Parallax

I'm sure that most regulars here could give a short explanation of the basic method whereby stellar parallaxes were determined in the past century or so suing ground based (or other) telescopes.

However, at University of London Observatory we are in the midst of sending our old plate archive to secure storage. These are plates from the Radcliffe Observatory parallax programme starting shortly after they got the 24-inch telescope [now at London] up and running in the first decade of the 20th century.

http://www.ulo.ucl.ac.uk/telescopes/radcliffe/

What is remarkable is the technique actually used. The plates were exposed as follows, as shown by our inspection and the log books:

Observation 1: three images, displaced by 30 arcsec in dec, over a few minutes, with selected parallax candidate near the centre.

Put the undeveloped plate away for six months, stored VERY carefully.

Observation 2 and 3: On the same night six months later, take two sets of three images as above on the same plate, but displaced by a few seconds of time in RA.

Put the undeveloped plate away again.

Observation 4: Six months on, take another three exposures, displaced in RA again. Then move a bit more, take a couple of shorter exposures so we can tell which set was the fourth.

Then develop the plate, and measure the relative displacements vs other stars in the field and compare to expected parallax displacements. You get, in effect, two independent pairs of data point sets for the parallax.

Think of the effort and patience required, and the care required to preserve plates safely for a year...

We are scanning some examples for our archives. Maybe remeasurement of digital scans could be an interesting student project...

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Mike Dworetsky

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