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 eliminate sources of stray light entering the telescope,
since the Earth, Sun, and Moon will always be in
the same area of sky as seen by SNAP, making it
easier to baffle the telescope.
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 three-mirror anastigmat (TMA), and is familiar
to 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.
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 near-infrared (NIR) sensors and visible light charged-coupled devices (CCDs). The CCDs are 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 infrared detector is 2048 x 2048 pixels (4.2 megapixels). Each visible light CCD is 3512 x 3512 pixels (12.3 megapixels). This means
SNAP will be like a 600 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
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
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
- what kind of supernova it is,
- measure the features that change from supernova
- determine the redshift of host galaxy of the supernova, and therefore the amount by which the universe has expanded since the star exploded,
- build a library of supernovae spectra that can be
used as a reference, and
- tie in the observations of
"standard stars" (stars with known characteristics)
to the observations of supernovae.
Technical documentation on the SNAP mission, the spacecraft, and
the instruments can be found
SNAP Lawrence Berkeley Lab website