Schematic of the source
The Source simulates all of the important
features of the real sky for this experiment. The Source provides
a simulated star field that represents:
- the same flux as will be expected in the
Kepler Mission (after accounting for the aperture of the
optics) of 109 counts/sec for mv=12 star
- the same spectral color as solar-like stars
- the same range in brightnesses (9<mv<14) for target stars
- the same density of stars for mv<19
- a field covering a large fraction of the
CCD
- several bright stars (mv=4) as is to be expected in the Kepler Mission
Finally, the source also has:
- the ability to generate Earth-size transits
in selected stars.
Lamp
The light source is made up of two Labsphere
integrating spheres with a quartz-tungsten-halogen (QTH) lamp
in the 15-cm sphere, connected to the 50-cm integrating sphere
with an iris, diffuser and spectral filters in between. The spectral
filters BG34, KG4 and an OCLI "hot mirror" (with a
750nm cutoff) selectively attenuate the light, especially in
the red to provide a fairly close match to the solar spectrum
(see the figure below). There is a slight deficiency in the blue,
since the lamp is not nearly as hot as the Sun.
The reason for using two spheres is to thermally
isolate the heat of the lamp from the star plate. Separate cold
strapping and thermal electric coolers are attached to the small
sphere to remove the heat from the experiment.
A closed-loop control to 0.1% of the lamp
brightness is provided by the Oriel controller and power supply.
The amplitude of these fluctuations is similar to what is expected
from the Sun and similar stars. However, unlike the real sky
where the fluctuations of the stars are uncorrelated, in this
simulation, all the fluctuations are all the same due to the
common lamp. Note though that this brightness variation is ten
times greater than an Earth-size transit. The use of ensemble
relative photometry reduces the effect of the lamp variations
by about a factor of one hundred. Maintaining a constant lamp
current is also important for maintaining the same color of the
light source.
Spectral Response Curves
Star Plate
The star plate has a large number of holes
of various sizes (used to perform time-variant relative photometry)
and they are placed in many locations across the field-of-view
to support the suite of tests described earlier. The plate is
made of 50-micron thick stainless steel and opaque (transparency
of less than one part in a million). The hole pattern was drilled
with a laser beam by Lenox
Laser, with some holes as small as 3 microns diameter (for
the mv=19 stars). There are
84 holes for the 9<mv<14
target stars in the uncrowded region of the plate. These are
used to isolate the effects of faint background stars, bright
stars, smearing, etc. Some of these have very nearby stars as
faint as mv=19 to demonstrate
that stars five magnitudes fainter than the target star are not
a problem even when spacecraft jitter is simulated. There is
a crowded portion of the plate with 1540 stars having the same
star field density to mv=19
as the actual Cygnus region to be viewed by the Kepler Mission.
This region was used to demonstrate the ability to perform the
high-precision relative photometry even in crowded fields.
The bright (mv=4)
stars were used to demonstrate the maintenance of the relative
photometric precision in the presence of an occasional bright
foreground star. These bright stars are produced using fiber
optic bundles driven by external bright LEDs.
Star plate
Wiring for the 42 transit stars is
in blue. The bright-star fiber optics are black.
An Image of Star-Plate Star Field
The image taken with the CCD shows
the set of 84 isolated stars used to test various parameters
as well as the vertical region of dense stars equivalent to the
star density in the galactic plane. The regions labeled "Bias
strip" and "Smear calculation strip" are used
to make corrections to the image.
Transit Generation
Earth-size transits cause a very small change
in brightness on the order of 8x10-5 for several hours
on individual stars. An innovative concept to simulate a transit
was developed to produce this small change. A fine fixed wire
was mounted across the star hole (see the figure below). To cause
the required small change, a current is passed through the wire.
The small resistivity of the wire causes it to heat to a few
degrees above ambient. Due to the heating and the small coefficient
of thermal expansion (CTE) of the wire, the wire temperature
increases by about 5°C and the wire expands by about 10 nm
to reduce the amount of light by the required small fraction.
Transit wires were mounted across 42 of the star holes, but only
about a dozen were used during any one test. The switching on
and off of the transits and monitoring were controlled with a
Macintosh computer running LabView software with National Instrument's
digital I/O boards.
Microscope photograph of a transit wire
A transit wire across an mv=9
star hole with an mv=14 nearby
background star in the upper left.
The Camera simulates all of the functions
performed by the photometer on the Kepler Mission spacecraft.
Schematic of the Camera
The camera consists of the optics, CCD, CCD
controller electronics, camera operation computer and software,
and a CCD cooling system shown schematically above and in cross
section below.
Cross sectional view of the Camera
The Optics
The optics are used to form an image
of the sky. The light from the star plate is first collimated
to make it parallel; similar to star light from a great distance.
A Cooke triplet is used to focus the light onto the CCD. All
of the optical surfaces, lenses and windows have been anti-reflection
coated. As in the Kepler Mission the f-number is small,
equal to 1.5. A central obscuration was added inside the triplet
to simulate the central obscuration of the Schmidt system for
the Kepler Mission. For the Testbed Facility a thermal
compensation sleeve was incorporated so that the image remains
at the same focus even if the optical components change temperature.
As in the space mission, the image was defocused to spread the
light over many pixels. The optimum defocused image has about
80% of the light in a five pixel diameter aperture.
The CCD
The key component in the demonstration was the CCD (charged coupled
device) and its operation. For the purposes of the test a commercially
available CCD was selected that meets the criteria for the Kepler
Mission. The device chosen is manufactured by Marconi (formerly
EEV) and is shown below.
The Marconi 42-80 CCD
The CCD has 2048x4096 pixels with the pixels
being 13.5 microns square. The overall size is 27 by 54 mm (about
1 x 2 inches). In actual operation the pixels are binned on the
CCD to 27 micron square. In effect it was used as a 1024x2048
device. The binning improves both the readout speed and the photometric
precision.
The most significant feature of the device
is that it is a "back-illuminated" device. Usually
CCDs are illuminated through the polysilicone wires on the front
of the CCD. The wires provide the electrical voltages to control
and readout the device. But these wires absorb light and modulate
the image, much like looking through a picket fence. To overcome
this problem several additional manufacturing steps are utilized.
A glass plate is bonded to the front; the device is turned over
and the backside is thinned; then it is doped and anti-reflection
coated. This provides a device with a much higher quantum efficiency
and much less photometrically sensitive to image motion.
The CCD can also be readout very rapidly.
Since there is not a shutter in the system, all images have some
amount of smearing effectively adding to the system noise. We
are currently reading out the CCD at 1 mega pixel per second
per amplifier.
Test results discussed on the following page
confirm that the end-to-end system performance meets the Kepler
Mission requirement.
How do CCDs (charge coupled devices) work?
CCDs are at the heart of each "HandyCam"
TV camera and special purpose CCDs are what we use for Kepler.
When light strikes a piece of silicon, it
produces electrons that are free to move about the silicon material.
These electrons form a charge or a current which is measured
to determine the amount of light that has fallen on to the silicon.
In a CCD, the silicon region is divided electrically
into small individual picture elements or pixels with about four
hundred elements per cm in each direction, like a very finely
divided sheet of graph paper. The free electrons are kept from
moving around by permanent channel stops (the vertical lines
in the figure) and externally applied voltages (the horizontal
lines in the figure). Each pixel can then be thought of as an
individual bucket or well that collects electrons.
As shown in the animation, first the CCD is exposed
to light from a telescope or camera lens. Overtime this produces
an image made up of electrons in the CCD.
To readout an image that has been captured
with the CCD requires shifting the information out of the pixels.
First, the columns of pixels are all shifted down one row. The
bottom row of pixels is shifted into a readout register. Each
pixel in the readout register is shifted out to an amplifier
and the number of electrons in each pixel is recorded. This produces
a series of 1's and 0's that represent the image. This is repeated
over and over until all the pixels have been read. The stream
of 1's and 0's is then digitally processed to reproduce the image
that is later displayed.
In the Kepler Mission the 1's and 0's
are recorded onboard the spacecraft and sent to the ground, where
the data are processed to look for changes in the brightness
of each star that may be caused by a planetary transit.
Controller
The CCD requires control and data acquisition hardware and software
to operate. The readout is performed by applying a sequence of
clock signals to the CCD. On the CCD chip itself are preamplifiers
to convert the charge read out from each pixel into a voltage.
The sequence of voltages is then feed into the CCD controller.
The CCD controller was built by Dr. Robert Leach's CCD group at San Diego
State University. The CCD controller is programmed to generate
the clock signals for the readout of the CCD. It takes the analog
voltages readout and digitizes them. These are then transferred
to a Sparc 5 computer where the data are accumulated and written
to disk. The software used to perform these operations is the
Lowell Observatory
Instrument System (LOIS) software (Taylor, et al., 2000).
The CCD is split down the middle (along the
long dimension) and the controller has two channels to control
and process each half of the CCD independently thereby doubling
the overall readout speed.
CCD Operations
To improve readout speed and mitigate pixel saturation, the Kepler
Mission uses relatively large pixels, on the order of 25
microns square. To achieve this with the 42-80 CCD which has
13.5 micron pixels, the CCD was binned 2x2 on the chip, that
is, as the CCD was clocked, the charge from each 2x2 bin group
was summed. Since only about half of the CCD was illuminated
by the star field, only one half of the CCD was read out each
time, providing an approximately square image. The binning and
use of only half the CCD and use of two readout channels resulted
in each channel handling one-half million pixels per 0.5 sec
read. The clock speed is 1 megapixel/sec, so one image takes
just one-half second to readout.
To prevent saturation of stars as bright as
mv=9, the CCD must be read
out every 3 seconds. However, to achieve the required shot noise
level to detect an Earth-size transit the signal must be integrated
for 6.5 hours. This is accomplished in two steps; the 3 second
CCD read outs are integrated for some length of time. (In the
lab this is selectable. We have chosen values of 3 or 15 minutes.
For the Kepler Mission this is 15 minutes.) In the ground
processing software these integrations are co-added for longer
times to match the transit duration being searched (2 to 16 hours).
To simulate the effects of cosmic ray hits
on the CCD on the overall system noise, this additional noise
factor was added to the data in software. Cosmic rays were optionally
added into the 3 sec readout data and then an independent cosmic
ray despiker algorithm was used to identify them and remove them
before the data from each 3-second integration are co-added.
CCD Cooler
To reduce the dark current in the CCD, it was cooled to well
be -40 °C. In previous lab setups we have used liquid nitrogen.
Although this is a fairly simple method, it has the disadvantage
of producing a varying mechanical load on the structure and thereby
causing small structural deflections. For the Testbed Facility
we chose a Cryotiger closed-cycle cooling system. By tying the
cold strap from the Cryotiger cold finger to a rigid insulating
isolator before connecting to the CCD we prevented any mechanical
vibration from the refrigerator from causing the CCD to vibrate.
The temperature of the CCD was controlled by a proportionally
controlled heater attached to the CCD. The CCD can be operated
below -60°C for many weeks at a time. The dewar has to be
pumped out about once per month to maintain these low temperatures.
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