Beijing-Arizona Sky Survey (BASS)

The Second Data Release (DR2)

  • News: including MzLS z-band data (total grz bands)
  • News: new observations as of July 2017
  • News: detecting sources on stacked images and running co-added measurements on single-epoch images
  • News: Kron aperture photometry for extended sources
  • News: Sky viewer for both images and catalogs
  • This page is public, so users don't need to register(无需注册).

Description

The BASS DR2 contains the BASS data taken during the past three yeas as of July 2017 and MzLS data as of June 2017. For more details about the two surveys, go to data release page. The data products include single-epoch images, stacked images, single-epoch catalogs, and co-added catalogs. There are 297,367 reduced single-epoch CCD images. Those images with zero point calibrations have corresponding catalogs. The full sky is equally divided into 96,775 blocks. These blocks are evenly spaced in declination. Each block has an area of 0.681×0.681 deg², equal to an image size of 5400×5400 with a pixel scale of 0.454“. There are overlaps of 0.02° in both right ascension and declination between adjacent blocks. There are 13419 blocks in total having stacked images. For each block, there are three stacked images for g, r, and z bands and one co-added catalog.

Data Access

Coverage and qualities for single-epoch images

The right figure shows the observed tiles for different passes and filters. DR2 footprint. The folloing table presents the median observational and imaging parameters for single-epoch images. The area gives the percetange of tiles that have been observed. The depth is the 5σ PSF limit estimated from single-epoch catalogs.

Area Seeing Sky Airmass Zeropoint Depth
g 75% 1.60 22.09 1.05 25.92 23.33
r 70% 1.42 21.27 1.05 25.81 23.06
z 76% 1.04 18.96 1.05 26.46 22.59

Data Reduction

The BASS and MzLS raw data are processed using the same pipeline. The basic reduction is almost same as the BASS DR1. We make some updates on imaging processing. Gaia DR1 is used for astrometry. The astrometric accuracy is much better than DR1. The general position error is about 0.03”. We use PS1 for external zeropoints, at the same time the internal zeropoint is calculated using the common objects of mutiple exposure more>>.

Detection and Photometry

Source detection is implemented in stacked images and photometry is made in single-epoch images. Circular aperture, Isophotal, Kron elliptical aperture, and PSF magnitudes are measured by a newly developed code. Coadded catalogs are generated by co-adding the measurements in single-epoch images. more>>.

Catalog Depth

The depth requirements for DESI are 24.0, 23.4, and 23.0 mag for g, r, and z bands, respectively, which are defined by 5σ extended sources with correction of Galactic extinction. The Galactic reddening map comes from Schlegel et al. (1998). We estimate the 5σ depths by both χ² error and standard error of co-added PSF magnitudes. The median depths are 24.05, 23.61, and 23.10 for g, r, and z bands, respectively. The depth for g band should be at least 0.1 mag deeper if we exclude 2015 data, which were shallower and problematic due to the bad A/C converter. We will reobserve all the data taken in 2015. The following table lists the median depths and fractions of objects for different exposure numbers.

5σ depth Fraction
Exposures g r z g r z
1 23.31 22.99 22.63 5.7% 9.6% 13.7%
2 23.74 23.44 22.98 11.9% 18.6% 28.5%
3 23.91 23.64 23.18 18.1% 43.2% 38.6%
>=4 24.14 23.80 23.38 64.3% 28.6% 19.2%
median 24.05 23.61 23.10

Color term

The PS1 magnitudes are transformed to the BASS photometric system using the following equations:

(g-i) = g_PS1-i_PS1
g_BASS = g_PS1 - 0.08826 + 0.10575(g-i) - 0.02543(g-i)² + 0.00226(g-i)³, 
r_BASS = r_PS1 + 0.07371 - 0.07650(g-i) + 0.02809(g-i)² - 0.00967(g-i)³, 
z_MzLS = z_PS1 + 0.10164 - 0.08774(g-i) + 0.03041(g-i)² - 0.00947(g-i)³.

These color terms are valid for stars with 0.4<(g-i)<2.7. The constant terms are kept to ensure no systematic offsets between two surveys.

User guidelines

  • For single-epoch images and catalogs, go here
  • For stacked images, go here
  • For co-added catalogs, go here

Known issues

  • The seeing statistics have some problems in DR2. Even though we have fixed it in the summary file of the ccd images, but it is not updated in the co-added catalog. So please don't use related columns in the co-added catalogs: Seeing, Seeing_Mean, Seeing_Min, Seeing_Max, Depth_Point, Depth_Extend.
  • Quite a few pixels in large extended sources are identified as cosmic rays
  • Parameters in the FITS header of single-epoch images, such as seeing and zeropoint, are not final one. One can find them in the summary file.

Discussion

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