with variable_flag \ge 3, about 1.5 million with \ge 4, and about 0.66 million with \ge 5, corresponding to about 2.8 million, 1.1 million, and 0.50 million unique variable candidates, respectively (Mingyang et al., 14 Jul 2025).

5. Released data products and scientific applications

The project releases WCS-calibrated FITS images, per-plate stellar catalogs, and associated metadata through the National Astronomical Data Center. Access is provided through the China Astronomical Plates Data Release at https://nadc.china-vo.org/res/r100742/, with a DOI-linked photometric release at https://www.doi.org/10.12149/103032. Query interfaces support retrieval by station, telescope, observation year, and celestial coordinates (Shang et al., 2024, Mingyang et al., 14 Jul 2025).

The photometric catalogs include calibrated magnitudes (MAG_CORR), instrumental magnitudes (MAG_AUTO), plate coordinates, Gaia DR2 and DR3 identifiers, BEST synthetic JKC magnitudes, Gaia DR3 synthetic $50\% \pm 5\%$9, $300 \,\mathrm{mm} \times 300 \,\mathrm{mm}$0, $300 \,\mathrm{mm} \times 300 \,\mathrm{mm}$1, BP/RP excess factor, calibration credibility flags, and variable flags. Each variable candidate also has a dedicated light-curve FITS file named by Gaia DR2 source_id (Mingyang et al., 14 Jul 2025).

The scientific rationale is long-baseline time-domain astronomy and astrometry. The century-scale temporal span enables reconstruction of pre-CCD light curves, study of secular variability in AGNs, cataclysmic variables, long-period variables, and pre-main-sequence stars, and extension of proper-motion baselines when combined with Gaia. The project papers specifically validate the calibrated products with a color–magnitude diagram for M4 and with an M31 supergiant whose historical $300 \,\mathrm{mm} \times 300 \,\mathrm{mm}$2-band light curve shows a strong brightening around 1985-11-13 and 1985-11-17 (Mingyang et al., 14 Jul 2025).

The project also preserves the heritage dimension of the archive. FITS images, plate envelopes, and catalog products are published together, allowing the archive to be used simultaneously as a scientific resource and as a record of Chinese observational astronomy. This dual status is typical of mature plate archives and aligns the Chinese project with broader historical-astronomy digitization practice (Shang et al., 2024, Lencinas, 2016).

6. Methodological position, limitations, and broader significance

Methodologically, the Chinese Plate-Digitizing Project belongs to a wider class of plate-archive digitization programs in which preservation, metadata recovery, scanning, astrometric solution, calibration, and public dissemination are coupled. Related efforts at Córdoba emphasize the need for a unified base record derived from plate borders, boxes, and logbooks, while the Hamburger Sternwarte/APPLAUSE project illustrates the value of WCS-calibrated scans, linked handwritten documentation, and science-driven validation through historical light curves (Lencinas, 2016, Wertz et al., 2016).

Several limitations remain explicit in the Chinese project. First, only the single-exposure subset has been fully treated in the present photometric release; multiple-exposure plates are handled separately, and grating plates and moving-object plates require additional methodology (Shang et al., 2024, Mingyang et al., 14 Jul 2025). Second, metadata incompleteness is substantial for emulsions and filters, which is why band selection is empirical rather than purely metadata-driven (Mingyang et al., 14 Jul 2025). Third, the calibration uses present-day Gaia $300 \,\mathrm{mm} \times 300 \,\mathrm{mm}$3 color for historical epochs; the authors argue that most color variations are smaller than the amplitude of the brightness variations of interest and that the color term is relatively flat, but the assumption remains a methodological constraint (Mingyang et al., 14 Jul 2025).

The project’s broader significance lies in its transformation of a dispersed and physically vulnerable archive into a modern survey-like resource. It combines centralized preservation, a purpose-built digitizer, Gaia-tied astrometry, Gaia-XP-based photometric calibration, and public dissemination at national scale. A plausible implication is that the Chinese archive is not merely a national historical resource but part of the emerging global century-scale time-domain infrastructure built from rescued photographic materials (Shang et al., 2024, Mingyang et al., 14 Jul 2025).