Research
Radio Astronomy
Introduction
The DSES 18-meter dishes represent a unique opportunity for our members
to do radio astronomy studies. Recently we have become equipped to observe
neutral
hydrogen (HI) in the Milky Way galaxy. Neutral hydrogen is known to
change its spin axis under certain stimuli thus causing it to produce
radio emissions at 21 cm (1420.406 MHz). This phenomenon provides
a means by which hydrogen clouds in the Milky Way can be observed.
It is also possible to determine the relative velocity between
the Earth and various hydrogen clouds by observing the doppler shift in
the received signals. One possible application is to use the doppler data
to study of the motions of the radial arms of the Milky Way Galaxy by
observing the HI emissions from various nebulae.
The equipment for this project consists of a feedhorn placed
at the focal point of the dish, two low noise amplifiers (LNA)
attached to orthogonal elements in the feedhorn, a coupler, a
low loss cable to our control room, and a
SpectraCyber Astronomical Gas/Molecule Spectrometer - a combination
1420 MHz receiver and spectrometer.
The system can be used either to track a given target or to
perform drift scans where the dish receives whatever passes overhead
while the dish is in its parked position pointing at the zenith.
An example of the type of data that can be recorded during a drift
scan is shown in this movie of emissions from the Crab Nebula (105k) that were received using SpectraCyber
receiver.
The procedure for calibrating our system is to observe various
flux calibration sources.
A critical component in the receiving system is the feedhorn. Its
design and positioning must be chosen so as to achieve the maximum signal
to noise ratio (S/N).
Illustrated here are some
of the feedhorn configurations we have been testing
along with records of their performance.
Another option we are considering is to observe emissions from
hydroxyl radicals (OH) which are also associated with hydrogen clouds.
HO radicals have spectral lines at 1665.402 MHz and other frequencies.
One approach for receiving equipment would be to use LNAs with down
converters to 1420 MHz to match our SpectraCyber spectrometer.
An alternative receiver system that we have acquired for observing
hydroxyl emissions, and one that is continuously tunable to over a
wide band of frequencies, consists of a combination of an ICom 8500
receiver and a SDR-14 software defined receiver. The Icom serves
as a front end and its IF output is connected to the SDR-14. We have
modified the Icom by disabling the AGC to allow it to be used to
for radio astronomy. The
SDR-14 is
made by RFSPACE and is a very versatile receiver.
DSES Research Proposals
Solar Astronomy
Simple Mapping Project
A project to produce radio brightness maps of the sky over head using the
DSES upper dish has been underway since early 2006. Measurements are
being taken at the neutral hydrogen frequency HI (21 cm or 1420 MHz)
and the map is being built up by doing 24-hour meridianal drift scans at
1 degree increments of declination each day. Details of the project plan
and a sample of the preliminary results are shown below.
Detection of Gamma Ray Bursts
Abstract: The VLF computer model proposed in this paper examines whether
gamma ray bursts (GRB) or X-ray transient flux from distant supernova can be
detected by amateur VLF radios. Arguments presented in this paper compare how
GRBs created from supernova events might cause detectable signatures similar to
magnetar or other local X-ray transient Sudden Ionospheric Disturbances (SID).
High-energy GRB and short X-ray transients of supernova (SN) origin affect the
upper ionosphere through Compton free electron interaction and not through
magnetic field reconnection as local solar plasma might affect the Earth’s
magnetosphere. Gamma ray and X-ray ionization of the upper F2 layer, or
thermosphere, should be a measure of ionizing radiation as small as 10E-6 ergs,
yet may not be detectable with amateur VLF radios. High-energy solar plasma
interactions causing ionization have larger energy regimes, which impact the
lower ionosphere layers. Local atmospherics such as lightening, and sprites
also confound detection of SN GRB. Only events of very long duration, such
as the nighttime ionosphere disturbance from SRG1900+14, or a recent 'super
flare' from SGR1806-20 located toward the center of our galaxy 45,000 light
years away from Earth, and GRB030329
http://www.konkoly.hu/cgi-bin/IBVS?5415
have been detected at Very Low Frequencies (Peterson and Price et al.
2004).
Mira Observations
The objective of this research will be to detect OH masers around old Mira
stars, which in the Northern Hemisphere can be found around: U Her, S
CrB, W Hya, and R Cas. For radio measurements at T-22 we'll have to use
dark skies as a control measure of cold sky between 2 and 3 degrees off
from these Miras. This will be a program for collecting data over a
couple of years or more. Mira stars are old variable red giants with an
average period of 331 days. However, their magnitudes may vary from a
visual minimum magnitude to greater than 4 or 5 magnitude increase (very
bright). The hope is that we can detect their OH masers at 1.665GHz and
1.667GHz with circular polarization during the entire cycle of
variability. If detection is possible with this receiver on the 18 meter
dish[es], then data can be collected on the weekends or whenever the
stars are above the horizon. Variable star observations are normally
done in the optical, however these OH masers have been rated greater
than 1 Jansky in flux. (see article) Previous calibrations with T-22's
1.420GHz receiver have shown that this is about the limit of sensitivity
for our 18 meter dish[es]. However, with suitable tracking capability
and dwell time, we may be able to detect these OH maser signatures.
Should our data be statistically significant, the hope will be to submit
our results to the AAVSO Mira program:
http://www.aavso.org/vstar/vsots/archive.shtml#mira
which would certaintly be a first, as there are no radio observations of
Mira stars in the AAVSO database of variable stars.
Radio Source Occultations by Comets
A potential capability of the DSES radio dishes is to observe
the occultation of radio sources by passing comets.
As comets approach the Sun their comas expand thus providing
a larger target and increased likelyhood that an occultation will occur.
Such events although rare, would permit us to measure the absorption
at hydrogen or hydroxyl frequencies during the occultation of the radio
source behind it.
During 2005, flybys of comet Machholz
(C/2004 Q2) and 9/P Temple 1 presented possibilities for such
experiments as did the encounter of the Deep Impact impactor
with 9/P Temple 1. Details of our efforts to find occultation
opportunities and to observe the Deep Impact encounter are
can be found here.
Radio Astronomy Measurements Technology and Signal Analysis
Abstract: The San Marino
Scale, an analytical tool for assessing the significance of transmissions
from Earth, was adopted by the IAA SETI Permanent Study Group in 2007.
This additive model, which encompasses estimates of both signal strength
and signal characteristics, remains a work in progress. It is
statistically valid only to the extent that the two terms are assumed to
be wholly independent discrete random variables. We question this
assumption of independence, and, in fact, find a strong negative
correlation between the two terms, though we further suspect that the
interaction is nonlinear. Thus, we propose to amend the San Marino Scale,
making it more statistically robust by capturing, and compensating for,
this interdependence between detectability and information content.
Abstract: The investigation of sampling properties of a signal sample
spectral density involves the statistical distributions of the real and
imaginary components of the sampled signal. The distributions involved
include the normal (Gaussian), the chi-square, the half normal, the
truncated normal, and the Rayleigh distributions. We first review some of
the properties of these distributions before and after (1) the absolute
values of the real and imaginary components are summed, (2) when the
absolute value of the sum of these components is taken, (3) when the
squares of these components are summed, and (4) the square root of the
sums of the squares is determined. These four distinct sums result in four
different distributions. We compare of the respective means and variances
of these distributions.
Simulations of sinusoidal signals added
to noise with a standard normal distribution at five different sinusoidal
amplitudes were evaluated for distribution type. The comparison shows that
the distribution of the square root of the sums of the squares of the real
and imaginary components has the smallest variance over those of the
absolute value of the sums or the sum of the absolute values, though the
statistical significance of the differences must be determined per sample
size. When the difference is is not significant, the most convenient
additive combination may be used.
Activity Reports
- 01/04/05 Report on
Soft Gamma-Ray burst detection; Hydrogen receiver calibrations using 3C033
- 12/26/04 Report on
radio astronomy and other work projects at T-22
- 11/13/04 Progress
Report on radio astronomy activities at DSES T-22
- 10/11/04 First
full-up test of tracking an extra galactic radio source under computer control
- 07/12/05 Deep Impact
Tracking - Final Report
Other Radio Astronomy Websites
Last update: Monday, 01-Jun-2009 16:02:38 PDT by Webmaster
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