Re: precession of mercury

On Dec 22, 3:50 pm, "Paul B. Andersen"
<paul.b.ander...@xxxxxxxxxxxxxxx> wrote:
Jerry wrote:

Hipparcos was designed for performing precise astrometry. Its
telescope was not designed to take images.

The 1 degree square field of the telescope (actually a split
screen facing two segments 58 degrees apart, but that's another
story) was masked by a grid of alternating opaque and transparent
bands with a period of 8.2 microns or 1.208", aligned precisely
parallel to the rotational axis of the satellite. The satellite
rotated slowly, about once every two hours. One object at a time
would be selected for monitoring. As the object crossed the bands,
it would generate a 139.7 Hz pulse train signal, and 20 minutes
later it would be monitored again. The phase of this pulse train
would be compared with the phase of pulse trains created by other
objects crossing the telescope field. In this way, the precise
angular relation between all the target objects in the area swept
out by the rotating telescope could be ascertained.

| | | | | | | | | | |
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| | | | | |*| | | | |
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| | | | | | | | | | |
| | | |*| | | | | | |
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| | | | | | |*| | | |

The rotational axis of Hipparcos was controlled so that it was
aligned at 43 degrees with respect to the Sun and precessed 6.4
rev/year. In this way, the entire sky would be covered multiple
times per year.

The highly eccentric orbit in which Hipparcos had been left by
failure of the apogee boost engine necessitated more accurate
monitoring of the orbit than originally planned. Nevertheless,
Hipparcos exceeded its design goal of measuring its 100,000
primary target objects to 0.002". The 120,000 stars in the
Hipparcos catalog are measured with median accuracy of better
than 0.001".

A second photomultiplier system designed for high accuracy
magnitude measurements in the B and V bands accessed the
telescope image via a beam splitter. The Tycho catalog contains
photometric B-V and astrometric measurements for over 1 million
stars measured to 0.03", and the Tycho-2 catalog contains B-V and
astrometric measurements for an additional 2.5 million stars.

Jerry

Not quite what I asked for, but an indirect answer.
Since the stars were drifting past the field as the measurements
were done, and the angular distance between pair of stars approx
58 degrees apart were measured, the correction for the different
stellar aberration could be done when the data were analysed.
The difference in stellar aberration due to the combined motions
of the spacecraft around the Earth and the Earth around the Sun
for two stars 58 degrees apart can be considerable.

BTW, have you seen this?
http://www.rssd.esa.int/SA/HIPPARCOS/docs/vol2_all.pdf

Thanks! It appears from the above that I made a bit of an error
in my description... the two images separated by 58 degrees were
not focused on a "split screen" as I stated, but were directed
towards the same -focal- surface. This allowed near-simultaneous
position comparisons with both nearby and distant objects, which
of course would provide opportunity for much increased accuracy
in the measurements.

Jerry
.

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