The WISE Survey

The Wide-Field Infrared Explorer Survey (WISE) is an all sky mid-infrared survey launched in December 2009. The mission is funded by NASA and managed by Jet Propulsion Laboratory (JPL). The initial mission lasted for approximately 10 months until October 2010, when it ran out of hydrogen coolant. After that a secondary mission continued for another 4 months. The satellite was put into safe mode following its completion. During the initial mission it performed an all-sky survey in the four infrared bands 3.4 μm (W1), 4.6 μm (W2), 12 μm (W3), and 22 μm (W4). Prior to WISE few other comparable missions were launched. The IRAS (Infrared Astronomical Satellite) was launched in 1983 and it scanned the sky in four infrared bands, 12 μm, 25 μm, 60 μm, and 100 μm [11]. Although WISE and IRAS had similar bands in the mid-infrared region, WISE could achieve sensitivity, which is 1000 times greater than IRAS [14]. Another infrared mission AKARI, launched in 2006 performed scan of the sky from mid to far infrared bands at 9 μm, 18 μm, 65 μm, 90 μm, 140 μm, and 160 μm. At the mid-infrared bands of 9 μm and 18 μm AKARI had sensitivities of 50 and 100 mJy respectively [15]. At the WISE bands of 3.4 μm , 4.6 μm, the only other all-sky survey that has been carried out was the Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite but at these bands WISE had 500,000 times better sensitivity than COBE

Optical Instruments

The WISE has an afocal telescope of 40 cm diameter. It has a four-channel imager operating in a single mode, snapping overlapping photographs of the sky. The imaging sensors have 1024 X 1024 pixels array, having a resolution of 2.75 arcsec/pixel. It provides an instantaneous field of view of 47 arcmin2. The first and the last four pixels in each column and row are used as non-illuminated reference pixels, so the effective size of the arrays is 10162 pixels. For the short wavelength of 3.4 μm and 4.6 μm, WISE employs Mercury Cadmium Telluride (HgCdTe) focal plane arrays (FPA), with cutoff at 4.2 μm and 5.4 μm and for the longer wavelength of 12 μm and 22 μm it uses Silicon Arsenide (Si:As) FPAs with 17.5 μm and 27 μm cutoff. Besides the telescope, WISE has a scan mirror mechanism, which is used to stabilize the line of sight while the spacecraft scans the sky, and two 2-band reimaging cameras, all of which are housed inside a solid-hydrogen cooled, two-stage cryostat. [14].
Each WISE frame has about 47arcmin2 field of view and there is about 10% overlap between 2 frames. The time taken to capture is frame is 11 seconds. It takes 1.1 second to read-reset the arrays, followed by read cycles totaling 8.8 seconds and then a final reset cycle of the arrays, during which the scan mirror flies back.

Source Detection

Below is a list of what each of the WISE band is sensitive to

W1 – 3.4 μm – detect galaxies and stars
W2 – 4.6 μm – detect thermal radiation from the internal heat sources of sub- stellar objects like brown dwarfs
W3 – 12 μm – detect thermal radiation from asteroids
W4 – 22 μm – sensitivity to dust in star-forming regions (material with temperatures of 70–100 kelvins) [15]

About SDSS Mission

The Sloan Digital Sky Survey (SDSS), one of the largest redshift survey ever conducted, is a multi-filter spectroscopic and imaging survey located at the Apache Point Observatory in New Mexico. It uses a 2.5 m telescope, a mosaic CCD camera, two fiber-fed double spectrographs, and an auxiliary 0.5m telescope for photometric calibration. It capture images in five broad bands ‘u’, ‘g’, ‘r’, ‘I’, and ‘z’, centered at 3551 Å, 4686 Å, 6166 Å, 7480 Å, and 8932 Å , respectively The imaging data are processed by the astro-metric and photometric pipeline and are photometrically calibrated to a standard star network[16].

SDSS began gathering data in 2000 and its final imaging data release covered just over 35% of the entire sky. It gathered photometric observation data of around 500 million objects and spectral data of more than 1 million objects. It has detected quasars with redshift of as much as z = 5 and the imaging survey is currently involved in detecting quasars with redshift beyond z = 6. The latest data release as of January 2011 is the DR8 catalogue, which includes photometric observations covering 14,555 square degrees of the sky[17].
For the purpose of this project we will be dealing with the quasar catalogue from the seventh data release (hereafter known as DR7) of the Sloan Digital Sky Survey. The DR7 catalogue includes 11,663 deg2 of imaging data, adding about 2000 deg2 of data to the previous data release, lying in regions of low Galactic latitude. The catalog contains photometric data for about 357 million distinct objects. It has completed spectroscopy of more than 9380 deg2, covering large area of the Northern Galactic Cap, adding considerably to the previous data release. There are spectral data from over 1.6 million sources, including 460,000 stars, 930,000 galaxies and 120,000 quasars.

About ROSAT Mission

ROSAT, which is short for Röntgen-satellit, was launched by German Aerospace Center on 1 June 1990 and was jointly developed by Germany, the UK and the US. Initially designed for 18 months of operation, it lasted for 8 years, ending its mission on 12th February 1999. It carried an X-ray telescope and a wide field camera. The wide field camera is an autonomous telescope with micro-channel plate detectors and is sensitive in the range 20eV – 0.2KeV. The X-ray telescope had a geometrical area of 1141cm2 and a field of view of diameter 2 deg2. ROSAT also had two multiwire position sensitive proportional counter (PSPC) and a high-resolution imager. Throughout its mission time it performed all-sky X-ray survey and formed a catalogue of about 150,000 sources. The satellite has also been used for the study of individual
sources and for performing deep surveys over small areas.