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Bungonia Optical SETI Australia

 

Dr. Ragbir Bhathal* and Leon Darcy

 

Photon-Counting Head

 

A brief report from Leon Darcy:

I am a member of Ragbir's OSETI team and helped design and build the photon detector/discriminator instruments.  Ragbir in turn is a member of my MSETI team where he has helped with the calculations and manual labor in erecting my Radio interferometer at Bungonia.  We will be observing 200 F, G and K-type stars within 1000 light years and observing each target star for a month.  While Ragbir is receiving light from the target star and looking for "laser" nano-second bursts, I will be observing the same star in the radio spectrum from 1GHz to 1.660 GHz.  All data will be scrutinized in real time.

The detector is compact and robust and housed in a extruded aluminum case in 3 separate modules.  Module #1 houses the optics, consisting of a 2 mm aperture that allows light from the target star to hit a 30 x 30 mm flip-mirror.  This star is viewed through an eyepiece when the star is centered, and the flip-mirror is switched to open, allowing the starlight to hit a 50/50 30mm x 30mm beamsplitter.  Each beam is then focused onto a Fabry lens which focuses a 10 mm spot onto the surface of a Photo Multiplier Tube (Module #2) type Hamamatsu R7400U-03 with cathode luminous sensitivity of  70.6 A/lm.   Anode luminous sensitivity is 68.1 mA/W.   Anode dark current 1.24 nA and the cathode sensitivity with a blue filter is 7.82 A/lm.  These PMTs are state of the art and have a rise time of 0.78 ns.  The detector high voltage supplies and control are housed in Module #3.

The signal of each PMT is then fed into the discriminator/counter (installed in a computer) the coincidence optical arrangement cancels system noise and heightens the signal to noise ratio. These photographs were taken by a friend and astronomy student Gregg Gibbons with a digital camera set to highest resolution.  The first picture is Ragbir and I checking the balance and the optical alignment. The photon-counting head only weighs a half a kilogram.  The second picture is the unit set up to receive data with (Plossl) eyepiece installed.  During first tests, the telescope slewed OK.  The small weight of the PMT module didn't affect the 16" LX200's smooth operation.  As it was a full moon, we didn't "power up" the unit at first.  This will be done during a new moon.

Leon Darcy, ETRC, Australia
First Posted: February 20, 2000
Revised: May 27, 2000

 

*Dr. Ragbir Bhathal is also co-chair for OSETI III.


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