[time-nuts] Measuring sidereal/solar time?

Bob Camp kb8tq at n1k.org
Fri Dec 30 17:40:48 EST 2016


> On Dec 30, 2016, at 4:53 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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
> gears.
> 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.

If by 3D print you mean a filament machine, pretty much all of the useful
filaments will soffen / deform / melt if left in the hot sun for a day. The few
that are more temperature tolerant require an unusual printer to work with 
and are a real pain to work with. 

One alternative would be a 3D printed mould and a metal casting process. 
It would take more than a few steps to get to metal. You also would need
to clean up the casting before it was of any use. 


>   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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