subject:"Re\: \[time\-nuts\] finding time astronomically."

On Mon, Jan 23, 2012 at 12:02 PM, Jim Lux jim...@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?

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.

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.


Chris Albertson
Redondo Beach, California

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Re: [time-nuts] finding time astronomically.


On 1/23/12 12:05 PM, Poul-Henning Kamp wrote:

In message4f1dbcc9.9040...@earthlink.net, Jim Lux writes:


How well could you do with something like the camera in the iPhone4
facing up. The front camera is VGA resolution.


Very badly.

The major trouble is actually not getting the light from the star,
but making sure your camera/telescope/transit-circle has a known
and stable geometric relationship to the planet Earth.



Say you had it in some sort of fixture to allow it to be placed 
repeatably with reference to your local earth position.


I can think of two general scenarios here.

One is where you lay the iphone on the table in a fixed position.  One 
could use the internal accelerometers to determine level, but I don't 
think you could tell orientation, unless, perhaps, you can see 
circumpolar stars?  That is, by watching the movement of the 
stars/planets through the field of view over some hours, could you 
figure it out?  Or is there some fundamental ambiguity.


(obviously, you can trivially see the moon/sun)

The other scenario is where you get an inexpensive camera (webcam, or 
perhaps some slightly better point and shoot) and build a precision 
mount (so you DO have accurate knowledge of sensor orientation and 
position) Could you, perhaps over time, do an insitu calibration?


I suppose any of these techniques is going to have issues with the 
uncertainty in when the image is actually captured (e.g. there's 
probably 10-100 ms you're not going to get away from).




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Re: [time-nuts] finding time astronomically.


On 1/23/12 12:29 PM, Chris Albertson wrote:

On Mon, Jan 23, 2012 at 12:02 PM, Jim Luxjim...@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
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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Re: [time-nuts] finding time astronomically.

 I can think of two general scenarios here.
 If you planet has air you will need to know how it refracts st
 One is where you lay the iphone on the table in a fixed position.  One
 could use the internal accelerometers to determine level, but I don't
 think you could tell orientation, unless, perhaps, you can see circumpolar
 stars?  That is, by watching the movement of the stars/planets through the
 field of view over some hours, could you figure it out?  Or is there some
 fundamental ambiguity.

No, you can point to any location and you can (in theory) figure out
where it's pointing given that you have a large enough field of view
to see many stars at the same time.   You can make a fixture easy
enough, just some epoxy and a large boulder.  I used lag bolts
onto my garage roof and it worked more than good enough.

If you can choose, straight up is the best aim point.  Refraction is
not much of an issue and there is less air to look through.  But
looking at the equator means there is less field rotation and the data
is easier to reduce.  We looked at the equator because we did not want
to deal with image rotation.   Motion blur is minimize down there too.

But if you want to know absolute time then you need more.  Looking
at any random but fixed location will get you the period of the
planet's ration to about a mSec with cheap equipment but to get
absolute time you need to measure the aim point relative to the local
meridian.   That is not as easy.  Star with a protrator and a plumb
bob.That is what I used.   But to refine that you need a good
source of time and for the purpose of this exercise we don't have
that.  Only the plumb bob which means a few seconds of error.  maybe
an precision level can do 10X better?

 (obviously, you can trivially see the moon/sun)

 The other scenario is where you get an inexpensive camera (webcam, or
 perhaps some slightly better point and shoot) and build a precision mount
 (so you DO have accurate knowledge of sensor orientation and position) Could
 you, perhaps over time, do an insitu calibration?

 I suppose any of these techniques is going to have issues with the
 uncertainty in when the image is actually captured (e.g. there's probably
 10-100 ms you're not going to get away from).




 ___
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-- 

Chris Albertson
Redondo Beach, California

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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Javier Herrero


El 23/01/2012 21:43, Jim Lux escribió:



One is where you lay the iphone on the table in a fixed position.  
One could use the internal accelerometers to determine level, but I 
don't think you could tell orientation, unless, perhaps, you can see 
circumpolar stars?  That is, by watching the movement of the 
stars/planets through the field of view over some hours, could you 
figure it out?  Or is there some fundamental ambiguity.



I don't know about the iPhone, but I've seen an HTC with a funny 
application that, when you point anywhere in the sky, it shows you the 
constellations that are there. Even if you point it to ground, it shows 
you the constellations in the other hemisphere :) I don't remember if 
the application is this http://www.google.com/mobile/skymap/ or 
something similar, but in any case, the phone knows its orientation 
quite good (well... also depends on the phone to have the right time, of 
course... :) )


Regards,

Javier


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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Doug Millar

A mercury mirror is better than a plumb bob.
  Doug 

 



 


 From: Chris Albertson albertson.ch...@gmail.com
To: Discussion of precise time and frequency measurement time-nuts@febo.com 
Sent: Monday, January 23, 2012 1:12 PM
Subject: Re: [time-nuts] finding time astronomically.
 
 I can think of two general scenarios here.
 If you planet has air you will need to know how it refracts st
 One is where you lay the iphone on the table in a fixed position.  One
 could use the internal accelerometers to determine level, but I don't
 think you could tell orientation, unless, perhaps, you can see circumpolar
 stars?  That is, by watching the movement of the stars/planets through the
 field of view over some hours, could you figure it out?  Or is there some
 fundamental ambiguity.

No, you can point to any location and you can (in theory) figure out
where it's pointing given that you have a large enough field of view
to see many stars at the same time.   You can make a fixture easy
enough, just some epoxy and a large boulder.      I used lag bolts
onto my garage roof and it worked more than good enough.

If you can choose, straight up is the best aim point.  Refraction is
not much of an issue and there is less air to look through.  But
looking at the equator means there is less field rotation and the data
is easier to reduce.  We looked at the equator because we did not want
to deal with image rotation.   Motion blur is minimize down there too.

But if you want to know absolute time then you need more.  Looking
at any random but fixed location will get you the period of the
planet's ration to about a mSec with cheap equipment but to get
absolute time you need to measure the aim point relative to the local
meridian.   That is not as easy.  Star with a protrator and a plumb
bob.    That is what I used.   But to refine that you need a good
source of time and for the purpose of this exercise we don't have
that.  Only the plumb bob which means a few seconds of error.  maybe
an precision level can do 10X better?

 (obviously, you can trivially see the moon/sun)

 The other scenario is where you get an inexpensive camera (webcam, or
 perhaps some slightly better point and shoot) and build a precision mount
 (so you DO have accurate knowledge of sensor orientation and position) Could
 you, perhaps over time, do an insitu calibration?

 I suppose any of these techniques is going to have issues with the
 uncertainty in when the image is actually captured (e.g. there's probably
 10-100 ms you're not going to get away from).




 ___
 time-nuts mailing list -- time-nuts@febo.com
 To unsubscribe, go to
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 and follow the instructions there.



-- 

Chris Albertson
Redondo Beach, California

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Re: [time-nuts] finding time astronomically.


On 1/23/12 1:18 PM, Javier Herrero wrote:

El 23/01/2012 21:43, Jim Lux escribió:



One is where you lay the iphone on the table in a fixed position.
One could use the internal accelerometers to determine level, but I
don't think you could tell orientation, unless, perhaps, you can see
circumpolar stars? That is, by watching the movement of the
stars/planets through the field of view over some hours, could you
figure it out? Or is there some fundamental ambiguity.



I don't know about the iPhone, but I've seen an HTC with a funny
application that, when you point anywhere in the sky, it shows you the
constellations that are there. Even if you point it to ground, it shows
you the constellations in the other hemisphere :) I don't remember if
the application is this http://www.google.com/mobile/skymap/ or
something similar, but in any case, the phone knows its orientation
quite good (well... also depends on the phone to have the right time, of
course... :) )



yes, Pocket Universe (pUniverse) does this quite nicely (esp on the iPad)

But it uses the magnetic compass (and GPS) as well as orientation.

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Re: [time-nuts] finding time astronomically.

On Mon, Jan 23, 2012 at 1:08 PM, Jim Lux jim...@earthlink.net wrote:
 On 1/23/12 12:29 PM, Chris Albertson wrote:

 On Mon, Jan 23, 2012 at 12:02 PM, Jim Luxjim...@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.

This is why I sugested using the sun. It is easy.  I know fisrt hand
that using camera pointed upward requires months and years of effort
and it is unlirly you will find one person who knows enough to pull it
off as a solo effort.

But a wire or better a slit that sweeps an image across a photo diode
is far simpler.

Yes the sun is huge angular extent but you measure the entire light
curve and fit a function to the curve to find the center of the fuzzy
shadow.   Also you can collect data every clear day for years and over
time see how close you can get.  I bet pretty good.

You don't want a pin hole or you'd be adjusting the aim every day

To get better data you can have multiple slits so you get three or
five light curves, say 15 minutes apart every day.

The hard part will be the simple things like designing the
instrument so dirt and bird poop does not block the photocell or slit
and rain doe not get into the electronics.   And build it sturdy
enough that it can last outdoors in the sun and rain for many years
with zero maintenance and not cost much.




 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.


 ___
 time-nuts mailing list -- time-nuts@febo.com
 To unsubscribe, go to
 https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
 and follow the instructions there.



-- 

Chris Albertson
Redondo Beach, California

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Re: [time-nuts] finding time astronomically.

The atmospheric issue is more differential refraction, than refraction per
say. A zenith pointing camera is likely the best choice. The zenith is the
direction of the least atmospheric depth also.

-John

===


 I can think of two general scenarios here.
 If you planet has air you will need to know how it refracts st
 One is where you lay the iphone on the table in a fixed position.  One
 could use the internal accelerometers to determine level, but I don't
 think you could tell orientation, unless, perhaps, you can see
 circumpolar
 stars?  That is, by watching the movement of the stars/planets through
 the
 field of view over some hours, could you figure it out?  Or is there
 some
 fundamental ambiguity.

 No, you can point to any location and you can (in theory) figure out
 where it's pointing given that you have a large enough field of view
 to see many stars at the same time.   You can make a fixture easy
 enough, just some epoxy and a large boulder.  I used lag bolts
 onto my garage roof and it worked more than good enough.

 If you can choose, straight up is the best aim point.  Refraction is
 not much of an issue and there is less air to look through.  But
 looking at the equator means there is less field rotation and the data
 is easier to reduce.  We looked at the equator because we did not want
 to deal with image rotation.   Motion blur is minimize down there too.

 But if you want to know absolute time then you need more.  Looking
 at any random but fixed location will get you the period of the
 planet's ration to about a mSec with cheap equipment but to get
 absolute time you need to measure the aim point relative to the local
 meridian.   That is not as easy.  Star with a protrator and a plumb
 bob.That is what I used.   But to refine that you need a good
 source of time and for the purpose of this exercise we don't have
 that.  Only the plumb bob which means a few seconds of error.  maybe
 an precision level can do 10X better?

 (obviously, you can trivially see the moon/sun)

 The other scenario is where you get an inexpensive camera (webcam, or
 perhaps some slightly better point and shoot) and build a precision
 mount
 (so you DO have accurate knowledge of sensor orientation and position)
 Could
 you, perhaps over time, do an insitu calibration?

 I suppose any of these techniques is going to have issues with the
 uncertainty in when the image is actually captured (e.g. there's
 probably
 10-100 ms you're not going to get away from).




 ___
 time-nuts mailing list -- time-nuts@febo.com
 To unsubscribe, go to
 https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
 and follow the instructions there.



 --

 Chris Albertson
 Redondo Beach, California

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Re: [time-nuts] finding time astronomically.


On 1/23/12 1:20 PM, Doug Millar wrote:

A mercury mirror is better than a plumb bob.
   Doug


Or Gallium?

But what sort of precision are we looking for here?

1 second of earth rotation is 1/240th degree (15 arc seconds), about 
0.07 milliradian.


So on a plumb bob a meter long, you're looking for a displacement of 
0.07 mm... Seems a bit challenging.


Even with an optical scheme looking for the reflection coming back from 
your mirror a meter away, that's just 70 microns..  Well, at least it's 
not a few wavelengths of light.


But I can see a lot of practical problems at that level of precision:

Vibration isolation?
Local gravitational anomalies.  (I seem to recall tens of arcseconds for 
this)


difference between local gravity vector and normal of the ellipsoid or 
geoid due to non spherical earth, etc.  (this one is calculatable)











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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Poul-Henning Kamp

In message 4f1dd9d3.6050...@earthlink.net, Jim Lux writes:
On 1/23/12 1:20 PM, Doug Millar wrote:

 A mercury mirror is better than a plumb bob.

But not much.

Both of them are subject to aberations of the local gravity vector
(any mountains, valleys near by ?) and in the case of a rotating
liquid metal mirror to a lesser degree to interaction with the earths
magnetic field.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Brooke Clarke


Hi Chris:

I would say you want an optimum hole diameter for imaging the Sun.
Sort of like the f/100 school of photography.

For a few years I drove brass tacks into a hardwood floor at exactly noon where the tack was placed at the center of the 
Sun's image using 3x5 cards with nested ellipsis of different sizes with a small hole in the centers.  I choose the pin 
hole diameter that was slightly larger than the hole size needed for good overall focus.  If the hole is smaller than 
needed for good focus you are getting a much dimmer image and much larger and the image gets fuzzy.  For this 
application maybe a hole somewhat larger that still has the same peak intensity as the in focus hole.


Another idea would be to use a photo sensor to read the spots from a 
Dipleidscope.
http://www.prc68.com/I/Dent.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html


Chris Albertson wrote:
. . .


You don't want a pin hole or you'd be adjusting the aim every day


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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Don Latham

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 Luxjim...@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
 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
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com



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Re: [time-nuts] finding time astronomically , Part 2

2012-01-23 Thread Brooke Clarke

Hi Jim:

I spent quite some time on looking at ways to optically tell the time, see:
http://www.prc68.com/I/StellarTime.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html

Jim Lux wrote:

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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Re: [time-nuts] finding time astronomically , Part 2


On 1/23/12 3:27 PM, Brooke Clarke wrote:

Hi Jim:

I spent quite some time on looking at ways to optically tell the time, see:
http://www.prc68.com/I/StellarTime.shtml




Oddly, I was *just* looking at that page...I mean, I closed the browser 
and opened mail and saw your email.



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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Mike S


On 1/23/2012 3:02 PM, Jim Lux wrote:

How well could you do with something like the camera in the iPhone4
facing up. The front camera is VGA resolution


A lower bound can be estimated.

A cell phone (iPhone 4 rear camera) camera sensor has a resolution of 
what? ~2600 pixels wide with a 45 degree field of view - that's ~ 60 arc 
seconds per pixel, which is about 4 seconds of time. The Dawes limit is 
about 1 second (17 arc-seconds) for a perfect .25 lens. Obviously worse 
with a VGA resolution camera.


Can such a camera even see stars?

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Re: [time-nuts] finding time astronomically.

I think you'd want a slit, not a pin hole.  The pin hole would be
better but it would only work one day a year.  And it could be plugged
up.

I'm thinking the best way to build this might be to paint a sheet of
glass after masking out a very thin strip with vinyl tape.  Face the
uncoated side to the sun. .  The glass would keep dirt and water out.
Aim it at the ecliptic and surround the glass with bird spikes.  Maybe
use a filter to reduce skylight but let IR in.  To make the blue sky
look more black.

I think the optimum width of the slit, or pinhole diameter to make it
match the width of the photo detector.   Making it wider does not put
more light on the detector.   The geometry when give you a nice raise
and fall.  You could place a full column of photo diodes in back of
the slit


On Mon, Jan 23, 2012 at 3:10 PM, Brooke Clarke bro...@pacific.net wrote:
 Hi Chris:

 I would say you want an optimum hole diameter for imaging the Sun.
 Sort of like the f/100 school of photography.

 For a few years I drove brass tacks into a hardwood floor at exactly noon
 where the tack was placed at the center of the Sun's image using 3x5 cards
 with nested ellipsis of different sizes with a small hole in the centers.  I
 choose the pin hole diameter that was slightly larger than the hole size
 needed for good overall focus.  If the hole is smaller than needed for good
 focus you are getting a much dimmer image and much larger and the image gets
 fuzzy.  For this application maybe a hole somewhat larger that still has the
 same peak intensity as the in focus hole.

 Another idea would be to use a photo sensor to read the spots from a
 Dipleidscope.
 http://www.prc68.com/I/Dent.shtml

 Have Fun,

 Brooke Clarke
 http://www.PRC68.com
 http://www.end2partygovernment.com/Brooke4Congress.html


 Chris Albertson wrote:
 . . .


 You don't want a pin hole or you'd be adjusting the aim every day


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-- 

Chris Albertson
Redondo Beach, California

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Re: [time-nuts] finding time astronomically.

On Mon, Jan 23, 2012 at 3:51 PM, Mike S mi...@flatsurface.com wrote:
 On 1/23/2012 3:02 PM, Jim Lux wrote:

 How well could you do with something like the camera in the iPhone4
 facing up. The front camera is VGA resolution


 A lower bound can be estimated.

 A cell phone (iPhone 4 rear camera) camera sensor has a resolution of what?
 ~2600 pixels wide with a 45 degree field of view - that's ~ 60 arc seconds
 per pixel, which is about 4 seconds of time. The Dawes limit is about 1
 second (17 arc-seconds) for a perfect .25 lens. Obviously worse with a VGA
 resolution camera.

The goal is not to create an image.   A blur is actually better and
I've read of people intentionally using de-focus.   What you do in
compute a best fit of the system point spead function (PSF).   Or with
many blobs in the field you do a convolution of the image with the
system PSF.

The end product is not an image but a table of X,Y coordinates of each
detected star.You don't need to detect every star.   Then you
search a star catalog and find thebest fit transformation matric that
takes you from  X,Y to the catalog.   The matric is your real
product.

Typically you should expect about 1/10 of a pixel resolution at the
end.   And then you take hundreds of images every night and average
them and you continue maybe for years.

If you were designing a camera for this purpose you make it so that a
typical star would cover maybe five pixels across so that the 5 by 5
pixel subimage would look like a Gaussian function.  The centroid of
the function is your X,Y for the star.   So you see that even with 5
pixel blurs you can likely find X,Y to much better than one pixel
width. This helps with noise too, noise would be a poor fit to a
2D  Gaussian function.  (and also there would be no catalog star for a
noise hit)





Chris Albertson
Redondo Beach, California

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Re: [time-nuts] finding time astronomically , Part 2

You might be able to track the reddish stars both night and day. If you
put a dark red filter in front of the sensor, it will get rid of much of
the sky. The sky and stars are very different optically, the former is an
area source the latter a point source. The energy from the sky varies
directly with the area being observed, the latter does not.

-John

=


 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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Re: [time-nuts] finding time astronomically.

 I think you'd want a slit, not a pin hole.  The pin hole would be
 better but it would only work one day a year.

Actually two days per year, unless it was adjusted for the summer or
winter solstice, then it'd be one.

-John

===




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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Hal Murray


albertson.ch...@gmail.com said:
 Actually two days per year, unless it was adjusted for the
 summer or winter solstice, then it'd be one.

 I still think it is one.  because there are not an integer number of days
 per year so you don't get and exact repeat in 6 months.   Maybe a pin hole
 would only work once ever?  I don't know.  To work the pinhole has to
 exactly line up with the detector at the exact same time of day. 

It could be zero or many, depending on the field of view.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.




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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Bill Hawkins

It might be useful to determine the rate of the sun's movement at
the ends of the analemma.

There is a passage grave north of Dublin, Ireland, that has a long
passage from a shadow box above the entrance to a spiral carving on
the rear wall. Light shines on the carving at the winter solstice.

The waiting list to see this event fills up with New Agers about a
year before the event. I asked our guide if that wasn't very hard
on people who could only see the event on one day if that day was
cloudy. Oh, no, she said. The event happens for 3-4 days on
either side of the solstice.

Of course, a passage grave is not the same as a shadow cast by a
fine wire on a microscope. It might take a few years to locate it
properly.

Are there any timenuts that want to be buried in a passage grave?

Bill Hawkins


-Original Message-
From: Chris Albertson
Sent: Monday, January 23, 2012 8:40 PM

On Mon, Jan 23, 2012 at 6:07 PM, J. Forster j...@quikus.com wrote:
 I think you'd want a slit, not a pin hole.  The pin hole would be
 better but it would only work one day a year.

 Actually two days per year, unless it was adjusted for the summer or
 winter solstice, then it'd be one.

I still think it is one.  because there are not an integer number of
days per year so you don't get and exact repeat in 6 months.   Maybe a
pin hole would only work once ever?  I don't know.  To work the
pinhole has to exactly line up with the detector at the exact same
time of day.

But I'm not liking slits either because I can't see how to adjust them
to exact vertical.

I'm back to the first thing I thought of,  a wire with a large weight.
 Then you measure the light curve as shadow of the wire sweeps over
the detector.



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Re: [time-nuts] finding time astronomically.

At the solstices, the derivative of the declination goes through just
about to zero, just like a sine wave.

-John

==


 It might be useful to determine the rate of the sun's movement at
 the ends of the analemma.

 There is a passage grave north of Dublin, Ireland, that has a long
 passage from a shadow box above the entrance to a spiral carving on
 the rear wall. Light shines on the carving at the winter solstice.

 The waiting list to see this event fills up with New Agers about a
 year before the event. I asked our guide if that wasn't very hard
 on people who could only see the event on one day if that day was
 cloudy. Oh, no, she said. The event happens for 3-4 days on
 either side of the solstice.

 Of course, a passage grave is not the same as a shadow cast by a
 fine wire on a microscope. It might take a few years to locate it
 properly.

 Are there any timenuts that want to be buried in a passage grave?

 Bill Hawkins


 -Original Message-
 From: Chris Albertson
 Sent: Monday, January 23, 2012 8:40 PM

 On Mon, Jan 23, 2012 at 6:07 PM, J. Forster j...@quikus.com wrote:
 I think you'd want a slit, not a pin hole.  The pin hole would be
 better but it would only work one day a year.

 Actually two days per year, unless it was adjusted for the summer or
 winter solstice, then it'd be one.

 I still think it is one.  because there are not an integer number of
 days per year so you don't get and exact repeat in 6 months.   Maybe a
 pin hole would only work once ever?  I don't know.  To work the
 pinhole has to exactly line up with the detector at the exact same
 time of day.

 But I'm not liking slits either because I can't see how to adjust them
 to exact vertical.

 I'm back to the first thing I thought of,  a wire with a large weight.
  Then you measure the light curve as shadow of the wire sweeps over
 the detector.



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Re: [time-nuts] finding time astronomically.

2012-01-23 Thread Neville Michie

For those unfamiliar with horology,  look at the Wikipedia under  
equation of time
This is the relationship between solar time and the average or mean  
time.
It is mainly the sum of two sine functions, one of 6 months frequency  
the other

one year. Amplitude 16 - 17 minutes.
This function allows a mean time clock to show the time of noon each  
day (passage of the Sun

through the meridian)
cheers,
Neville Michie





On 24/01/2012, at 3:59 PM, J. Forster wrote:


At the solstices, the derivative of the declination goes through just
about to zero, just like a sine wave.

-John

==



It might be useful to determine the rate of the sun's movement at
the ends of the analemma.

There is a passage grave north of Dublin, Ireland, that has a long
passage from a shadow box above the entrance to a spiral carving on
the rear wall. Light shines on the carving at the winter solstice.

The waiting list to see this event fills up with New Agers about a
year before the event. I asked our guide if that wasn't very hard
on people who could only see the event on one day if that day was
cloudy. Oh, no, she said. The event happens for 3-4 days on
either side of the solstice.

Of course, a passage grave is not the same as a shadow cast by a
fine wire on a microscope. It might take a few years to locate it
properly.

Are there any timenuts that want to be buried in a passage grave?

Bill Hawkins


-Original Message-
From: Chris Albertson
Sent: Monday, January 23, 2012 8:40 PM

On Mon, Jan 23, 2012 at 6:07 PM, J. Forster j...@quikus.com wrote:

I think you'd want a slit, not a pin hole.  The pin hole would be
better but it would only work one day a year.


Actually two days per year, unless it was adjusted for the summer or
winter solstice, then it'd be one.


I still think it is one.  because there are not an integer  
number of
days per year so you don't get and exact repeat in 6 months.
Maybe a

pin hole would only work once ever?  I don't know.  To work the
pinhole has to exactly line up with the detector at the exact same
time of day.

But I'm not liking slits either because I can't see how to adjust  
them

to exact vertical.

I'm back to the first thing I thought of,  a wire with a large  
weight.

 Then you measure the light curve as shadow of the wire sweeps over
the detector.



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time-nuts

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Re: [time-nuts] finding time astronomically.