[time-nuts] finding time astronomically.
jimlux at earthlink.net
Mon Jan 23 16:08:04 EST 2012
On 1/23/12 12:29 PM, Chris Albertson wrote:
> On Mon, Jan 23, 2012 at 12:02 PM, Jim Lux<jimlux at earthlink.net> 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
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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