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Norfolk
State University Astronomical Observatory
Summary
Norfolk State University (NSU) Astronomical
Observatory is housing a 24” Ritchey-Chrétien telescope. It is named the Rapid
Response Robotic Telescope (RRRT), as it was designed for rapid slew and
to be operated robotically. The goal of the project is to enhance NSU
participation in astronomical research, education and public outreach.
This project intend to increase diversity in the astronomical
community and provide astronomy outreach for the Hampton Roads area and
the rest of Virginia. The RRRT will be an important complement to an
Astronomy education initiative at NSU directly to improve the offerings
in a recently created Minor in Astronomy. It will also enhance our
general public outreach trough collaborations with local public school
systems and local amateur groups.
The telescope is a Optical Guidance Systems’
24” RC telescope with equatorial fork mounting and open truss. It is
equipped with a CCD camera for UVBRI imaging photometry and polarimetry.
It can be operated on-site, remotely or robotically; controllable trough
the web, allowing its operation from NSU’s campus and from the Hampton
Roads area.
For the operation of the RRRT we collaborate with the Department of
Astronomy of the University of Virginia, Charlottesville VA, and the Back
Bay Amateur Astronomer, Virginia Beach, VA. The RRRT is located at the
Fan Mountain Observatory, near the Blue-ridge Mountains, Covesville, VA. NSU’s RRRT research
activities are centered on the early photometry and polarimetry of
Gamma-ray Optical Afterglows (GRB-OA) and photometry and polarimetry of Blazars.
This telescope is part of the Swift
ground follow-up team, tracking data produced by NASA’s Swift and
GLAST missions. Intermediate-sized scopes with
low inertia (fast slew), as the RRRT,
play a crucial role in GRB observing programs. Moreover,
polarimetry capability on the ground is important to study the physics of
the electromagnetic nature of the GRBs. The study of GRB-OA provides
a natural evolution for NSU current high-energy astrophysics program,
introducing more traditional observational astronomy that NSU would like
to develop on the curriculum.
The RRRT can also be used for other
research and educational projects. Undergraduate students, from several science
disciplines at NSU, can participate in research and students of
introductory courses are able to enhance their education thought direct
astronomical observations. Another important educational component targets local high school science
teachers and students. The Virginia Beach Public School System (VBPS),
the largest in Virginia, has recently introduced an astronomy course in
its curriculum (at the senior year). We plan a close collaboration with
the VBPS to allow those students to use the RRRT. Moreover, BBAA astronomers
are also interested in using the RRRT for Near Earth Objects (NEO),
Variable Stars studies and Supernova searches.
We are also encouraging the use of the RRRT
as a “bench-table” for the design and construction of hardware and
software in the forefront of observational astronomy. We plan to involve
a minimum of two NSU undergraduate students in the operation of the RRRT.
NSU is one of the leading HBCU in the country and its main campus is
located at about 180 miles from Fan Mountain. The RRRT will strengthen
the national participation of undergraduate minorities in astronomical
research. Throughout the use of this telescope we will make special
efforts to attract historically underrepresented groups and to popularize
the concepts of modern astronomy in the Hampton Roads area of Virginia.
 
A - Research Activities
The main research activity
proposed for the Rapid Response Robotic Telescope (RRRT) is the
photometric and polarimetric study of Gamma-ray Bursts Optical Afterglows
(GRB-OA). We intent to measure UBVR and near Infrared magnitudes and
polarization during the first few minutes after the gamma ray burst was
detected by the Swift or GLAST missions.
Gamma-ray bursts are currently
one of the most exciting topics in high-energy astrophysics, and perhaps
of all of astrophysics. Their fundamental importance to cosmology and
particle physics can be appreciated from the fact that GRBs are
temporarily up to 106 times more luminous in the optical than
their host galaxies. They are detectable to the limits of the observable
Universe. Cosmologists appreciate the potential of the luminous
GRBs to probe further back in space-time than even supernovae, to the
first eras of star and galaxy formation [1]. Once their physical
mechanisms are better understood, they may eventually surpass type Ia
supernovae as the “standard candles” for probing high redshifts, thus
helping to constrain the fundamental parameters in Cosmology [2], [3], [4].
The liberated energy in a GRB explosion and its afterglow phase can be
extreme, assuming narrow jet opening angles, about 1051-52
ergs in an interval of tens of seconds.
The
most popular class of models for the GRB phenomenon goes by the name
“hypernova” – in analogy with the well-studied supernova classes – or
“collapsar” [5], [6]. Such scenarios envision formation of a black
hole near the time of supernova collapse, with the GRB following within
seconds to a few days, the timescale depending on immediate or delayed
accretion of residual debris from stellar collapse. The burst
emission is envisioned to result from ultrarelativistic particles that
are accelerated in a narrowly collimated jet emerging from the spin axis
of the nascent black hole. We detect the cone of gamma-ray emission
only if the jet axis is closely aligned to our direction. Thus, in
GRBs we may be seeing one of the most exotic processes in the Universe –
the creation of a black hole.
GRBs
are detected across the whole electromagnetic spectrum, with global
alerts triggered by space-based gamma-ray and X-ray
instrumentation. In principle, transient alerts and positions, once
telemetered to the ground, are distributed within seconds to ground
observatories around the world operating in the optical, infrared, and
radio bands. The GRB Coordinates Network (GCN) operated at GSFC is the
center for GRB message distribution [7].
Space-based alerts provide
ground observatories with a relatively large GRB localization error
region – in the past, degree-sized; in the future with Swift,
arc-minute (after 10–20 seconds) to arc-second-sized (after 1
minute). The Swift’s Burst Alert Telescope (BAT) will be
able to quickly determine the GRB position to within 4 arcmin radius and
then distribute the information to the GCN within 12 seconds. The Swift’s
XRT image coordinates (accurate to within 5 arcsec) would arrive within
62 seconds later for distribution via GCN. The time scale is critical,
since some GRB afterglows are believed to decay below
detectability in tens of seconds [8]. Without detection of the very
accurate position of an optical/IR or radio afterglow, the host galaxy
and redshift will not be found.
![Figure 1: R magnitude light curve of the GRB030329 [9].](images/bandlightcurves.gif)
Figure 1: R magnitude light curve of the GRB030329 [9].
Smaller scopes making the
detection of a quickly decaying counterpart then pass the refined
position to larger telescopes, which may observe the host galaxy’s
redshift, even if the GRB itself has faded past the detection limit. Thus,
intermediate-sized scopes with low inertia (fast slew) will play a
crucial role in future GRB/OA observing programs. Moreover,
polarimetry capability on the ground is important to study the physics of
the electromagnetic nature of the GRBs. Phenomenally high
polarization (~ 80%) has recently been reported in the prompt gamma-ray
emission of one burst, and optical polarization of order 1–3% has been
reported for several bursts. [10] Polarization measurements as a function
of time throughout the afterglow are expected to help reveal the evolving
structure of the fireball [11].
Of the roughly 190 GRB
high-precision X-ray/gamma-ray positional alerts since 1997 (see J.
Greiner’s website, http://www.aip.de/~jcg/grbrsh.html), only ~ 40 have
led to discovery of optical afterglows and only about 24 have been
reported to have radio afterglows. These detection success rates
have varied according to the capabilities of each particular
spacecraft/instrument detection and alert system. The bottom line
is that only 32 redshifts have been associated with a GRB, either through
the afterglow itself, or via the host galaxy. Thus a large fraction of
GRB afterglows have gone undetected, with ~ 65% not seen at the longer
wavelengths, and more than 80% with no redshift.
That most GRB sources have gone
unidentified is due to a conspiracy of logistics, limits to present
instrumentation, and the large dynamic ranges of GRB afterglow
luminosities and decay time scales. The optical/IR afterglow
typically fades with a power-law dependence (see figure 1). There
is a distribution of power-law indices and we only know that part of the
distribution for which afterglows are detected. There could be a
substantial tail in the distribution on the steeper, i.e., faster decay
end. So just to detect the afterglow, rapid acquisition is required in
some cases.
Also, from spectroscopic and
image information, we understand that at least some GRB sources lie in
obscured parts of their hosts – star forming regions – where optical
extinction is high. Many GRBs could be hidden in very high
extinction regions, such that little optical emission penetrates to the
exterior. This argues for coverage into the infrared where
extinction due to dust is significantly less than in the visible.
Most infrared telescopes are fairly large, with high inertia; hence, very
rapid acquisitions in the optical with intermediate-size telescopes may
serve to alert larger infrared facilities before decays beyond detection
occur.
Two separate classes of GRBs
exist, long and short duration, with the transition near 2 seconds [12],
[13]. Afterglow detection statistics are available for “long”
bursts only (durations longer than ~ 2–3 s). “Short” bursts do not
trigger BeppoSAX, and only two short bursts have recently been detected
by the IPN and reported to the Global Coordinates Network (GRBs 000326
and 000408: [14]). No afterglow was found in either case. One
hypothesis, based on the tenuous statistics of these two non-detections,
is that afterglow decay timescales are related to burst durations – total
released energy – in which case very rapid acquisition on the ground will
be essential for obtaining the redshifts of short bursts, assuming they
also lie at cosmological distances?
The proposed RRRT will fulfill
a need for faster, more sensitive telescopes for acquiring GRB
afterglows. Implicit in the above discussion is that very large
telescopes do not move fast enough to acquire GRBs when they are at their
brightest, as we know from an example of one, GRB 990123 which reached 9th
optical magnitude during the burst [25]. The large inertia of large
telescopes and their observing programs are not geared towards
interruption to immediately observe these unexpected transients with
power-law decays. Since at gamma-ray energies some GRBs may last
for only seconds to a few minutes, a network of ground scopes distributed
in longitude/latitude is essential for efficient acquisition and
follow-up studies
The proposed RRRT will be
located at Fan Mountain, Virginia [15] (latitude = +37.9o and
longitude = -78.69o) and a height of about 800 ft. There are
very few RRRTs in the USA East coast; the proposed telescope will fill
this need for the GRB Coordinates Network. It will be also one of the few
in making efforts to provide GRB-OA polarimetry during the very first
minutes after the burst.
The proposed telescope will be
directly connected to the GCN, such that a GRB detected over the
telescope horizon will take priority over any other program, and the
telescope will be ready in a few seconds to start photometry and
polarimetry of the GRB-OA object. We expect to study afterglows up to
17-18 visible magnitudes.
The GRBs themselves are brief,
lasting from about 1 sec to about 100 sec. They are almost unbelievably
energetic bursts of short wavelength radiation. The initially bright
optical afterglows can last for days to months, decaying with power-law
dependence in time [16]. The Swift mission [17] will be
dedicated to detect GRBs, rapidly localize them in-orbit and relay this
information to ground-based observatories in intervals of seconds. Swift
is now scheduled to be launched by October 2004, and it is expected to be
fully operational by January 2005. Swift localizes GRBs in stages
(gamma rays to X‑ray to ultraviolet and optical) using a gamma-ray
telescope with a wide field of view, and narrow X‑ray and optical telescopes
with fine resolutions. The proposed timescales associated with Swift
are shown in table 1. Our telescope design has been done taking into
account these parameters. The ground based rapid response telescopes will
be most important during the first 150 sec (when the UVOT
telescope in SWIFT is not activated).
|
0 sec
|
GRB
|
|
10 sec
|
Slew begins
|
|
12 sec
|
BAT (Burst gamma telescope)
position relayed
|
|
~50 sec
|
GRB acquired
|
|
55 sec
|
X ray image
|
|
62 sec
|
XRT location, BAT light
curves
|
|
150 sec
|
Optical (UVOT)
|
|
200 sec
|
Optical finding chart
distributed
|
|
1200 sec
|
XRT spectrum
|
|
1210 sec
|
X spectrum distrubuted
|
|
7200 sec
|
UVOT complete
|
Table 1: Swift’s time
scales [17].
During
the initial few tens of seconds, the optical afterglow begins to fade
with power law dependence. The brightest optical emission occurs during
the GRB, not hours after. The range of optical magnitudes at 100 s delay
from GRB onset is of order 10th to 20th magnitude, accessible
to a 24” telescope. The prompt optical emission is millions of
times brighter than the light of the host galaxy. Larger
observatories will be needed to study the host galaxy that is usually a
very faint, > 23th magnitude, highly redshifted
galaxy. In conclusion, earliest times (<100 sec) are the best
to study GRBs directly.
As discussed before, invaluable
physical insight into the GRB mechanism will be obtained by studying this
prompt optical emission. Since a relatively coarse (4 arcmin radius)
gamma-ray position will be provided by Swift and distributed in
about 20 seconds to ground observers, these parameters determine the
needed slewing and positioning precision. Most GRBs will still be
in progress when the position is relayed to ground observatories. Thus,
the optical observer will need to make a fast determination of the
possible position of the GRB source in this coarse positioning.
An automated acquisition system
combined with a relatively large field of view, rapid-slew telescope with
polarimetry capability appears to be a good strategy for extending the
study of GRBs. Perhaps most important to this proposal is that
GLAST will also have GRB localization capability, but it will produce
much larger error boxes than Swift. Since GLAST [18] is
scheduled for launch in about 2006, and the two-year Swift mission
may (or may not) overlap with GLAST for about one year, GLAST will
definitely need ground-based assistance to determine accurate
localizations, host galaxies, and GRB redshifts, and to study afterglow
properties. The GLAST schedule is perhaps more fitted for this proposal,
since we plan to have the proposed RRRT completely operational,
calibrated and debugged at that time.
The expected rate of GRBs
detected by Swift and GLAST over the RRT horizon is of about one every
two-three days. Most of the times the telescope will be remain available
for other type of research and/or educational purposes.
There is a large local amateur
organization, the Back Bay Amateur Astronomers Association, which will
collaborate with us. They plan to hunt for new objects that come near the
Earth, called, NEO's, (Near Earth Objects). The research goal will be to
find and track Asteroids and occasional Comets. They will use photometry
(CCD camera) and the method known as "blinking" to identify
Asteroids in the star field we are taking images. This program is
naturally complementary to our main research objective since will use the
same instrument and will allow a fast change of program. The group is
also active in variable stars (including supernova searches) observations
that are also among the most common among the amateur community and
complementary to the other programs.
B - Description of the
Research Instrumentation and Needs
The RRRT studies GRBs in the 10th
to 17th magnitude range. It is equipped with a CCD camera for
UVBRI photometry and polarimetry. The telescope is housed in its own
enclosure and is fully operational through the World Wide Web. The
telescope is robotic and completely automatic using the ACP Observatory
Control software by DC3-Dreams. Fan Mountain is about 180 miles
from NSU’s campus. We have weekly maintenance
visits from NSU’s. There is a continuous people presence at Fan Mountain
by UVA staff.
The University of Virginia
currently operates three telescopes at the Fan Mountain Observatory,
about 15 miles south of Charlottesville. The Observatory complex was
built in the mid 1960's. The Observatory was used extensively for
research up to the late 1980's and then fell in disuse as the astronomers
moved their research to larger aperture telescopes. There are currently
three (40”, 30” and 10”) telescopes at Fan Mountain but are not
robotically operated. In recent years, extensive hardware upgrades and
instrumentation efforts have transformed the observatory into a more
modern research facility. Efforts are underway to expand the capabilities
to include IR imaging with the 31” and low-resolution spectroscopy with
the 40” telescope.
Figure 2: Fan Mountain
Observatory with the location
of the NSU‘s rapid response robotic telescope (RRRT).
Figure 3: UVA’s Fan Mountain
Observatory with the location
of the NSU‘s rapid response robotic telescope (RRRT).
In cooperation with the University
of Virginia (UVA), we are able to use the current installations at Fan
Mountain (small machine shop and astronomer’s accommodations) and to get
power and fast Internet access for our telescope. UVA in turn will be
able to share the use of the telescope as schedule permits.
Specifications
for the Rapid Response Robotic Telescope
The telescope is capable of local (manual/real
time) and remote (scheduled and real time) robotic operation. It is a 0.6 m (24”) aperture,
Ritchey-Chretien, f/8 optics with an equatorial fork mounting. Fast
slewing, precision positioning and tracking are the most important characteristics
for the telescope and are specified accordingly to our research goals
(GRB-OA , Blazars photometry and polarimetry in the Swift
time and GLAST scales).
To maintain
precision on the telescopic focus and position after fast slewing, the
RRRT require very small (zero) optics and mechanical mounting
shifts, small detectable focal plane shifts versus focus position and
small focus changes with temperature, zenithal distances and instrumental
positions. These specifications are achieved with good focal compensation
or computerized focus control.
The telescope is fully
automatic. All controllable elements are Ethernet addressable, or “web
based controlled”, such as: optical covers, telescope positioning,
instrumentation (photometer and polarimeter controls), cameras, filter
wheel, dome, scheduling, weather observation (cloud and seeing
monitoring) and other observatory related hardware.
Positioning
specifications
20 arcsec (absolute) pointing
precision within 45 deg zenithal distance.
5 arcsec (relative) pointing precision for more than 10
degrees.
1 arcsec (relative) pointing repeatability for offset move
of less than 2 degrees
Tracking
specifications
Track objects at sidereal rates
(open-loop) better than 0.3 arcsec in Right Ascension and Declination per
100 seconds
Tracking jitter less than 0.3 arcsec (rms) in Right Ascension and
Declination
Slewing
specifications
Fast Slew rate: greater or
equal to 5 deg/sec (better than 90 degree in 20 seconds)
Slow Slew Rate Selectable from 0.1 deg/sec to 0.5 deg/sec
Fine Set rate: Selectable 0.01 deg/sec to 0.05 deg/sec
Guide rate 1 arcsec nominal, adjustable zero to 10 arc/sec
Accelerations
0.5 degree/sec2 nominal
1 to 2 degrees/sec2 in special applications
Filter
Wheel - 8 Position Filter Wheel for 2-inch diameter
filters
The RRRT has equatorial fork
mounting made of steel elements with stainless steel roller drives
component. Gears and drives are be able to withstand fast slew and low
maintenance. All mount and telescope’s structural elements are constructed
from like thermal coefficient materials for matching thermal expansion
properties to better pointing and tracking.
We have two main CCD cameras, with very high quantum
efficiency (i.e., Apogee Alta E42 – Back illuminated) without gaseous
cooling (for easier robotic capabilities) and a STL-1001 Santa Barbara.
The telescope operating system is
Windows based (for better in-house programming accessibility) by
Bisque (TCS) and TCP/IP link to
the Internet.
The RRRT enclosure is an
Astro-Haven fiberglass clam-shell dome (about 16’ Dome). This enclosure
allows the RRRT to point to any place in the sky with altitudes of 30
dgrees or more at any time with no further mechanical tracking or motions.
A more protected enclosure (dome) will allow us to operate in higher wind
conditions.
Polarization evolution will
provide one of best diagnostics of the magnetic field and jet dynamics of
GRBs. To perform polarimetry, the fewer the optical elements in the light
path, a.k.a. the simpler the optical design, the better off you're likely
to be as less corrections need to be performed on the acquired data. An
equatorial mounting is required.
We will use the same CCD camera
for imaging and polarimetry. For polarimetry, we would need an
additional module in front of the band pass filter wheel to carry the
polarimeter analyzers - a pair of calcite Savart plates would be the
simplest and least expensive (this will be done “in-house”). A
tailpiece could be designed with both filter wheels in place and with
blank space(s) in the analyzer wheel for direct imaging. It would
need to be carefully designed to avoid vignetting, but that would not be
a major difficulty.
The measured GRB-OA
polarization decay times are on the order of hours to days and have been
done with bigger telescopes up to larger magnitudes. We want to measure
brighter and in shorter times. It is not clearly know the degree of
polarization in the early stages of a GRB-QA (~150 sec). If the degree of
polarization is of a few to several percent, something like 10 to 20
minutes of integration time will give the necessary precision at
magnitude 12 with a 24” in aperture. Beyond that, the data can be
continuously streamed to disk for later analysis. You get one
complete measurement of the polarization degree and position angle for
every revolution of the analyzer, which are around 10 per second.
It is entirely possible to go back and analyze the recorded data stream
by binning any time interval or any number of subsets of time intervals
you like and simply adjusting your choices to get any desired level of
precision and look for variations. In other words this method of
measurement is based on the "signal averaging" technique, where
the precision improves according to how many measurements you
combine. The polarization parameters, when measured this way, are
completely independent of the brightness of the object. Also by streaming
data continuously to disk you have a record of brightness that can be
analyzed separately from the polarization to look for variability.
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