Beijing-Arizona Sky Survey (BASS)

The First Data Release (DR1)

# Description

The BASS DR1 contains the data taken during the first two semesters of 2015A and 2016A. About 41% of the whole survey has been completed (up to July of 2016). For more details about the survey, go to this page.

# Data Access

Search forms at CAsDC

Information files

• Information file for single-epoch images: bass-dr1-ccdinfo.fits | (Column description)
• Information file for co-added catalogs: bass-dr1-blocks-prior.fits | (Column description)
• The BASS footprint is covered by three passes (dithered by 1/4 camera FoV). There might be multiple magnitude measurements for the same object.
• The whole sky is divided into homogeneously distributed blocks. Each block has the size of 0.681°x0.681° and overlaps of 0.02° in both right ascension and declination (so objects are not unique). There is a one-by-one correspondence between the co-added catalog and the block. The catalog name is defined as XXX.XXXsYY.YYY_IIIIII: where s is the sign of the declination, XXX.XXX and sYY.YYY give the block center in J2000, and IIIIII is 6-digit block ID. The co-added catalog for each block is generated by cross-matching the catalogs of single-epoch images related to this block.

# DR1 coverage

The following figure and table give the observation progresses and median observation parameters for all passes and filters.

• Median observational parameters for different passes and filters
Filter g r
PASS Pass 1 Pass 2 Pass 3 All Pass 1 Pass 2 Pass 3 All
Completed 62% 58% 21% 47% 47% 46% 12% 35%
seeing (arcsec) 1.55 1.83 1.90 1.71 1.30 1.61 1.52 1.45
sky (mag/arcsec²) 22.05 22.06 21.97 22.04 21.39 21.15 21.02 21.24
airmass 1.05 1.04 1.02 1.04 1.03 1.03 1.01 1.03
zeropoint (AB mag for 1e/s) 25.90 25.91 25.93 25.91 25.82 25.79 25.72 25.80
depth (5σ) 23.38 23.33 23.31 23.35 22.97 22.89 22.80 22.92

# Imaging pipelines

The imaging pipelines benefited from the SCUSS pipeline include the detrending, astrometric calibration and photometric calibration. The image processing have some updates relative to the SCUSS data reduction. The astrometry is tied to SDSS/2MASS. The general position error is about 0.15″. The Pan-STARR 1 point-source catalogs are used for deriving the photometric solutions. The median zeropoint The median zeropoint RMS for both g and r bands are about 0.03 mag more>>.

# Single-epoch catalogs

SExtractor is used for source detections and aperture magnitude measurements in single-exposure images. This table presents some key parameters as the SExtractor configuration more>>.

 aperture radii (arcsec) aperture radii (pixel) 1.36 1.82 2.27 2.72 3.63 4.54 5.9 7.92 9.08 11.35 13.62 18.26 3 4 5 6 8 10 13 17.44 20 25 30 40

A new code for the PSF fitting is specially developed, which is based on the source positions from SExtractor and PSF models from PSFEx more>>

Both astrometric and photometric residuals relative to the Pan-STARR1 are corrected in the catalogs more>>.

We divide the whole sky into homogeneously distributed blocks. Each block has the size of 0.68°x0.68° and overlaps of 0.02° in both right ascension and declination. The single-epoch catalogs overlapped with the blocks are are cross-matched to form co-added catalogs. Note that there are a lack of sources close to these limits, mainly because we detect objects in the single-epoch images and generate the co-added catalog by cross-matching. In the future, we will detect objects in stacked images and measure the magnitudes in single-epoch images with the prior information of positions. more>>.

# Depth

The magnitude limits for single-epoch images are estimated by the PSF magnitude error at about 0.21 mag, which is corresponding to the S/N of 5. The median single-epoch depths are calculated as below:

• g = 23.35
• r = 22.92

All depths are corrected with the Galactic extinction of SFD98 by using the following extinction coefficients:

• $A_g$ = 3.303
• $A_r$ = 2.285

It should be noted that it is hard to separate point sources from extended ones with faint magnitudes, so the depths might be underestimated for point sources but overestimated for extended ones. The BASS footprint is covered by three passes, so each object could be observed at least three times. The standard deviation of magnitudes for the same object gives an actual measurement of the photometric error. The depths estimated by the standard deviation give similar numbers as shown above.

The full depths can be estimated from the co-added catalogs in the area covered by three passes with typical observational conditions. The co-added PSF magnitude error is used for the calculation. The full depths at S/N=5 are presented below:

• g = 24.06
• r = 23.46

Note that the depth estimation are also dependent on which photometric system the data related to. Our depths are tied to the PS1 system, which has systematic magnitude offsets of about 0.14 and 0.03 mag for g and r bands relative to the SDSS.

# Color term

We also obtain the transformation equations to transform the PS1 magnitudes to the BASS photometric system. First, the zeropoints are estimated without any systematic transformation. After being applied these zeropoints, the magnitude difference as function of the $g - i$ color is derived. We use a 3-order polynomial fit to determine color terms in the color range of $0.4 < (g_{\rm PS1}-i_{\rm PS1}) < 2.7$, which are presented in the following equations: \begin{eqnarray}

  (g-i) = g_{\rm PS1}-i_{\rm PS1}, \\
g_{\rm BASS} = g_{\rm PS1} - 0.08826 + 0.10575(g-i) - 0.02543(g-i)^2 + 0.00226(g-i)^3,  \\
r_{\rm BASS} = r_{\rm PS1} + 0.07371 - 0.07650(g-i) + 0.02809(g-i)^2 - 0.00967(g-i)^3.

\end{eqnarray}

## Discussion

, 2017/01/13 22:19

Are sources unique in co-added catalogs?

, 2017/01/13 22:24

No, the co-added catalogs are overlapped by 0.02° in both R.A. and declination (in a block with size of 0.681°x0.681°). See dr1_coaddcat

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