Short Duration Exposures of Rapidly Variable White
Dwarf Stars via the Whole Earth Telescope
Ian Radtke (Minnesota State University, Mankato)
Dr. Steve Kawaler (Dept. of Physics and Astronomy, Iowa State University)
Dr. Reed Riddle (Dept. of Physics and Astronomy, Iowa State University)
Abstract
We describe a new CCD-based time-series photometer currently under
development for use with the Whole Earth Telescope and compare this
emerging system to the current three-channel PMT photometer.
of
I. Introduction
Texas
Astronomy
Department
In astronomy, as in all walks of science,
established a world-wide network of
improvements in technology are coupled
cooperating astronomical observatories
with improvements in theory. As time-
to
series photometry has matured over the
measurements of variable stars.
last two decades, advances in the
“Whole Earth Telescope”, hereafter
theories
stellar
referred to as WET, currently consists of
pulsations and stellar seismology have
23 observatories in 16 countries around
quickly
the world.
of
stellar
interiors,
outpaced
instrumentation.
advances
in
obtain
uninterrupted
time-series
This
This cooperative optical
This discrepancy is
observing network is distributed in
primarily due to the lack of hardware
longitude around the Earth and is
capable
of
coordinated from a single site so that it
“uncluttered” resolution that is fine
acts as one instrument. Nightly data sets
enough to provide direct observation of
from each telescope are sent to the WET
current theoretical predictions (Nather et
headquarters via email allowing for real-
al, 1990).
time data reduction, combination and
of
providing
a
level
analysis, maximizing the effective use of
In 1986, to fill this gap in hardware
the
entire
distributed
instrument
capability, scientists from the University
(http://wet.iitap.iastate.edu). By making
proper use of the WET, gaps in time-
brightness of white dwarfs can be most
series
be
readily thought of as “starquakes”.
minimized or completely eliminated,
Temperature fluctuations within the star
allowing for greatly improved sensitivity
result in the vertical movement of dense
and resolution.
material above its equilibrium point.
photometric
data
can
This high-density material is relatively
II. Science
hotter, therefore brighter, than the
Photometry is, literally, the measurement
surrounding low-density material which
of light. Viewed from an astronomical
causes variations in the amount of light
perspective, photometry generally refers
emitted across the surface of the star.
to
stellar
The small vertical motions of surface
Time-series
material also generate larger, horizontal
photometry extends these measurements
motions (e.g. waves across the surface of
to observe time dependent variations,
the star) (Kawaler & Dahlstrom, 2000).
such as the multi-periodic oscillations
To fully understand such rapid and
observed in rapidly variable white dwarf
complex
stars. Simply put, the goal of time-series
observational coverage is essential.
the
measurements
magnitudes and colors.
of
oscillations,
extended
photometry is to extract, from the light
curve of a variable star, information of
The
collection
and
study
astrophysical interest about its internal
information,
composition and behavior.
asteroseismology, has been the driving
now
of
known
this
as
force behind the WET since its inception.
Pulsations of variable white dwarf stars
With a distributed instrument such as
reveal information about their internal
WET, it is possible to minimize or
structure and composition, age, rate of
remove the gaps in the time-series
rotation, magnetic field, mass and
photometric data acquired.
distance. These stars often pulsate in a
WET run will produce roughly ten times
hundred or more modes simultaneously
the amount of data that a series of
and have periods on the order of two to
observations at a single site.
twenty
phenomenal
minutes
(Kleinman,
1996).
These periodic changes in the relative
gain
in
A typical
The
capability,
compared with that of a single site,
resulting in such a high data density has
sky brightness, it became possible to
virtually limitless possibilities. (Nather
observe
et al, 2000).
telescopes than were originally thought
dimmer
stars
on
smaller
useful. Such a run is controlled by a
laptop PC which records the data and
III. Equipment
astronomy
plots a real-time light curve of the target
observation is always the equipment
star, comparison star and sky. (Nather et
used.
al, 1990).
The
backbone
of
any
Improvements in equipment
almost always lead to improvements in
the quality of data collected which, in
The next evolutionary step in WET
turn, leads to improvements in analysis
hardware
and understanding as well as more
comparatively bulky PMT photometer to
accurate theories.
a much svelter CCD-based time-series
is
to
shift
from
the
photometer. A CCD photometer offers
Although the distributed nature of the
the ability to track multiple companion
WET allows for continuous coverage of
stars and has a higher sensitivity than its
a target object, it also adds the difficulty
PMT predecessor. Again, a laptop PC
of multiple, often overlapping data sets.
controls the instruments, records the data,
To
time-series
and plots near real-time light curves of
observations from multiple telescopes
the target star, up to four companion
into a single, comprehensive light curve,
stars, and sky brightness.
best
combine
the
uniformity of hardware, software and
observing procedure is a must. When
As this system is aimed at taking short
the WET was created, a two-channel
exposures (on the order of 10-15 seconds)
photomultiplier tube (PMT) photometer
of rapidly variable stars, keeping the
was adopted as the hardware standard.
camera exposed to the target is of utmost
As the WET has matured, the original
importance. To do this, the CCD must
two-channel PMT photometers were
read out its information as quickly as
retrofitted with a third channel allowing
possible in order to minimize exposure
for a continuous measurement of the sky
down-time. Unfortunately, this read out
brightness. By keeping better track of
latency is the bottleneck of the current
hardware.
The CCD system currently
IV. Software
under development runs on the Linux
As the hardware of an observing system
operating system with an Apogee AP7
is as the backbone of the body, the
camera. While the AP7 is an excellent
software driving such a system is its
camera, it is severely limited by its ISA
nervous system. The software must be
interface which throttles read out speed.
agile enough to juggle the tasks of data
While a faster PCI-based interface is
acquisition
available, Apogee has yet to write
simultaneously. Thanks to the excellent
microcode to drive an AP7 at these
work of Dave Mills from The Random
higher speeds. As it now stands, reading
Factory (http://www.randomfactory.com)
out an unbinned 512x512 pixel image,
a software development kit (SDK) for
roughly 1 MB of data, takes about ten
Apogee CCD cameras under Linux is
seconds.
available in the public domain.
For single image exposures,
and
rough
analysis
the ten second delay is a small hindrance
but for time-series photometry it is a
The Apogee SDK is coded in a mixture
serious problem. Thankfully, Apogee is
of TCL/TK for the user interface and
currently developing a new line of CCD
basic
cameras that incorporate an RJ-45
SAOImage DS9, also written in TCL/TK;
10BaseT Ethernet connection.
http://hea-www.harvard.edu/RD/ds9)
With
image
manipulation
(via
such a lofty read out speed, taking full,
while the “heavy lifting” of camera I/O
unbinned images will become a trivial
in done in C.
task. If the image size remains the same,
interface (i.e. the light curve and
reading out 1 MB of data on the new
comparison windows) are written in
camera will take just under one second.
TCL/TK while the image correction,
Such a dramatic improvement will
centroiding, and count extraction are
eliminate
written in C.
virtually
all
gaps
in
All additions to the
observation. Technical specifications on
the Apogee AP7 CCD camera can be
Using the routines provided in the SDK,
found
direct interaction with the AP7 is easily
http://www.ccd.com/apseries.html.
at
accomplished. In addition to low-level
I/O, the SDK provided a framework for
specific requirements of our time-series
Preliminary
Image is Taken
photometry, a variety of routines were
written to extend the capabilities of the
base package. While the generic control
window was little modified (as seen in
Fig. 2), entirely new structures for light
Subregions are
Selected
curve plotting and manipulation have
been coded. As seen in Figure 1, the
process
of
image
acquisition
and
analysis can be explained through a
Image
Correction
simple flowchart.
As with any observing run, a series of
bias frames, dark frames and flat field
frames must be taken to allow proper
Centroiding on
all Stars
image correction.
Through a simple
pull-down menu, these three libraries are
very simple to create. A bias library is
created simply by inputting the number
Count
Extraction
of frames desired. The dark library is
one step more complex, requiring not
only a number of frames but also an
exposure time no less than the exposure
time to be used in observing, optimally
Next Image is
Taken
identical.
Flat fielding is equally
complex, requiring both an exposure
time and the number of frames desired.
Once these libraries are created, a
Figure 1: Basic process of image
acquisition and rough analysis.
preliminary image is taken and saved as
a standard FITS file. From this image,
a rudimentary graphical user interface to
ease operation of the system. Due to the
the target star, up to four companion
stars and a sky region are prompted for.
robust telescopes. From these corrected
The software allows for either a Box
subregions, photon counts are extracted
subregion or an Annulus subregion to be
and plotted. The next exposure is then
selected. Using either subregion type,
taken and the above process repeats.
the aperture is adjusted on the displayed
image and sized by the observer to best
As the light curve is produced, direct
fit the target star. Any companion stars
manipulations of the resulting plot are
that the observer elects to use are forced
available to the observer. (Figure 3)
to use an identical aperture size to
Both the X- and Y-axes can be moved to
provide for direct comparison between
view a particular region of the data and
plotted light curves.
If the Box
both are independently scalable to allow
subregion has been chosen, a Sky
for a maximum of functionality. Other
subregion is prompted for. The Annulus
options include viewing the entire data
subregion will not prompt for a discrete
set simultaneously, a “live” view that
Sky subregion as it is already accounted
will adjust the plot as new data points
for through the outer annulus of each
are created and an option to display the
chosen star. After subregions have been
last corrected image with associated
selected, the frame is corrected by the
subregions in DS9.
bias, dark and flat libraries which will
minimize any intrinsic flaws in the CCD
hardware.
While not necessary after
only one exposure has been taken, each
chosen star undergoes a centroiding
process which adjusts apertures to
correct for unavoidable drift between the
camera and the sky. Regardless of the
quality of stellar tracking inherent to the
telescope used for observation, the
centroiding process is an easy step to
take
which
greatly
improves
the
observing capabilities of small, less
Although the software supports up to
five simultaneous stars, it is often
beneficial to directly compare only two
of the light curves.
The “Compare”
button from the main light curve window
prompts the observer to select which two
targets are to be viewed and new, larger
plot is created.
Both light curves are
combined in the new window with a
similar set of control and zooming
options are available.
In this “Light
Curve Comparison” window, alterations
to the X-axis will affect both plots while
instruments, the increased portability is
the Y-axes are both independently
still a benefit.
movable and scalable. Again the “All”
and
“Live”
buttons
are
available.
(Figure 4)
As the summer winds to a close, the last
two and a half months of development
have shown TCL/TK to be a less than
V. Summary and Future Plans
optimal development language for our
While the CCD system is still under
purposes.
development, a few distinct advantages
development is to rewrite the entire code
over the PMT photometer are apparent.
base in a more robust, lower level
Firstly, up to four companion stars can
language such as C, C++ or Java. This
be selected where the PMT can only
will allow for much faster image
accept one companion star in addition to
processing simply due to improved
a discrete sky brightness reading. These
memory
additional companion stars allow for
handling solutions that can be found
more precise tracking of the target star,
outside of TCL/TK. More functionality
especially when mounted on an Alt-
can be gained by fully multithreading the
Azimuth telescope which will cause the
code to allow image acquisition to
image to rotate as the observing run
continue
progresses. Secondly, not only is a light
encountered in analysis (e.g. a star
curve produced but all of the CCD
moving outside its aperture or off the
images are saved allowing for a much
field entirely). Once the error has been
more thorough post-processing. This is
corrected, the images taken since the
an advantage over the PMT system
error occurred can be processed and
where only the light curve is retained.
added
Lastly, the difference in size and weight
Multithreading will also decrease overall
between the PMT system and CCD
latency by allowing the acquisition
system is significant. While not an issue
thread to run unhindered by any analysis.
of major importance, as each telescope
All processing will occur independently
houses the majority of their own
of the image acquisition process.
Our first priority in further
management
regardless
to
the
and
of
light
memory
any
errors
curve(s).
While much has yet to be done to
finalize the CCD time-series photometer
as the new WET standard, large strides
were taken in the past ten weeks,
development continues and, with some
perseverance, the new hardware standard
will be available to the WET for its next
observing run.
References:
Kawaler S.D, Dahlstrom M., 2000,
American Scientist, 88
Kleinman S.J., Nather R.E., Phillips T.,
1996, PASP, 108, 356
Nather R.E. et al, 1990, ApJ, 361, 309