[time-nuts] finding time astronomically , Part 2

Jim Lux 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 
camera lenses).

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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