[time-nuts] finding time astronomically , Part 2

Brooke Clarke brooke at pacific.net
Mon Jan 23 18:27:37 EST 2012


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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