[time-nuts] Measuring sidereal/solar time?

Chris Albertson albertson.chris at gmail.com
Fri Dec 30 16:53:49 EST 2016

Yes, the below is basically correct.  But you save a ton of time and
get better results if you simply bolt the telescope down to the Earth
so that it can't move at all.   The aim point just needs to be "close"
and then later you determine where it is aimed.    If you are only
measuring period you don't need a surveyed location.   If measuring
absolutely time you do.

Using a fixed mount is what makes this affordable by amateurs.  Epoxy
the camera to a fixed masonry building.  This removes an unknown and
dramatically simplifies the processing and also saves most of the cost
witch is always the mechanical stuff.  One package of JB Weld epoxy
replaces thousands of dollars of motors and encoders and precision

With a fixed mount camera you have two kinds of "tine", that observed
by the camera and a second from your GPSDO.  If they diverge then you
deduce that it must be the Earth's rotation that changed.   But maybe
you wonder of maybe the camera moved or some effect you forgot to
remove.  So it is but to have some buddies running the same setup in
different cities around the world and check that you all see the same
results.   That is what we did.  It is FAR EASIER to do this kind of
replication when the setup is very inexpensive.

Today you could build a camera for a LOT less then we did.  I'm
thinking of a surplus used lens from a 35mm film camera.  A 250mm lens
or so and a 3D printed plastic part that holds this to a cheap point
and shoot camera.   We used epoxy to held the lens to the camera,  it
meant you'd never be abler to take it apart again but it was going on
a roof top, rain and all.

On Fri, Dec 30, 2016 at 3:16 AM, Ilia Platone <info at iliaplatone.com> wrote:
> Bruce,
> I think that you refer on prjects like Astrometry plate solving. I think one
> should got a reference to get a time reference instead of scope "pointing"
> reference, so, once one's got local coordinates in encoder positions, for
> example the values of the north pole with an alt/az mounting, can use a
> sub/arcsec plate solver to obtain good sidereal timing reference. using two
> encoders helps much.
> The problem can be visibility of the reference points, however.
> Best Regards,
> Ilia.
> On 12/30/16 10:59, Bruce Griffiths wrote:
>> Attila
>> 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.
>> Bruce
>>      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:
>> http://astro.corlan.net/gcx/
>> http://www.clearskyinstitute.com/xephem/
>> http://starlink.eao.hawaii.edu/starlink
>> http://astro.corlan.net/avsomat/index.html
>> http://rhodesmill.org/pyephem/
>> HTH
>>              Attila Kinali
> --
> Ilia Platone
> via Ferrara 54
> 47841
> Cattolica (RN), Italy
> Cell +39 349 1075999
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Chris Albertson
Redondo Beach, California

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