Hi Jim:
I spent quite some time on looking at ways to optically tell the time, see:
http://www.prc68.com/I/StellarTime.shtml
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html
Jim Lux wrote:
So, to summarize the chain so far..
You need to solve two problems:
What's my camera orientation with respect to the stars.
Where is the Sun (or something else) as it moves across the field.
Conceptually, if I have my camera fixed and look at stars over some hours, they'll follow a path that's an arc (think
of pictures pointed to north star). That will give me the orientation of my sensor with reference to the celestial
pole, and the instantaneous positions of the stars gives me rotation around that axis.
But that's not sufficient to tell me what time it is, just how I'm oriented
relative to the stars.
So then, I look for something that moves, and by occultation or some other means, I can tell what time it is. (I
suppose this is basically what the celestial nav method of lunars does, but, of course, the moon has to be visible)
But, given that 1 second time accuracy requires 0.004 degree kind of measurements, that's tough with a wide field of
view camera with megapixel kinds of resolution.
And, it's going to be hard to detect stars with a small sensor, because they're not very bright. I was fooling with my
old iPhone 3G, and it can see Jupiter pretty easily, and maybe Sirius, but you're not going to see even 0 magnitude
stars.
However, maybe a small inexpensive reflector to increase the aperture and a webcam would do. You could replace
optical perfection with calibration, etc. (I suppose that's what Chris was doing with the camera lenses).
There's a whole FOV aim point tradeoff here.
Going with "sun only" schemes.. you get solar noon (and you apply the equation of time in some other way) by fitting a
curve to light intensity vs time.
Aligning with vertical can be done with a plumb bob or equivalent, and then a slit/photodiode can work, with curve
fitting. Is this something that is "arduino-able"? (at least the data collection.. the reduction might be done with
post processing)
How do you align the slit vertically, relative to the sensor? (to the required
seconds of arc)
I guess I should go look at some descriptions of zenith sun detectors. it's
probably obvious once you know.
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