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Spacecraft and orbit

SNAP could be launched on a Delta IV or a Soyuz SREGAT.

The spacecraft itself is rather compact for such a large telescope. The current design calls for a length of 6 meters and a width of 2.5 meters. Unlike the usual configuration of solar panels extending outward like "wings", SNAP's solar panels will lie along the body of the spacecraft. This simplifies the design and makes the panels more reliable.

SNAP's orbit will place it at a gravitational balance point between the Sun and the Earth, located about 1.5 million kilometers (1 million miles) from the Earth in the direction away from the Sun. This region, called the L2 Lagrange point (or colloquially as a "halo orbit") has many advantages. For one, very little fuel is needed to keep the spacecraft in position.the combined gravity of the Earth and Sun act to keep the spacecraft in place, much like a ball sitting at the bottom of a groove.

Also, at L2 the telescope can be "thermally stable". Since the Sun shines on it all the time, the temperature does not change much, putting less stress on the telescope and optics (as opposed to low-Earth orbit, where the spacecraft experiences 18 sunrises and sunsets every day). The relatively remote placement of the spacecraft also makes it easier to point, since the Earth, Sun, and Moon will always be in the same area of sky as seen by SNAP, making it easier to avoid them.

Optics

SNAP's main mirror will have a diameter of about 2 meters (about 6 feet). While small compared to some ground-based telescopes (which have mirrors as large as 10 meters across) this is still a powerful mirror for a space-based telescope. The primary mirror will collect the light and reflect it to a secondary mirror, which in turn reflects that light through a hole in the primary mirror. Two additional smaller mirrors then direct the light to the "focal plane platform", where the filters and instruments are mounted. This type of optical setup is called a Cassegrain, and is familiar to amateur astronomers: it is a tried-and-true way of packing a lot of telescope into a relatively small volume. While the focal length (the distance from the mirror to where the light is focused) is 22 meters, the actual physical length of the optical package is only about 3 meters.

Imaging Camera

Most space telescopes have many cameras on board which perform different functions. SNAP, however, has a very different design. Instead of filters mounted on a wheel and a complicated series of mirrors to direct the light into the detectors, SNAP mounts the detectors on a metal plate, and the filters are mounted directly onto the detectors! In a sense, SNAP will have 72 different cameras on board: 36 which can detect visible light (the kind our eyes can see) and 36 that "see" in the infrared.

The detectors are CCDs, similar to what are used in retail digital cameras. However, unlike their earthbound cousins, the CCDs on SNAP are far more sensitive to light, and produce much higher quality images. Each visible light CCD is 3512 x 3512 pixels (12.3 megapixels), and each infrared CCD is 2048 x 2048 pixels (4.2 megapixels). This means SNAP will be like a 510 megapixel camera!

SNAP's field of view is truly huge: each image it takes will be about a square degree, or four times the size of the full Moon. Hubble, for comparison, has a maximum field of view of only 1/400th that size. This will allow SNAP to look at a large part of the sky at once. Since SNAP is designed as a survey telescope, the wide field of view greatly enhances its abilities versus other observatories.

The visible and infrared CCDs together will detect light from about 350 nanometers (roughly blue) to 1700 nanometers (well into the infrared). There are 6 different filters used for the visible light CCDs and three for the infrared. This means that SNAP will have color vision, allowing scientists to better study the type of light emitted by astronomical targets.

SNAP's resolution (the ability to distinguish between two closely-spaced objects) will be about 0.2 arcseconds in the visible and 0.3 arcseconds in the infrared. An arcsecond is 1/3600th of a degree (for comparison, a person with typical "good" eyesight can distinguish objects as small as about sixty arcseconds, meaning SNAP's vision will be very sharp, about the same resolution as the Hubble Space Telescope.

For more technical data on the filter and CCDs, please see the Technical Specifications.

Spectrograph

A spectrograph is a device that sorts incoming light according to its wavelength (in visible light, the wavelength corresponds to color). The resulting spectrum yields a wealth of information about an astronomical target, including its temperature, chemical composition, rotation, and in some cases even its distance. SNAP's spectrograph will have two detectors (like the imaging camera), one visible and one infrared. It will cover a wavelength range of 350 to 1700 nanometers, or from the visible blue to the infrared. It uses what is called an image slicer to divide the observed part of the sky into 60 strips, and produce spectra of each of those slices. After slicing, each incoming beam of light is split so that one beam is sent to the visible detector and one to the infrared detector. The main purpose of the spectrograph is to measure the spectra of supernovae and determine

1. what kind of supernova it is,
2. measure the features that change from supernova to supernova,
3. determine the redshift of host galaxy of the supernova, and therefore the amount by which the universe has expanded since the star exploded,
4. build a library of supernovae spectra that can be used as a reference, and
5. tie in the observations of "standard stars" (stars with known characteristics) to the observations of supernovae.

Technical Specifications

Technical documentation on the SNAP mission, the spacecraft, and the instruments can be found on the SNAP Lawrence Berkeley Lab website.