On 1/23/12 12:29 PM, Chris Albertson wrote:
On Mon, Jan 23, 2012 at 12:02 PM, Jim Lux<[email protected]> wrote:
This chat of zenith cams, etc. is interesting.
How well could you do with something like the camera in the iPhone4 facing
up. The front camera is VGA resolution.
Say you're on another planet?
You can use a stick pounded into the ground and wait until the shadow
has minimum length. But I assume we need better accuracy?
An interesting approach, because it could conceivably get
"magnification" without using lenses or mirrors. Imagine the shadow tip
of a 2 meter long stick, and I have the camera positioned so that I only
see about 20cmx20cm. (of course, the shadow isn't that well defined,
because the angular extent of the sun is huge)
A similar scheme if i use a pinhole to project an image of the sun, and
image that, instead.
If you use a camera, accuracy will be limited by your knowledge of
where you are aiming the camera. If you are off by one degree then
the error is about 1/360 times the length of the day on your planet.
So finding the time is really about discovering where you have aimed
the camera. This is best figured out at night when you can see
stars. You can actually aim the camera at random, so long as you
measure the aim point and don't let it move.
That said, I think if you were to leave a cell phone in a fixed
position, un-moved all night you can likely get to 1/10th of a pixel
angular resolution. So what is the angle subtended by one pixel
on your phone divide that by 10 then multiply by one day. A
total guess is "about 1 mSec" if you use a full night's data. Just be
warned that reducing the data is not simple there are many steps
involved just one of then is matching your data to a good star catalog
and this implies having a good catalog.
iPhone cameras (and most webcams, etc.) seem to have a FOV about 45
degrees, so one pixel is around 0.1 degree. At 4 minutes time per
degree, that's about 24 seconds per pixel.
(It's not a monochrome sensor, either, so although it's NxM pixels, that
doesn't mean that you could actually resolve a planet to that scale,
depending on color, and how the image is processed)
You really can get to 0.1 pixel. You fit a function to the "fuzzy
blob" image of each star and then maybe 100 pixels contribute to a
solution.
tricky on a iPhone type camera, since star images are one pixel at best.
On the cameras I've seen that were designed to do this, they have a
cleverly designed optical system that blurs the image. (and another
scheme uses a camera with a multi pinhole mask in front, to render the
image in multiple places across the sensor.
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