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 <[email protected]>
wrote:
On Fri, 30 Dec 2016 10:59:03 +0200
Anders Wallin <[email protected]> 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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