Transcript Demonstration of Adaptive Optics in a

Demonstration of Adaptive Optics in a
Laboratory Setting
Jianing Yang, Elizabeth McGrath
Department of Physics and Astronomy, Colby College, Waterville, ME
Introduction
Abstract
Construction
Turbulence in the Earth’s atmosphere bends and spreads the light
coming from the outer space, causing a blurring effect when celestial
objects are viewed through a ground-based telescope. This effect
creates a blob in the image and reduces the resolution, largely limiting
the performance of ground-based telescopes. On the other hand, the
resolution of space telescopes is limited by their size of apertures. For a
certain wavelength, the greater the aperture, the greater the angular
resolution. Here is a diagram of resolutions of Hubble telescope, JWST,
Hubble’s successor and a ground-based telescope without blurring.
We constructed a simple adaptive optics system to demonstrate how
a laser guide star coupled with a deformable mirror and wavefront
sensor can be used to correct for distortions caused by turbulence in
the Earth’s atmosphere. Adaptive optics (AO) systems are currently
implemented at a number of national astronomical observatories,
including the W. M. Keck Observatory, Gemini, and Subaru, and is a
key design component for the next generation of very large (30meter-class) ground-based telescopes. Adaptive optics is crucial for
improving spatial resolution of ground-based imaging in astronomy.
Using AO, we are able to achieve better image quality with the
largest ground-based telescopes than we can achieve with spacebased telescopes such as the Hubble Space Telescope and its
successor, the James Webb Space Telescope. The laboratory
setting of our device allows us to understand the principles required
to correct for wavefront distortions before implementing such a
system on a telescope. The system includes a mirror with a 6x6
array of actuators that can deform its shape up to 5.5 microns from
its reference position. A software package receives and processes
signals from the wavefront sensor and sends corrections to the
mirror at a rate of 15 Hz. We will finish alignment of the optics in the
coming weeks, and will begin testing by introducing artificial
distortions to examine overall system performance.
The process of construction mainly has two parts. The first one is
building the AO system. We first put the small parts of every component
together. Then we had the laser pen, optics, the deformable mirror, the
beam splitter, and the wavefront sensor mounted and at certain positions
relative to one another according to the instruction manual. The second
part is to align the system. We carefully measured the height of each
component and their distances. Then we turned on the laser for finer
alignment, and made sure the beam spot was centered at the lenses,
mirrors and finally the wavefront sensor.
Therefore, if we can use adaptive optics to correct for the interference of
the atmosphere, ground-based telescopes have huge potentials of better
performance than space ones.
The principles of adaptive optics (AO) are illustrated in the figures below.
Image credits: Lawrence Livermore
National Laboratory and NSF Center
for Adaptive Optics.
Third,
the
measurement of the
wavefront sensor is
sent to a computer to
calculate the shape to
apply to a deformable
mirror. This mirror
cancels
out
the
distortion due to the
turbulence. Last, light
from both the guide
star and the target is
reflected
off
the
deformable mirror to
the main camera. The
image of the target
celestial object is
therefore sharpened
by the AO system.
First, we shine a laser beam
to the sky, creating an
artificial star near the target
of observation. Second, light
from both the guide star and
the target object passes
through
the
telescope’s
optics. Light from the guide
star is sent to a wavefront
sensor, a special high-speed
camera that can measure
how the star’s light is
disturbed by the atmosphere
at a rate of 450Hz.
Below is a picture of the aligned AO system in the optics lab.
Laboratory Set-up
Image
credits:
ThorLabs
AOK1
Adaptive
Optics Kit
User
Guide
DM Calibration
After alignment, the next step is to use the software ThorLabs AO Kit to
take in information from the wavefront sensor and control the deformable
mirror (DM) to make adjustment.
Before it’s possible to put adaptive optics techniques into use on Colby’s
telescope, it’s necessary to test the system in a laboratory setting. The
schematics of the system is shown in the figure above. The source,
which is a laser beam, goes through the collimation optics, composed of
two mirrors and four lenses, reflected by the deformable mirror, goes
through the relay optics and goes to the Shack-Hartmann wavefront
sensor.
Acknowledgement
Image credits: Lawrence Livermore
National Laboratory and NSF Center
for Adaptive Optics.
Funding for this project is provided by the CARA Program. We also
acknowledge Claire Max (UC Santa Cruz) for providing background
information about the use of adaptive optics in astronomy.
The picture above shows a screen capture of the software during the
process of calibrating the DM. The left-hand side shows the shape of the
wavefront measured by the wavefront sensor, along with the peak-tovalley amplitude in microns. Ideally, we want the DM to correct the
wavefront to better than 0.6 microns, which is the wavelength of the
laser source. The upper-right window shows the deflection of individual
actuators of the DM. The lower-right window shows the higher order
correction terms for the wavefront which we will use when actively
correcting the wavefront.