In Fall 2009 I executed successful Lick and Keck AO second-epoch
observations of Hyades main-sequence stars.
With the LOCI algorithm,
I am conducting a survey for binary fraction
and mass function, down to the L/T-transition.
LOCI constructs a reference PSF by calculating least-squares-minimized
weights to sum over a catalog of images;
point-sources are found in the residuals after PSF subtraction.
Observing techniques employed to improve the PSF stability include
pointing the same pixel to each star and grouping stars with similar
magnitude whilst minimizing changes in elevation.
However, the latter may be less critical than presumed:
in examining the weights, I see PSF contributions distributed
throughout the night, with only slight preference for time adjacency.
Modifications I made to the algorithm include tuning the size of the
local optimization areas and dithering them during different iterations to mitigate edge effects.
The figure shows the 10^4 contrast I obtained at close separations.
In the Laboratory for Adaptive Optics (LAO), I conducted experiments
to see whether MEMS deformable mirrors could,
in a woofer/tweeter arrangement,
correct for real Kolmogorov turbulence without saturating in terms of stroke.
(That is, whether the tiny mirror actuators have enough "throw"
to correct the full scale of the turbulence in the atmosphere.)
The actuators on the device I tested have up to 1.5 microns stroke each,
when pulled down in tandem.
The results were published in
Optics Express.
I found that using a woofer with a MEMS tweeter
mitigates stroke saturation by an order of magnitude.
Furthermore, there were no instances of up-down-up-down neighboring actuator saturation,
implying that the highest spatial frequencies of Kolmogorov turbulence could be
well compensated by the MEMS,
therefore a woofer correcting low-to-mid spatial frequencies
is effective in mitigating stroke saturation.
The plot shows MEMS-actuator stroke saturation as a function of woofer actuator pitch,
for Kolmogorov atmospheres of 10cm and 15cm r_0 (Fried parameter).
The LAO experimental data are the points (with lines fit to them),
whereas the predictions (from fitting error) are the steep curves.
This work helped specify the pitch of the GPI woofer and the stroke of the GPI tweeter-MEMS.
This figure shows the stroke as a function of spatial frequency
for two different MEMS devices.
The limit of the influence functions are seen in the location
of the "knee."
This is also published in the
Optics Express paper.
A stellar occultation occurs when a nearby body (such as a solar system
planet or moon) passes in front of a distant Milky Way star.
The shadow formed by the planet or moon blocking the distant star
falls on the earth, and its size is equal to the width of the occulting body
(e.g. the planet or moon).
By observing the light curve (with high time-sampling), we can study
the diameter of the occulting body as well as its atmosphere.
In fact, this technique is how Professor Jim Elliot (now of MIT),
his student Dr. Ted Dunham (now of Lowell Observatory),
and other collaborators and competitors
discovered the rings of Uranus
from the flying telescope in the Kuiper Airborne Observatory in 1977.
I have assisted in observations of three stellar occultations:
Triton
with Jim Elliot in 2001 from the South African Astronomical Observatory at Sutherland,
Pluto
with Scott Severson in 2007 from Lick Observatory's 3m Shane telescope with adaptive optics,
and a KBO occultation
with Amanda Zangari and Bryant Grigsby in 2009 from Lick Observatory's 1m Crossley telescope.
This is the most exciting type of observing and I hope to become
more involved in these projects in the future!
I modeled Lyot-coronagraphic PSFs in IDL with Fourier optics (without aberration),
determining the occulting- and Lyot-stop size combinations that give the best
throughput and contrast at 8-12 λ/D.
These were then machined for the ExAO testbed.
I also tested machining and alignment tolerances,
finding that it is more important to focus on machining and aligning the occulting stop well,
as it is much less tolerant than the Lyot stop to both machining and alignment errors.
While at Lowell Observatory in the summer-fall of 2001,
I conducted preliminary site-testing for the
Discovery Channel Telescope
and worked on the
Planet Search Survey Telescope,
a transit-search telescope for exoplanets.
It was also while I was at Lowell that I travelled to South Africa for the Triton occultation.
As an undergrad, in the summers I worked in the Non-proliferation
and International Security division at Los Alamos National Laboratory.
We had five satellites in geosynchronous orbit
collecting data on the number density of ions in the magnetosphere.
The ions orginiate in the solar wind.
As the solar wind encounter's the Earth's magnetosphere in the bow shock,
charged particles are driven around to the night side, where they
are injected into the magnetosphere (positive ions going the opposite
direction from negative electrons).
For this project I looked for substorms of ions.
As we notice them tracking from one satellite to another,
we can map out their trajectory.
See our
paper
for more detail.