NASA’s Wide-field Infrared Survey Explorer (WISE)

Today Mark Kahan, ORA’s Chief Electro-Optical Systems Engineer, has contributed a short Guest Blog on NASA’s Wide-field Infrared Survey Explorer (WISE).   

Mark is a Member of the WISE Standing Review Board, and, among other areas, he was involved in a number of different aspects of the Hardware, from Design through Test, and on to Cover Deployment and On-Station Performance. Many of the WISE Lessons-Learned J, over various Engineering & Managerial aspects, will be covered at the SPIE’s Annual Meeting in San Diego on 1 August 2010 as part of a broader Session entitled “An Optical Believe It or Not: Key Lessons Learned”.

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The WISE mission was competitively selected under NASA's Explorers Program which is managed by the Goddard Space Flight Center (GSFC), Greenbelt, Md. The Science Instrument was built by the Space Dynamics Laboratory, Logan, Utah, using a Cryogenic Telescope, Aft-Optics, and Scan Mirror (shown to the right) built by SSG-Tinsley/L-3 Communications, Wilmington, MA, and the Spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena, under the watchful eye of Ned Wright, the WISE Mission Principal Investigator (PI). Caltech manages JPL for NASA, and JPL was the Lead NASA Center relative to the responsibility of insuring the Hardware was built-to, and is performing to, Specifications (see http://www.jpl.nasa.gov/wise/)

WISE launched Dec. 14, 2009, from Vandenberg Air Force Base in California, and it will spend nine months scanning the sky in infrared light, revealing all sorts of cosmic characters—everything from near-Earth asteroids to young galaxies more than ten billion light-years away. Before the Mission ends it should uncover hundreds of thousands of asteroids, and hundreds of millions of stars and galaxies. Its vast catalog of data will provide astronomers and other missions with data to mine for decades.

Because so much has been written about WISE, this Blog will concentrate on providing a bit of added insight into the WISE First Light Image, and in doing so we can see why WISE has astronomers so excited.

Before WISE launched, this is what we could see at 3.5 microns in one area of the sky about three times the size of the full moon:

We could see a bit more at 12.5 microns:

 
Then we opened up the WISE Cover, and this is what we saw in the same area of the sky:

 
The 1st light WISE picture shown above was taken looking at the constellation Carina which is near the Milky Way (about 5 degrees out of the plane of our galaxy called the ecliptic). We were Nadir pointing  (i.e. we were pointing away from the earth, which is important so as to keep WISE cold), and we were in the Southern Hemisphere, probably over South America. We were staring at one region of the sky by controlling the S/C reaction wheels. Though this first image was captured as the spacecraft stared in a fixed direction in order to help calibrate the WISE pointing system, the mission's survey is being done while the satellite continuously scans the sky, and an internal scan mirror counteracts the S/C motion to create 8.8 second freeze-frame images.

The "first-light" picture shows thousands of stars and, as was noted above, it covers an area ~ three times the area of the full Moon (WISE’s FOV is about ¾ of a degree by ¾ of a degree – actually 47 arc-sec, and the Moon is ~ ½ Degree). Eventually, if nothing goes wrong, WISE will take more than a million similar pictures covering the whole sky.

The first light picture shows infrared light from three of WISE's four wavelength bands: blue, green and red correspond to 3.4, 4.6, and 12 microns, respectively. There’s a 25+ micron band too, but it wasn’t included for convenience and to speed up the processing to get the 1st image out-the-door (3 color printing is common; sometimes folks add a couple of bands into one of the 3 colors to add further information, but that wasn’t done in the first-light image).

The star labeled V482 is a bright M2 Giant [RA (2000.0): 093023.66, Dec. (2000.0): -582143.2] located about 1,000 (1,058 +230/-160) light years away from Earth, and is about 6th Magnitude (5.9, actually) in the visible wavelength region. Also, since it is a red giant star, it is quite a bit brighter in the infrared, almost first Magnitude by the time one gets to 2 microns (it is ~ 0.5 Magnitude in Bands 3 & 4, around 12 microns and 25 microns, respectively). A hot star like Vega will have the same optical and IR Magnitudes, while a cooler star will have brighter (smaller number) Magnitudes in the IR than the visible.

One question that comes up a lot on the WISE 1st Light Image (because it sort of sets a sensitivity threshold for comparison) is “Would the bright star be visible with the naked eye?” The answer is “Yes”. V482 Carinae is barely visible to the naked eye from a dark site. Bigger Magnitudes are fainter, and we usually say the naked eye limit is 6, though some people can see 6.5 and a few can even see 7 (sadly I can see less every year!). Each Magnitude is about a factor of 2.5 in brightness, and 5 magnitudes is exactly a factor of 100, so a Magnitude 1 or first Magnitude star (which is about what stars like the Belt of Orion or the Big Dipper are) are 100 times brighter than 6th Magnitude stars.

With 5-sigma surety, for hot stars with no IR “excesses” (a long discussion in and of itself), WISE can see down to < 16th Magnitude in Band 1, < 15th Magnitude in Band 2, < 11th Magnitude in Band 3, and under 8th Magnitude in Band 4.

Because this V482 star is at declination -58 degrees, you need to be at latitude -58 degrees for it to come overhead, and you can't see it all from the ground if you are north of 32 degrees latitude on Earth (which is the case for both Pasadena at 34.15 degrees, and Boston at 42.32 degrees).

Finally, WISE Pixels are 2.75 arc-seconds in size at the shorter 3 wavelength bands, but we agglomerate 4 pixels (so we’re 5.5 arc-sec by 5.5 arc-sec) at 25+ microns. The FWHM in the shorter bands is ~ 6 arc-sec and it varies over wavelength (in Noise Pixel/NP Terms we run from 12 to 40 NPs over wavelength band, at least when we’re staring; things go up a smidge when we’re scanning).

It’s great to see thousands of the tiniest of details come together in such a great way. Go WISE! J

 -Mark Kahan

Comments

[Ruby at Science Camp]

My kid has always been fascinated with astrology. He's too young to understand things about NASA but I'm glad to find this so I could somehow explain to him a bit even though I do not completely understand it all.