![]() There are many surveys concerning the identification of quasars such as the large Bright Quasar Survey ( Hewett et al., 1995), the 2DF Quasar Redshift Survey (2QZ, Croom et al., 2004), the quasars from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Luo et al., 2012) and Solan Digital Sky Survey (SDSS, Pâris et al., 2018 Lyke et al., 2020). On the other hand, quasars are also a critical pathway to explore the evolution and mergers of galaxies in astrophysics ( Begelman et al., 1980 Shen et al., 2021). ![]() In astrometry, a large number of evenly distributed quasars can be used to establish a celestial reference frame ( Ma, 1997 Ma et al., 2009 Mignard et al., 2018 Charlot et al., 2020) because they have almost zero proper motions and point-like shapes. Since the discovery of the first quasar in 1963 ( Schmidt, 1963), this type of extremely distant active galactic nuclei (AGN) has gradually become the focus of astronomical research. They are potential candidates of quasar pairs. We obtain two catalogs containing 155 sources and 44 sources, respectively. We propose a series of criteria for selecting abnormal quasars based on Gaia astrometric data. In this work, we aim to find these quasar candidates from Gaia EDR3 astrometric data combining with Sloan Digital Sky Survey (SDSS) spectroscopic data to provide a candidate catalog to the science community. These quasars may come with astrometric jitter detectable with Gaia data, or significant changes in the position at different wavelengths. However, some special quasars may have abnormal astrometric characteristics, such as quasar pairs, lensed quasars, AGNs with bright parsec-scale optical jets, which are scientifically interesting objects, such as binary black holes. Given their extremely large distances and small apparent sizes, quasars are generally considered as objects with near-zero parallax and proper motion. 2School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing, China.1Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China.We present a sample of 386 BL Lacertae candidates identified from 2860 deg2 of the Sloan Digital Sky Survey (SDSS) spectroscopic database.Qi-Qi Wu 1,2, Shi-Long Liao 1,2*, Xiang Ji 1,2, Zhao-Xiang Qi 1,2, Zhen-Ya Zheng 1,2, Ru-Qiu Lin 1,2, Ying-Kang Zhang 1,2 and Tao An 1,2 Further characteristics of the data are described, as are the data products themselves and the tools for accessing them. All data included in the SDSS Early Data Release and First Data Release are reprocessed with the improved pipelines and included in the Second Data Release. The spectroscopic software now repairs a systematic error in the radial velocities of certain types of stars and has substantially improved spectrophotometry. The spectroscopy extends from 3800 to 9200 Å at a resolution of 2000. The result is that model magnitudes are now a good proxy for point-spread function magnitudes for point sources, and Petrosian magnitudes for extended sources. The imaging data have all been processed through a new version of the SDSS imaging pipeline, in which the most important improvement since the last data release is fixing an error in the model fits to each object. The imaging data reach a depth of r ≈ 22.2 (95% completeness limit for point sources) and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. This data release consists of 3324 deg2 of five-band (ugriz) imaging data with photometry for over 88 million unique objects, 367,360 spectra of galaxies, quasars, stars, and calibrating blank sky patches selected over 2627 deg2 of this area, and tables of measured parameters from these data. The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its Second Data Release. Towards the end of this work, we demonstrate one way to increase the tessellation resolution with artificial generating points, which would be useful for analysis of rare objects with small number counts. A validation of our method is demonstrated by the agreement with the $Gaia$ proper motions, to the precision better than $0.1$ mas yr$^20% more objects than the existing method in a mock catalogue of a white dwarf-only solar neighbourhood with Pan-STARRS 1-like characteristics. The exquisite depth and the nearly $12$ years of time baseline between HSC and SDSS enable high-precision measurements of statistical proper motions for stars down to $i\simeq 24$. In doing this we use $3$ million galaxies to recalibrate the astrometry and set up a common reference frame between the two catalogues. We present proper motion measurements for more than $0.55$ million main-sequence stars, by comparing astrometric positions of matched stars between the multi-band imaging datasets from the Hyper Suprime-Cam (HSC) Survey and the SDSS Stripe 82.
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