[time-nuts] Measuring sidereal/solar time?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 30 05:59:52 EST 2016

Lookup "Stellar compass" as used for determining space probe attitude.Can also be used to determine the direction of the centre of an image of a field of bright stars.Subarcsecond accuracy is fairly routine.Pattern recognition techniques combined with measures of the relative brightness of the stars is used to identify them.Subpixel accuracy in determining the location of the stellar image centroids is also routine. 
There is at least one US PhD thesis on such stellar compass techniques.A stellar compass technique has been used to determine the pointing direction of small portable telescopes without requiring precision axis encoders etc.

    On Friday, 30 December 2016 11:43 PM, Attila Kinali <attila at kinali.ch> wrote:

 On Fri, 30 Dec 2016 10:59:03 +0200
Anders Wallin <anders.e.e.wallin at gmail.com> wrote:

> out of curiosity, are there any amateur/semi-pro experiments that can
> measure the length of the solar or sidereal day to sub-millisecond
> resolution?
> To reproduce data like this:
> https://upload.wikimedia.org/wikipedia/commons/5/5b/Deviation_of_day_length_from_SI_day.svg
> Something in the sky that goes "ping" every day - detected with a pointing
> accuracy of < 1ms/24h or <0.01 arc-seconds (!?). Or perhaps two
> satellite-dishes pointed at the sun and noise-correlation/interferometry??

I don't know of any such experiment already performed, but I am not up
to date on what's going on in the hobby astronomy community.

I am not sure whether sub-milisecond resolution is feasible, but
I think the "easiest" method would be to do a "modern" version of
an meridian telescope:

Using a camera fix mounted (ie not moving and if possible vibration isolated)
on a pedestal pointed at the sky, approximately looking south. A simple
webcam would be probably enough for first experiments, as long as you get
a good picture of the stars. A good compact camera which allows to use
a remote shutter with a proper lens and exposure control should be better.
Probably the best resource here are the people/websites that deal with
book scanning, as they tend to automate the whole picture taking process.
Using magic lantern (http://magiclantern.fm) with Canon cameras might
give additional features needed for the task.

>From the pictures taken, calculate the positions of the stars (by fitting
circles onto the bright pixels) and figure out which star is which (using
astronomical list of stars). For this step there is a plethora of open source
astronomical software available, but I don't know how well they fit the task
of figuring out what the position of the stars relative to the camera reference
frame. After that, it's just some simple math of calculating the difference
between the position of the stars and where you would have expecteded them
at the time when the picture has been taken.

Some usefull software projects are:


            Attila Kinali

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