[time-nuts] finding time astronomically.
jimlux at earthlink.net
Mon Jan 23 15:43:13 EST 2012
On 1/23/12 12:05 PM, Poul-Henning Kamp wrote:
> In message<4F1DBCC9.9040900 at earthlink.net>, Jim Lux writes:
>> How well could you do with something like the camera in the iPhone4
>> facing up. The front camera is VGA resolution.
> Very badly.
> The major trouble is actually not getting the light from the star,
> but making sure your camera/telescope/transit-circle has a known
> and stable geometric relationship to the planet Earth.
Say you had it in some sort of "fixture" to allow it to be placed
repeatably with reference to your local earth position.
I can think of two general scenarios here.
One is where you "lay the iphone on the table" in a fixed position. One
could use the internal accelerometers to determine "level", but I don't
think you could tell orientation, unless, perhaps, you can see
circumpolar stars? That is, by watching the movement of the
stars/planets through the field of view over some hours, could you
figure it out? Or is there some fundamental ambiguity.
(obviously, you can trivially see the moon/sun)
The other scenario is where you get an inexpensive camera (webcam, or
perhaps some slightly better point and shoot) and build a precision
mount (so you DO have accurate knowledge of sensor orientation and
position) Could you, perhaps over time, do an insitu calibration?
I suppose any of these techniques is going to have issues with the
uncertainty in when the image is actually captured (e.g. there's
probably 10-100 ms you're not going to get away from).
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