[time-nuts] reply re Harrison's timing method - #13 in Vol 176, Issue 44 digest

[Bruce Griffiths]

In principle one could time tag each individual photon with subnanosecond resolution.

Bruce
> On 29 March 2019 at 09:00 Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> 
> --------
> In message <236772484.9174006.1553757616865 at webmail.xtra.co.nz>, Bruce Griffith
> s writes:
> 
> >However when used with a CCD camera or equivalent the accuracy
> >should improve somewhat much as adding a TV camera to a transit
> >circle improved its accuracy.
> 
> You know ... there *is* an official time-nut way to do this.
> 
> You want is a chevron shaped 'Høg grid' because that is
> objectively a very, very, very smart way of converting precise
> time to precise geometry.
> 
> I don't know of any popular explanations, but look at page 10 here:
> 
> 	https://www.hs.uni-hamburg.de/DE/Ins/Bib/AG2012AK1.pdf
> 
> The illustration on page 10 shows the original concept (from 1925!):
> 
> By modulating the starlight with a non-uniform pattern, and sampling
> the modulated light at high rate, the transit time of the star can
> be determined on the order of the sampling frequency.
> 
> Notice that the photon detector does not need high geometric resolution,
> I belive Strømberg, 17 year old at the time, used a simple photo-cell
> or possibly a photo-multiplier.
> 
> Now, if you want to measure both coordinates, you move to the chevron
> shaped grid illustrated on page 11, the "Høg grid".
> 
> You still get a precise measurement of the transit along the logitudal
> axis, but the width of the signal now also tells you where the star
> was on the transverse axis.
> 
> This is how the Perth 1970 catalog was made, and if not for a loose
> bolt, it would have been the most precise catalog on both axis instead
> of just one axis.
> 
> The Høg grid still leaves rotation as source of error, so look at
> page 2 here:
> 
> 	https://pdfs.semanticscholar.org/27f4/16df19441874fcd3b1bf52c477c889ca8045.pdf
> 
> Imagine the light-curve you get when a star transits that slit system
> in various directions, including, crucially, with a rotation[1].
> 
> About 12 years ago I did some ad-hoc experiements on my 5" telescope,
> with various simple slit geometries, and it works a treat.
> 
> I made the slits by taping mylar tape on a neutral filter, and cut
> slits with a scalpel and a steel ruler, the detector was a large
> area PIN photo-diode from the junk box and a digital oscilloscope.
> 
> While you can prove the concept, as I did, with portable tripod
> mount, to get usable data you have to bolt the telecope to a cubic
> meter of concrete or bedrock.
> 
> Poul-Henning
> 
> [1] This becaue very important for the Hipparcos satelite which a
> rocket failure left stranded in the parking orbit ... but they still
> completed their science objectives.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.