A pulsar machine (or pulsar detection system) is used at
many of the large radio
observatories around the world. Some of these pulsar
machines are well known for the
work that has been done such as the Penn State Pulsar
Machine at Arecibo. Others are
just coming on line, such as PuMa, located in the
Netherlands. This paper presents a low
cost Pulsar Machine for use at smaller observatories just
entering the field of pulsar
research.
·
Why Study Pulsars?
Why would any one want to study pulsars? They are for the
most part very distant objects
that can be seen only with large telescopes (radio or
optical). What effect could these
possibly have on us? Questions I am often asked. A pulsar,
for it’s size, around 10 to 20
km in diameter (6 to 12
miles) is one of the most energetic objects in the universe. It is
literally a physics laboratory in space. Reference 1
has additional information pulsars.
Now let’s apply this to one of our most pressing problems
today - clean, efficient, low
cost energy. The study of these objects could lead us to a
much more efficient source of
power. Maybe a tiny pulsar in a shoe box that can power you
car and never needs
refueling that is so strong that, when your car wears out,
you just unplug it and put it in
you new car. Speculation? Yes it is, but we will never find
these new sources unless we
look, and I believe that pulsars may be the right place to
look.
·
What is a Pulsar Machine?
A radio telescope has several parts. The antenna, which
collects the signal, a low noise
amplifier (LNA) that amplifies the signal, the receiver (or
converter) that converts the
signal to a lower (or intermediate frequency), and finally,
the backend which converts
the intermediate frequency to some type of audio signal. A
pulsar machine (or pulsar
backend) is an additional piece of equipment that attaches
to the radio telescope and
provides additional processing to allow the detection of
pulsars. A pulsar machine may
generally be defined as hardware and/or software that allows
the detection of pulsars.
·
Past Pulsar Machines
Pulsar machines have been developed at Pennsylvania Statue
University as well as
Berkeley Caltech for almost 30 years. They are deployed at
the major observatories
around the world. Reference 3 has additional information
on some of early pulsar
machines.
·
Current Pulsar Machines
One of newest is the
PuMa Westerbrok Synthesis Radio Telescope in the Netherlands.
There are also major
Pulsar Machine at Green Bank and Arecibo.
Pulsar Machines for the Amateur Radio Astronomer:
There have been
several efforts by amateur radio astronomers to build such machines.
For some of us, the detection of pulsars is the “Quest for
the Holy Grail” of amateur radio
astronomy. There are a number of notable efforts:
- James C. Carroll (A Post Detector
Pulsar Extractor – SARA Paper)
- Robert
M. Sickels (Pulse Catcher – SARA Paper)
Current Amateur Radio
Astronomy Pulsars efforts (including the author):
- P. Ibelings and M. Wheatley at the
PARI Observatory.
- Jim Van Prooyen and Rich Nagel at
the Grand Rapids Radio Observatory.
·
The low cost Pulsar Machine
The design of the pulsar machine involved a number of design
trade-offs to arrive at a
system that was low cost and usable with a small radio
telescope. The parameters that
were studied in the design included:
- Observation frequency
- Analog vs. Digital electronics
- Band width
- Dispersion issues
- Algorithms for processing the data
- Filter design
- Search methods
- Low Cost Processing Technologies
·
Observation Frequency
The selection of an observation frequency presents a unique
set of problems for the
amateur radio astronomers. The issues include:
1. More signal to work with at the
lower frequencies.
2. Less distortion of the signal at
higher frequencies.
3. Equipment costs are less at the
lower frequencies.
4. Much more gain for a given size
dish antenna at higher frequencies.
There is no one good choice, pulsars have been observed at
frequencies from a few
megahertz to the high gigahertz.
·
Analog vs. Digital
The first pulsar
machines had a large number of analog signal processing elements.
Today most of this is
done using digital technology. In general a digital system will have
a lower cost of
acquisition then analog systems.
·
Wide Band vs. Narrow Band
Many pulsar machines
use wide bands, and it makes sense from a systems view. The
more signal you can put into the system, the greater the
sensitivity. Some are as wide as
100 MHz. But there are problems here,
such as how you are going to digitize a wide
signal band and keep the cost under control. The other
problem is interference from other
sources, when you are not observing in a radio quite zone.
We use a narrow band
approach to the problem. This allows us to keep the cost
very low and it keeps out the local radio and TV stations. However, this has a
cost in that it we need to observe for a
long period of time before we see a detection. Generally,
one hour of observation is used
to make sure we have the data needed for a detection.
·
The Dispersion Issue
The narrow band approach means that dispersion is not an
issue. We use a bandwidth
of 75 kHz. We are currently studying
bandwidths of only 30 kHz for the next generation
receiver. This receiver front end will be build by Radio
Astronomy Supplies some time in
the winter of 2008/2009.
·
Fast Folding Algorithm (FFA)
This is a computer algorithm that is the heart of the
system. It allows for the detection of
periodic events within time series data. The algorithm was
developed by David H. Staelin
in 1969. References 4
and 5 have additional information on the development of the
FFA.
·
Filter Design
The pulsar machine has a filter that processes data after
the FFA. The filter is unique to
this pulsar engine. It is called a Jakeway filter, named to
honor Jarry Jakeway, who was
my mentor during its design in 2002/2003.
This is an efficient recursive filter for use with
noisy data sets.
·
Search Methods
The pulsar machine
supports two search methods, targeted and non-targeted. The
targeted search has
been fully developed and is optional. The non-targeted processing of
the pulsar machine
will be the subject of future papers. It will use a Parameter Space
Search Algorithm
(PSSA) that is still under development. This version of the Pulsar
engine will require
the use of advanced computer technologies that are now available,
such as Beowulf,
GPU’s, and other Parallel Processor technologies.
·
Deployment of the Pulsar Engine
One of the major
issues has been how to deploy the Pulsar Engine (i.e. put it in to use by
other members of SARA). Due to the fact that it is still
under almost continuous
development and the regulations on use of such technology
(ITAR), we have arrived at
the following model for it use:
- Members of SARA may send
observations to an FTP server for processing by the
Pulsar Engine. Data sent to the FTP server must be in one of
the supported data
formats. There will be more information on this in the SARA
Journal some time
this Fall.
- Processing produces histograms,
using of MS Excel or JPG files and will be
posted to the FTP server for downloading. This processing
may take anywhere from
24 to 48 hours for turnaround. The following is a example of the output from
the Pulsar Engine:
Summary of the observation histogram of pulsar B0031-07
pictured above:
This is a pulsar with a period of 0.9429509945998
seconds. This pulsar has three modes
of drifting sub pulses. This can be seen between index 95,
and 103, (on the x-axis) and
again between 161, and 172.
It also produces giant pulses approximately once in 800
periods (~ 754.3608 seconds). Due to the folding
of the data over an observation period
of 3600 seconds there should be several
giant pulses visible in the plot, this is supported
by the data at index 118 and 183
(on the x-axis).
·
Conclusion
The study of pulsars is fascinating and do-able for the
amateur astronomer. And, being
incredible energy-producers for their size, they may have
implications for energy
production on our own planet in the future. In this paper,
the reader has been brought upto-date with current work and given a glimpse of
the future in the study of Pulsars. If
you have a radio telescope and would like to be part of the
project please send an e-mail
to me at grro1@dnx.net.
