[Laser] Variable Stars

[TWOSIG at aol.com]

I attended the local astronomy club meeting last week.  Their speaker  had an 
interesting presentation on the methods that he had used twenty years ago  
and now.
 
What might be of value to this listing, is that he is now using a CCD  
imaging device.  The setup is to find a variable star, and so far he is  limiting 
his work to brighter than 12.5 magnitude be cause that is what is in  his 
database, that has a reference star and a check star of similar magnitude  close by. 
 In the example he used, they all fit within a circle about 4  minutes of arc 
( 1.16355 mR).  He then setup his telescope to track these  on the imager all 
night.  A single frame took about 120 seconds of exposure  ( he described the 
CCD device like a grid of wells, as the photons  fall into the wells, the 
wells increase in charge, and he did not want to  overfill the wells because it 
causes problems with the image) and 45 seconds to  download the image before 
starting the next frame.  At the end of the  observing night, he has a digitized 
video clip.
 
Now the good part.  He has software to convert the digitized pixels  into 
numbers on a spreadsheet, and eventually to a plot of the difference in the  
magnitude between the variable star and the reference star.  A similar plot  is 
done for reference and check stars.  With this system he has been able  to get 
excellent results even on full moon nights, or nights with high thin  clouds.
 
I think there is a lot of potential here for light communications.   And 
there may be a lot of technical hobbyists around that can help with  developing 
it.  You do not need to use a telescope that will track objects  as they move 
across the night sky. Astronomy clubs often have "loaner  Dobsonians" available 
to their members.  It is often the case that Amateur  astronomers typically do 
not go out on cloudy or full moon nights.  If one  in your area has a CCD 
imager, and you are flexible to work within his schedule,  you should be able to 
get some sample video clips.  You may be looking for  a large indistinct patch 
of brightness near the horizon, or you may be looking  for a line that passes 
from the horizon to overhead.  Once you know what  area you are looking for, 
then you can decode any modulation.
 
Working out a coding scheme would depend on the receiver sampling  rate.  
Even without synchronization, Morse sent a little faster  than one sample of each 
dit or element space should work as long as there  is a dah here and there to 
calculate the approximate start stop times.  For  a coding scheme that 
requires detecting frequencies might work better, but bear  in mind that you will 
not be taking instantaneous values of sine wave, but the  integration of a 
square wave.
 
Full duplex may not be practical even if the transmitter and receiver are  
separated.  For NLOS backscatter is probably stronger than forward  scatter.  On 
the other hand, if you have a means to synchronize alternating  
transmissions, download time, if not already eliminated by different hardware  than the 
above example, could be done while transmitting.
 
The imager need not be a CCD integrating device, or samples that take more  
than two minutes.  A webcam with thirty samples per second might have the  
sensitivity needed once it is coupled to a telescope.  At least until you  develop 
the software and techniques to the limit of the hardware.  I think  that 
there was a post on this list some time back with the idea of pointing a  
computerized camera at multiple modulated light sources.  Then set the  cursor on a 
source and have its text put on the display, sort of like selecting  a waterfall 
trace on PSK.  And if it was buffered, you could read the  previous parts of 
the message.  Maybe it is worth another look.
 
The idea of a signal, reference, and check light sources, might have  value.  
I am not suggesting that Johnny LaserCommunicator send out three  
simultaneous 1mR beams on 3.5 mR centers so that Biff the Astronomy Buff can  image three 
streaks of light from beyond the far horizon.  (Hey, if it  should work, and 
I thought of it first, put my name and call letters in a  footnote when you 
write about what you build.)  The technique is used by  astronomers so they can 
get useful data under less than ideal conditions, light  clouds, haze, 
moonlight, city lights.  If we can learn how to deal with  these problems from 
another group of technical hobbyists, I say go for it.
 
There might even be a way for the benefit to flow the other way.  In  the 
field of variable stars, there are all sorts of time scales.  Our sun  has an 11 
year or so sunspot cycle that has relatively minor magnitude  changes.  More 
common ones have periods of months or weeks.  The  example above was from an 
amateurs observations that proved what the  "professional" astronomers thought 
was a period of more than a day and a half,  was in fact closer to 40 minutes.  
Some stars are known to have periods in  the tens of milliseconds.  The 
discussions I have seen on this list have  included information about spectrum 
analysis using readily available software  and home computers.  Just maybe, there 
is an amateur astronomer out there  that can adapt some of the hardware and 
software we use for light  communication.  Then astronomy might get some really 
interesting data,  never before recorded, about what variable stars do when 
they are  changing.
 
 
James
N5GUI