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 <att...@kinali.ch> wrote: On Fri, 30 Dec 2016 10:59:03 +0200 Anders Wallin <anders.e.e.wal...@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 -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ 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. _______________________________________________ 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.