[time-nuts] Determination of the placement of the first pps

Chris Albertson albertson.chris at gmail.com
Mon Jan 23 12:54:51 EST 2012

On Mon, Jan 23, 2012 at 8:56 AM, Attila Kinali <attila at kinali.ch> wrote:
> On Mon, 23 Jan 2012 08:43:01 -0800
> Chris Albertson <albertson.chris at gmail.com> wrote:
>> It's a transit telescope (one that looks up at the local meridian) but
>> they don't use the sun.  It looks at every start that passes in front
>> of it, thousands of them every night.  Then they reduce the data by
>> knowing the exact location of every star and the time is pasted
>> directly overhead.  This way they get thousands of measurements every
>> day.  If you use the Sun you get only one per day.
> Not to mention the solar noon varies by +/- 15min over the year
> (don't ask me how this is called...old knowledge from my high school days)
>> A transit solar scope would be a fun Time Nut project.  I think a
>> primitive one would be a photo cell and a length of wire.  Measure the
>> time when the shadow of the wire sweeps across the cell.  The trouble
>> is that with only one measurements per day it would time years to
>> build up uSec level data.    Even if you placed the cell behind a pin
>> hold mask and used a fine piano wire, shades and baffles and so on.
> Why not use a real telescope with a CCD and get more reliable data?
> A friend of mine is into hobby astronomy and uses special type CCDs
> for it (ie not even the hobbist look directly into the telescope anymore).
> I guess it shouldn't be too hard to rig up some gear, a PC that collects
> the pictures at precise instances and calculates the ephemeris time to
> UTC/GPS difference. The only issue is that you'd need to put that telescope
> somewhere with little light polution, otherwise you'll only get the brightest
> stars.


I did this a while back.  It is not as easy as it sounds.  No it can
run from a house in the city.  Even very small telescope will be able
to see hundreds of stars.   But it is NOT as easy as you might thing.
first yu need to know _exactly_ where the scope is pointed and and
which stars are recorded on your CCD and unless you work full time
this has to be automated.  I was part of a group that did this.  We
used lenses from 35mm film camera as "telescopes".  We got a "group
buy" on a load of 135mm f/2.8 lenses.

Here is an example taken with a 135mm f/2.8 camera lens and custom
CCD.   This si a "worst case" image taken from the city.  Bright stars
are a problem because they saturate the CCD and and break the
centrioding algorithum. (we fit a 2D Gaussian "pint spead function"
the the image of each star and get near micro arc second resolution)

The 'scope was aimed at a fixed point in the sky using a solid
non-tracking mount as these are by far the easiest to build.   But
unless you track the sky the image of the stars move on the ccd and
blur.   So we moved the CCD.  Or I should say we moved the image
electronically by shifting the charge the match the moving image,
electronic image stabilization.       It works and a dozen or more of
thee were run by the group.   But when you get into the fine details
you see that atmospheric refraction (and other effects) is wavelength
dependent so now you need to measure color so you need multiple
cameras and calibrated color filters and the math involves matrix

A wire and a photocell is something you could set up, and yes the
solar day is not constant because we orbit the sun in an elliptical
>                                Attila Kinali
> --
> The trouble with you, Shev, is you don't say anything until you've saved
> up a whole truckload of damned heavy brick arguments and then you dump
> them all out and never look at the bleeding body mangled beneath the heap
>                -- Tirin, The Dispossessed, U. Le Guin
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Chris Albertson
Redondo Beach, California

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