Revisiting the Itty Bitty Telescope (IBT)

Revisiting the Itty Bitty Telescope (IBT)
The IBT has been around since 2000. Originally developed by former SARA President Chuck Forster.
While it is not a true radio telescope, it is an excellent radio astronomy teaching tool utilizing a 1,200
MHz DirecTV dish and LNB with a modified Satellite finder used as the radio. Output may be sent to
Jim Sky's Sky-Pipe program for data capture. Chuck's original price point was under two hundred
dollars ($200.00). The IBT with a little imagination can now be built for under fifty dollars ($50.00).
While we are on the history of the IBT, Kerry Smith past SARA VP took Chuck's basic IBT and
expanded on it mounting the dish on a photographic tripod and modifying the Channel Master Model
1004IFD tuning meter. To provide a serial interface for input to Sky-Pipe. Unfortunately this meter is
now out of production. Kerry built over 20 of these units, ten went to SARA members and another ten
were provided to NRAO Green Bank for outreach under the Navigator program.
We've talked a bit about the history of the IBT, now let's take a look at what can be done with it as a
radio astronomy teaching tool.
Simple IBT Experiments:
•
Detect the Sun
•
Detect trees, buildings and people (all are at 300 degrees Kelvin)
•
Measure dish gain
•
Determine antenna pointing accuracy
•
Determine antenna beam width
•
Plot the Sun passage utilizing Sky-Pipe
•
Detect Geo stationary satellites
•
Utilizing a larger dish the Moon may be detected.
◦ Plot the Moon temperature changes at different phases of the Moon
•
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Plus many more
Components required to build an IBT
Last year ten IBT kits were provided for a session at Star Quest the Green Bank star party that precedes
the SARA annual conference. Used dishes were rounded up and the kits sold for twenty five dollars
($25.00) each. With the reduced cost of technology and technology changes. 2000. So lets take a look
at the components required to build the new low cost IBT:
The low cost IBT requires the following components:
Component
Source/Part #
Cost
DirecTV dish/LNB
Amazon Various sources
$23.48 + $14.49 shipping
Satellite finder
WS International SF-99
$24.99 (EBAY)
1/8” Mini-Stereo panel mount
Radio Shack 274-249
$3.49
1/8” stereo cable
Radio Shack 42-2387
$14.99
3 ft coax with F connectors
Radio Shack 15-019
$6.99
10 AA batteries
Radio Shack 23-858
$11.99
6” lazy Susan bearing
Home Depot SKU 144916
$4.49
8 1” sheet metal/wood screws
Home Depot 30-622
$7.77 (100 lot)
2 plywood 12 inch square
Home Depot 3/4”x24”x24”
$8.62
Total Cost
$121.30
The worst case cost is 121.30 dollars ($121.30) plus tax. We provided the IBT kits with all the above
parts and already modified Satellite finders for Star Quest this year at a cost of twenty five dollars
($25.00) each. Plus an additional twenty five dollars ($25.00) for a new DirecTV dish.
The 18 inch DirecTV dishes are available for free or a few dollars from neighbors no longer using them
or perhaps you may cut a deal with a local DirecTV installer. A lot of the parts required may be formed
in your own “junk” boxes. So the cost may be kept down.
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WSI SF-99 Kit
Current IBTs are using the WSI SF-99 sat finder kit. This kit provides almost all of the necessary
components for the IBT receiver.
Let's take a look at what's in the WS International SF-99 kit:
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•
SF-99 satellite finder
•
10 AA battery holder with 9V battery adapter
•
9 v battery adapter cable with F type connector
•
short F connector coax
•
military style Compass
•
Case to contain above
Added to this is a 6 ft. stereo cable and 3 ft RG6 coax with F type connectors
An Introduction to Satellite finders operation
Satellite finders are low cost receivers which are capable of receiving 800-2,400 MHz signals. The
typical design features a high gain op-amp and a Schottky diode detector. The Schottky diode detectors
are capable of operating as RF detectors up to 3 GHz. The DirecTV dish and LNB used with the 18”
dish are designed for Ku band reception (12-18 GHz). The LNB does a down convert from KU band to
the 800-1,400 MHz frequency for input into a DirecTV receiver. It is important to note the LNB
amplifies and down converts all frequencies within it's range. We are utilizing the SF-99 sat finder to
receive the in coming signal. The SF-99 is capable of detecting signals from -40 to +10 dBm which
when added to the antenna gain of 30-40 dB and the LNB typical gain of 50 dB. The total system gain
is approximately 120 dB. While low for a radio telescope, it's adequate for a teaching tool to detect the
Sun and other objects as discussed.
How to modify the SF-99 satellite finder
The SF-99 utilizes SMT (Surface Mount Technology), hence soldering is quite different than soldering
other printed circuit technologies. A low temperature soldering iron is best for tacking the SMT.
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•
Pry open the back of the SF-99
•
Drill a ¼” hole in the top. offset from the front and toward the side that has more open space.
•
Install the "1/8 “ stereo panel mount
•
Solder one wire from the ground side of the mini stereo panel mount to the ground near the F
connector
•
Solder a wire from the stereo panel mount tip (left or right doesn't matter Sky-Pipe will sort it
out) to the top left pin on the 16 pin IC. Note this is SMT (Surface Mount Technology)
carefully using a low temperature soldering iron tack the wire to the top left pin
•
Now install the 2.5 mm stereo cable for operation with Sky-Pipe
Building the IBT Base
The IBT base may take several forms, from simply mounting the dish base to a board at least
12”x12”x3/4” (30.5 cm square). To building a swivel mount which allows free motion in azimuth.
Currently a good trade-off between cost, size and weight is using a 12”x3/4” (30.5 square base) with a
11.5” diameter circle to mount the dish base to. This simple design utilizing a lazy Susan bearing
allows the dish to rotate freely in azimuth.
Cut a 12” square ¾ “plywood base and another 12” square, cut into an 11.5 “circle. If you don't have
the tools to cut a circle then just cut the second piece small enough to rotate on the base without
exceeding the 12” base or what ever size base you decide to use. These dimensions are not critical at
all. Next mount the lazy Susan between the two pieces of plywood and finally mount the dish base to
it.
If you cut the 11.5' circle then a 36” tape measure may be attached to the edge of the plywood circle to
provide a 360 degree azimuth reference. Why 11.5” diameter circle, easy! Pi times 11.5 = 36.1”
More detailed instructions may be found at:
http://www.aoc.nrao.edu/epo/teachers/ittybitty/procedure.html
Using the IBT
Now that you have the IBT built, let's take it outside and try it out. After setting the cable from the
LNB to the SF-99 satellite finder, connect the battery pack to the satellite finder. Now turn the knob on
the satellite finder until the sat finder begins to “squawk” .
Start by pointing the dish toward some trees, the reading on the meter should increase, now point at
cold sky, point toward the zenith is usually safe, no bright objects or satellites. Now that you have seen
the meter deflect by detecting the 300 degree kelvin temperature of the trees, point the dish at the Sun.
This is done by setting the shadow of the of the LNB on the lower lip of the dish. You should now see
an increase in the meter movement. If not move the dish off the Sun and the meter movement should
drop, if not increase the gain until moving on and off the Sun shows an increase in signal strength on
the meter.
Experiment with other objects building, people, almost everything will radiate black body radiation at
12 GHz.
Connecting the IBT to Sky Pipe is done by connecting the mini stereo plug from the IBT satellite finder
to the microphone/line in on your PC or laptop. Set Sky Pipe's input to the sound card right channel, if
that does not produce an output then use the left channel. An easy and fulfilling experiment is to set the
IBT on the Sun, move it slightly to the East of the Sun and allow the Sun to “drift” scan through the
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dish. This will produce a very nice bell curve displaying the Sun's energy as the sun drifts though the
focus point of the dish.
We have talked about the 300 degree Kelvin temperature, perhaps a little explanation is needed so we
may better understand the reference. In the USA we use the Fahrenheit system for measuring
temperature. The rest of the world uses the SI system which for temperature measurement uses Celsius.
and for scientific work the Kelvin scale is often used. Radio telescope may be thought of a
thermometers.
In radio astronomy we talk in terms of antenna temperature and sky brightness. When we point the
IBT dish at a building or trees, the antenna temperature detects approximately 300 degree Kelvin.
Which is close to 70 degrees Fahrenheit or what ever the outdoor temperature is. Below is a chart
showing the relationship between the three common methods of measuring temperature.
Scale
Absolute Zero
Freezing (H2O)
Room
Temperature
Boiling (H2O)
Fahrenheit
-459
32
70
212
Celsius
-273
0
21
100
Kelvin
0
273
294
373
The following formula can be used to convert a temperature from its representation on the Fahrenheit
( F) scale to the Celsius (C) value: C = 5/9(F - 32). The Celsius scale is in general use wherever metric
units have become accepted, and it is used in scientific work everywhere. The exception is radio
astronomy where Kelvin is utilized. Converting from Celsius to Kelvin is simply subtracting 273 from
the Celsius value.
Using the IBT as a radio astronomy teaching tool
Lets make it perfectly clear the IBT is not a scientific instrument to be used for research. However it is
an excellent tool for teaching the concepts of radio astronomy. To that end lets take a look at a few of
the things that may be done with your IBT.
Utilizing the IBT in an introduction to radio astronomy there are several discussion points that may be
used:
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•
Illustrate the electromagnetic spectrum
•
Discuss what Black body radiation is
•
Compare the size of wavelengths to everyday objects
•
Describe thermal emission and how to predict sources of thermal emissions
•
Demonstrate detection of thermal sources with the IBT
•
Describe formula for predicting beam dish beam width and demonstrate the formula by a drift
scan of the sun
References:
http://www.setileague.org/articles/lbt.pdf
http://www.aoc.nrao.edu/epo/teachers/ittybitty/procedure.html
http://gb.nrao.edu/epo/ambassadors//ibtmanualshort.pdf
http://www.phasorlabs.com/waterhole2001-2002.pdf
http://www.antenna-theory.com
NRAO/SARA Navigator Program
http://wsidigital. com
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