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

[Poul-Henning Kamp]

--------
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.