# [time-nuts] finding time astronomically.

Don Latham djl at montana.com
Mon Jan 23 18:16:47 EST 2012

```You may run into diffraction problems before achieving the sought accuracy?
How about measuring the motion of a tracker against a clock?
Don

Jim Lux
> 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
>
>
>>
>> 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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--
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
R. Bacon
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP