Beijing-Arizona Sky Survey (BASS)

Table of Contents

Telescope

The Bok telescope is an equatorial mounting one with an aperture in diameter of 2.3 meters, which belongs to the Steward Observatory. The corrected primary focal ratio is f/2.66 and the telescope pointing error is about 3″. The site is Kitt Peak (111°36′01″.6W, +30°57′46″.5), 56 miles southwest away from the Tucson city. The elevation is about 2071 m and the typical seeing is about 1.5″ (Zou et al. 2015).

Bok telescope Bok dome and Kitt Peak

The Mayall Telescope is the largest optical telescope at Kitt Peak National Observatory. The 4-meter (158-inch) Nicholas U. Mayall Telescope, named after a former director of the observatory, was completed in 1970. The telescope is located at the highest peak of the Mountain, close to the Bok telescope. Its mirror diameter is about 4 m and clear aperture diameter is about 3.8 m.

Mayall telescope domeMayall telescope

Camera

The BASS uses the prime-focus imager, 90Prime, with corrected focal ratio of f/2.98. The optical design includes a four-element corrector and six position filter wheel. The focal plane array is a mosaic of four 4k x 4k CCDs which have been processed for back illumination by the University of Arizona Imaging Technology Laboratory. These CCDs were optimized for the ultraviolet response and built with the funding support from the South Galactic Cap u-band Sky Survey (SCUSS). The quantum efficiencies at both g and r are larger than 80% (see the following figure). The edge-to-edge field of view (FoV) is about 1.08°x1.03° and the pixel scale is 0.454″. There are gaps between CCDs along both right ascension and declination, which are 2.8′ and 0.9′ in width, respectively. The average gain is about 1.5 e/ADU and average readout noise is about 8 e. The CCD layout is shown in the following figure.

The MzLS uses the Mosaic-3 imager deployed at the prime focus. It is a new wide-field imaging camera installed in mid-2015, replacing the old Mosaic-2 one. There are four 4kx4k 500-micron thick deep-depletion CCDs, which significantly improved the z-band observing efficiency. The CCD pixel size is about 0.26″. The camera FoV is about 36′x36′. Each CCD is read out through four amplifiers simultaneously. The readout time in the normal mode is about 30 s. The average gain and readout noise are 1.8 e/ADU and 9 e, respectively. The CCD quantum efficiency is optimized for red wavelengths, which can reach 85% at 900 nm. The dark current is 0.95 e/hr. More information about Mosaic-3 can be referred to the camera manual.

Filters

The filters are required to be similar to those used in the DES, because the photometric data for the DESI target selection should come from consistent photometric systems. The BASS g band is the existing SDSS g filter deployed on the Bok telescope. The filter is close to the DES g band. The BASS r filter was newly purchased. It is almost the same as used in DES, but fairly different from the SDSS one. The cut-on and cut-off wavelengths are 5680 and 7110 Å. Figure 2 shows the response curves of both BASS g and r filters. The response curves include the filter transmission, CCD quantum efficiency (QE), and the atmospheric extinction at airmass of 1.0. The atmospheric extinction coefficients come from the Kitt Peak National Observatory and the molecular absorptions are not considered. The effective wavelength, and bandwidth for g band is about 4776 and 848 Å, respectively. Those for r band are about 6412 and 833 Å. The BASS r filter is over 200 Å redder and has a broader passband than the SDSS one.

In order to achieve homogeneous spectroscopic target selections for DESI, photometric systems of the imaging surveys should be as similar as possible. The DECaLS uses the DECam system, which is also used for the Dark Energy Survey. The BASS g band is the existing SDSS g filter. It is very close to the DECam g band. The BASS r filter is newly purchased by the Lawrence Berkeley National Laboratory, which is almost same as the DECam r band. The MzLS uses a newly purchased DECam z-band filter. The absolute transmission in passband is larger than 95%. The following figure shows the filter responses of these three filters. The response includes the CCD quantum efficiency, filter transmission, and atmospheric extinction at airmass of 1.0. The effective wavelengths for BASS g and r and MzLS z bands are 4776, 6412, and 9203 Å. For comparison, DECam filter responses are also plotted in this figure.

Filter parameters for different surveys:

BASS g BASS r MzLS z DECam g DECam r DECam z
Effective wavelength (Å) 4776 6412 9203 4842 6439 9172
Bandwidth (Å) 848 833 826 966 895 941
FWHM (Å) 1435 1410 1410 1290 1470 1470

The filter response curves are provided below:

The Galactic extinction coefficients are k_g=3.214, k_r=2.165, and k_z=1.211, which are come from the DECaLS webpage.

datarelease/telescope_and_instrument/home.txt · Last modified: 2018/07/19 15:04 by Zou Hu
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