[time-nuts] finding time astronomically , Part 2
jimlux at earthlink.net
Mon Jan 23 18:12:00 EST 2012
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
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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