Trip Reports
to Allen Telescope Array (ATA)
42231 Bidwell Rd., Hat Creek, California
Google Maps
W 121, 28' 13" - N 40, 49' 36" (gate on Bidwell) - 1009 meters altitude
W 121.47028 - N 40.82667
Or in the astronomer's Right Assention nn.nn.nn or something = 121/15.nn.nn
I visited Wednesday June 7, and Wednesday October 31, 2007
by Ed Thelen ed@ed-thelen.org
Goal
Organization of this document
Arrival at ATA
ATA is about 13 miles from Burney, the route is well marked. Driving along Bidwell Road we notice an antique looking 6 foot dish
atop a weathered looking 30 foot wooden tower. This place seems to have quite a history. A quarter mile further on we arrive
at a lockable gate stating the visiting hours.
An associate sets up chairs to form a little theater in the entrance hall
and visitors are shown two introductory movies from a DVD about the history of the Hat Creek
facility and the current ATA - Much of the following section is from History of HCRO.
History of Hat Creek in Radio Astronomy
In the 1970’s Berkeley astronomers developed a two dish centimeter wavelength radio interferometer at HCRO. This evolved into an array of
10 dishes operating at wavelengths of 1 to 3 millimeters. The 10-element array was called BIMA, named after the consortium of
universities that operated and helped fund the instrument (Berkeley, Illinois, and Maryland Association).
Around 1993, a wind of over 100 MPH lifted the 85 ft. antenna out of its supports and dumped it crumpled on the ground.
SETI
and Berkeley Radio Astronomy Lab proposed a big (350 antennas) array of 20 foot (6.1 meter) radio telescopes
connected in an array - with up to 16 simultaneous studies going on at once - basically 16 somewhat independent
data streams coming
from the electronics - limited to a circle in the sky of about 5 degrees diameter at any one time - the (frequency dependent)
beam width of a single antenna. The above is a simplification of many options, including
splitting the array into parts for more data streams.
There are also a number of other donors of significant cash and useful products. Susie mentioned that Xilinx has donated
several million in the form of FPLAs (Field Programmable Logic Arrays) and technical help. (Both LaFarr and I had purchased
Xilinx chips and development systems. :-))
Commercial Credits
Our guides :-))
Visitor Problem - Information Overload
Antenna, Offset Gregorian Telescope
Cooled Amplifier, Fiber optic link to a Node
Node, then to central building
Splicing optical fibers
Central Electronics
Visit with Rick Forster
Roaming the Telescope Area
Assembly Building and Jigs
Noise
Interesting search keys
Maybe we can understand their technique of delaying the signals of each antenna to make 16 independent beams.
(These 16 beams must be:
Then of course there is:
for comments, corrections, additions, ... please contact ed@ed-thelen.org
I (Ed Thelen) have been to ATA twice, with friends. The main contact at the site is
Susie Jorgansen (tel: 530 335-2364) (site manager).
In 1962, an 85 foot radio telescope was constructed here, and several very significant discoveries took place here
- including first interstellar maser, detection of molecules of water and ammonia in "outer space".
Much of the BIMA array was moved to CARMA in Cedar Flat at 7,200 feet in the White Mountains near Big Pine, CA in 2003 to minimize water vapor
absorbing the high frequencies. View in Google Earth
All except this old warrior - abandoned as just too old and worn.
Reading:
Each antenna costs about $150,000 installed.
There are a number of donors of $150,000, each of whom get to have his/her name placed on an antenna.
Other than some help in return for telescope time by the ONR (Office of Naval Research), this ATA is largely privately
funded. (University of California Berkeley also helps) Here are the commercials posted by the Computation Center:
Vendor
Donation
Agilent
A signal generator and spectrum analyzer used for testing and monitoring ATA protoype test array signals
Sun Microsystems
Sun workstations, LCD monitors, disk arrays and tape units used in SETI's Prelude system, and the massive
storage system for ATA imaging data
Trimble Navigation
The differential GPS positioning system used to establish the ATA antenna locations for the 350-element array
Xilinx
FPGA (Field Programmable Gate Array) chips for the ATAA's BeamFormer, Imaging Correlator and SETI PDMs
Programmable Detection Modules)
Since our announced purpose of visiting was to view techie stuff, Susie Jorganson always had a technical person
available to answer our questions :-))
Rick Forster (Resident Astronomer, with overview of technology) at his desk overlooking the array.
The top screen shows the web cam image which he can point and zoom
for a view of outside and antennas. Local temperature and wind is also available.
Jeff Chero, [jeffchero at berkeley dot edu] - electronics, our October guide, in the processing room. His other lair
is the electronics work room.
(not shown, Jeff Kaufman, Power Electronics Technician, our June guide.) I also talked with
Geoff Bower [GBower at astro dot berkeley dot edu] in Campbell Hall in Berkeley.
Pointing the Telescopes, Brakes
The antennas are offset Gregorian Telescope.
The big dish (the primary reflector) is 6.1 meters (20 feet) in diameter and
is a segment of a paraboloid. The center of the paraboloid in approximately at the bottom
edge of the dish - just like DishTV. A big
advantage of the offset is that the secondary reflector (the smaller dish (about 6 feet in diameter) does not obstruct the
view of the primary. Radio "light" bounces off the primary onto the secondary, then off the secondary to the Log Periodic antenna,
not shown, but at the V of the blue lines. An advantage of the large size of the secondary is its ability to reflect and focus lower
frequencies than the usual radio telescope. Note that the aluminum is not shiny - as a condition of usage, the aluminum
is made dull by "soda blasting". Dimensioned drawing
The above is a bit of a schematic diagram. The actual telescopes have added components.
from Acr48F.pdf
This is the motor control box on the back side of each antenna. This communicates with the Control Building
via fiber optic lines. Nice way to protect the Control Building incase lightning hits an antenna ;-))
It receives azimuth and elevation targets, and transmits back actual antenna pointing, and a large number
of other status values.
This is inside the protective cover in the antenna. The LogPeriodic antenna inside in about 2.5 feet long.
It can efficiently receive a VERY wide range of
frequencies - from about 400 megahertz (wavelength about 30 inches) to 11,000 megahertz (wave length less than
3 centimeters, about an inch) The little fins at the pointy end are about 1/4 inch long.
This converts the radio waves reflected by the dishes into electrical signals. The base of the LogPeriodic is
on the left, and the tip is connected to the Low Noise Amplifier inside the copper structure. Notice that fins are in planes
90 degrees apart - this permits receiving both vertical and horizontal polarized waves. After amplification,
both vertical and horizontal RF is converted to laser light and transmitted to the Control Building.
more pictures
This box is sitting on the base of the LogPeriodic antenna. Inside the LogPeriodic is a little cryogenic unit to make
*VERY* low temperatures, to cool the Low Noise Amplifier.
Apparently this (or every??) antenna has a vacuum monitor for the pump/refrigerator cooling the Low Noise Amplifier.
Inside of the base is a little motor to extend or retract the LogPeriodic a range of about 10 inches for
optimal focusing - see next lower box.
The optimal position of Log Periodic antenna is somewhat frequency dependent a focusing environment, such as a Radio Telescope.
The elements of this extreme log periodic are spread over a length of over 2 feet.
So, depending on general frequency you wish to receive, a stepper motor (Focus Drive) can slide the correct part of the antenna
into the focal point. Fortunately
the position is not very critical. Rick Forster says that improvement in setting gives a maximum improvement of about 20% :-))
An Overview of this "front end" is given at the SETI web site
Technical Overview
and diagram
Here is a local copy
I asked about the change from stepping motor antenna drive to an AC drive that Jill Tarter (SETI) had mentioned.
Jeff showed a smoked connector on a stepping motor.
Apparently the stepping motor currents were much higher than the connectors and drive circuitry could reliably handle.
He then showed the drive motors now used to drive the antennas in azimuth and elevation.
Each motor (above) drives a new (1950s) type "gear box"
called "harmonic drives".
Inventor's page This relatively new invention provides very high gear ratios
- on the order of 30-1 to much higher, very compactly and with almost no backlash/hystersis.
This photograph does not show the electronic position sensors.
(Local copies of harmonic drive documents)
page 1,
page 2,
page 3,
diagram,
And here is the standard drive brake - about the size of the palm of your hand. With no electricity, springs force
the center rotor (sintered metal) to contact the steel and cause a breaking action - to a limit.
If external forces (such as wind at 100 mph) occur, the brake will slip - and in azimuth permit the antenna to point down wind.
If you wish to move the antenna, you supply current to the break.
A refrigerator is used to permit the Low Noise Amplifier (I did not get a picture)
to have even lower noise, the Low Noise Amplifier is cooled to well below the boiling point of liquid nitrogen.
The refrigerator (called a Pulse Tube Cooler) is contained inside the pyramid of the log periodic antenna above.
The general idea is similar a household refrigerator, compressing and expanding a gas,
with but with changes for smaller size and longer life. This is a reference to a
Pulse Tube Cooler, which unlike a Sterling cooler,
has no moving parts on the cold end. also.
A SETI Block Diagram.
After the cooled Low Noise Amplifier, the signal is strong enough to send to this
Amplifier box via coaxial cable. Here it is further amplified and routed via coax to the board with the laser. The laser
is connected to the blue fiber optic shown for transmittal to a node box which handles 12 antennas (24 channels)
Note that there are 2 data channels in this box, one for the vertical polarity radio waves, and the other for the
horizontal. The big green board in the middle is the amplifier control (suitably shielded.
The control signals and data signals from each antenna pass through a "Node", which is an organizing convenience,
no known technical use? The current organization is 12 antennas are 'serviced' by one Node. There is more
physical capacity in each node if the need arises. The chimney is for a ventilating system.
Jeff points out the optical fiber entry and exit.
and lots of spare power and space.
Optical signals in the Central Building
This might be called an Optical Time Domain Reflectometer :-)) How about that for a mouthful ;-))
(as opposed to frequency domain).
It is useful in determining many characteristics of a fiber optic cable - such as how good are any splices or junctions,
where are they - as measured in time converted to length, various other imperfections, and very vital in this application,
how long in time to the other end as accurately as possible. With the game being to phase match every thing,
this is vital knowledge for initial setup of delay parameters to point the
electronically steerable sub-beams with in the physically steerable main beam.
There are miles of optical fibers already in the 42 antenna array. There are several splices in each fiber
between the control room and each antenna. Each splice must be carefully made or additional signal attenuation
and reflections will be introduced, causing potential or real problems. To make a low attenuation, low
reflection splice requires considerable care. The two faces of the splice are cut at 7 degrees from
perpendicular, carefully cleaned and polished, and then fused together.
The RF carrying fibers are of a special glass with especially low loss and low thermal expansion (delay change)
characteristics. The actual signal carrying cross section is (7 mills?) so cleanliness and alignment are
especially important.
The left hand image is a good clean surface, ready for fusing
Here we are in techie & mathematical heaven -
However, lets get started - This is where the field fiber optics, carrying the vertically and horizontally polarized RF information,
come into the building and are joined to the patching system.
And onto distribution (I think)
And somehow the data (both polarities) gets into these local mixers
- with inputs from 4 separate local oscillators to begin generating the 16 separate data streams available for users.
Detail on the mixers
Mixers
Low Pass Filters
(A correction)
The cards pictured here: -
are actually IBOB boards with 2 adcs attached. They use a very similar FPGA
to the BEE2 but are actually a different board. Much more info available
at:
Cheers,
Andrew Siemion
Dear Friends,
Clearly I've lost it :-((
The unfortunate part of all of this is that I came up to find out about the delay lines and correlators, as instructed by
the Berkeley folks, but that expertise seems to be elsewhere :-((
This is a weather reporting station and GPS Trak clock.
(Sidereal time is derived from GPS ( or ?) by a card and steers array and time tags data flowing into storage.
The site has a rubidium clock - about 10^-12. A cesium clock (10^-15) would be for
VLBA work - not currently planned.)
And precision pulses to ...
Here are SETI processors that were previously used at the Arecibo radio telescope (analog inputs)
I found this diagram, specifying a 42 antenna system so I figure it is relatively current - I doubt that a year ago
they knew that this phase would be 42 antennas. The image here is large (124 K bytes) but still
hard to read but maybe worth the struggle if you have a tutor handy. I was particularly confused as it indicates that
the output of the digital beam former is analog, and goes to more ADCs. But then again, I'm guessing at the whole
thing anyway -
But then I found this representation of probably brightness at some radio frequency of M-33 on the wall.
If you get tired of O-H radicals, Doppler shift, atmospheric absorption, calibration problems,
people problems, interference problems, scheduling problems, funding problems,
environmental problems, and all the other hazards of life, you can gaze at this
and feel good :-))
Susie Jorgansen then introduced us to Rick Forster (Resident Astronomer).
For instance, when the sun shines on one side of the steel tube center post,
it can tilt the top up to 1/2 degree (the angle of a full moon). Rick called this effect "sunflowering", as in flowers following the sun.
This may not sound like much, but it is 1/5 of the average beam width
of the reflector telescope (much greater error if working at higher frequencies. There are other aspects of this that contribute more
errors, such as shifting the focal point of the antenna on the order of a wave length. As this whole array tries to have errors less than
1/8 wave length, this one problem makes a big contribution to the entire error budget of the array.
Rick said that this center post tilt error is difficult to model, and therefore difficult to correct in software or by other means.
I imagine that "random" breezes, patchy clouds moving about, and similar effects are most difficult to measure and work into a model.
I suggested that painting the gray (galvanized steel) center posts and supports with
titanium dioxide paint (very white and reflective)
might help - The Nike precision tracking antennas I worked with so many years ago used titanium dioxide paint to help fight this same problem.
Maybe the reason this is not done is that the glare white, desired for engineering reasons, is very noticeable in the surrounding woods
(a National Forest?) and offensive to the natural beauty of the area??
Of course we had to ask Rick how so many antennas could be used (in phase) together - it sounds like a mind numbing problem.
Rick said that you basically pair each antenna with every other antenna in the group, and that you want as many different distances
as practical. That this was why the field of antennas looks so random.
The goal, in part, is to place antennas so there is as large as practical range of differences in distances. There is of course some minimum
practical difference between two antennas, and a maximum practical distance between two antennas - but you really do *NOT* want the antennas
to be placed like rows of corn or a checker board or any regular order. For a very helpful article on the placement of the antennas and
why it isn't as random as it appears, read Dr. Seth Shostak's
"Spreading Antennas Around". They even use
heavy duty computing to help do a proper job.
This is the back view of the ATA style antenna. The "dish" is "hydroformed" in Idaho by a firm that specialized in this operation.
As part of the deal, the "dish" is "soda blasted" to remove the shine. This has two benefits
Concrete pads which support the antenna weight and are designed to keep the antenna in place in a 100 mph wind.
These pads are poured into (I forgot how deep?) holes drilled by contractors. Nothing is simple. There are two sizes of pads.
Susie told us drilling contractor stories.
Notice the ground wire bolted to the antenna foot and leading into the ground. (Grounding and lightning protection is another specialty.)
All signaling to and from the antenna is by fiber optics,
but I imagine that a lightning hit could cause interesting power supply problems. The dark thing on the top left is the aluminum
preventing radio radiation from the warm earth from getting into the receiver. Part of the white antenna assembly building is visible
in the upper left.
This is the steel pipe that comes up from the ground to the center of the center post. All power, signals, and even cooling air
come into the antenna through here.
Detail of the foot and hold down screw. Note the square threads, and the jam nut on top to prevent loosening.
Going up, here is what we think is an electric ?motor? control box. We saw a little bird fly out near the thing that looks
like two eyes. There must be more practical problems than I wish to think about. Like trying to keep the fiber a constant
length - there is apparently a cooling air flow from the node (12 antennas each) to keep the optic fiber as close
to rather constant earth temperature as practical. Here is another view of the metal to keep radiation from the earth out.
Side view of the antenna
Detail of the rear. The wires going from strut to strut provide wind vibration damping. Also visible is the azimuth drive
at the top of the center pole, and the elevation screw.
This is a rolling center pole upon which a partially completed telescope can be placed for further assembly or
"system integration" ;-)) We did not understand the cut out near the top.
This circle (and more) posts bolted to the concrete floor are carefully leveled so that the hydroformed 20 foot primary
reflector will not be deformed during further assembly. A ring of aluminum is welded to the outer edge and supports for the
struts welded to the ring. The goal is to have the final reflecting surface with an error of less than 1/8 wave length
at 11 gigahertz - less than 1/8 inch from a paraboloid sector over the 20 foot diameter surface.
This orange jig probably helps lift the horizontal partially assembled 20 foot primary reflector for further processing.
Note the white ?Teflon? pieces on the jig which probably help protect the reflector from scuffing. There are two little
white pieces (one hidden by the on the foreground post) mounted on a rotating shaft controlled by a screw arrangement.
This likely secures ?something? during movement. We did not figure out how the reflector gets rotated from horizontal to
vertical for mounting on the black vertical post.
This is the fork lift that carries the assembled primary, secondary, struts, log periodic antenna, wiring, azimuth drive,
elevation drive, ...
I took these images of photos hung on the walls of the control building
The great outdoors (and lightning) is powerful stuff. This is ground wire that connects a steel leg of each telescope
to the earth ground. Looks like 19 strands of 4/0-1C soft drawn copper. Both LaFarr and I had past experience with lightning.
LaFarr was standing near a tractor when it was hit, and I had strung an informal interbuilding squawk box system through a tree
that got hit. (We forgot to ask details of the earthing.)
A reason for selecting this location for a radio telescope is the relatively low radio noise leaking in from civilization.
The surounding hills and mountains largely block noise from the earth outside. The rural location, and being in a National Park
provides low ambient noise. As you come into the ATA area, you are asked to turn off your cell phones - they all operate in
the frequency band (500 mHz to 11 gHz) observed by ATA. Unfortunately, EVERYTHING radiates radio waves, including you and me.
Carl Sagen is quoted as saying that all the energy ever received by all the radio telescopes in the world is less than that
of a snowflake gentle falling onto your hand. Keeping external noise out, and using low noise techniques inside a radio telescope
is vital to successfully observing the very remote signals that are so weak arriving at the earth. Noise is often quantified
as an equivalent temperature in Kelvins - from absolute zero. Liquid nitrogen at 77.2 K (-320 degrees F) is hot and too noisy.
Adding new finds here, as of Sept 22, 2007, new stuff near top (here)
Interesting web pages
My Understanding
adjusting the delays to allow for
This is serious stuff :-|
Integrated Technical Information - a running technical integration of what I think I know :-|
HatCreekATA-StrugglingOn
Pointing/tracking the Synthesized Beam "Array"
Information Please
