Re: precession of mercury

On Dec 20, 5:18 pm, "Paul B. Andersen"
<paul.b.ander...@xxxxxxxxxxxxxxx> wrote:
Dr. Henri Wilson wrote:

You don't know what you are talking about.
Because Hipparcos orbits in the ecliptic plane and does not spin around a
tilted axis like the Earth, it is insensitive to stellar aberration altogether.

Henri, your ignorance of what stellar aberration is, is long since
thoroughly documented, you don't have to repeat it over and over ad nauseum!

Why don't you learn what it is in stead of making
a fool of yourself over and over again?

 > It might detect a very small diurnal aberration that is easy to correct out.

Indeed a satellite in geostationary orbit will experience a diurnal
component to stellar aberration.
If you had known what stellar aberration is, you would also
have known that the semi-major axis in this ellipse is u/c where
u ~= 3km/s, u/c = 1E-5 rad = 2".
It is but a small perturbation of the ten times bigger ellipse.
The stellar aberration ellipse will have 365 small 'wave periods'
along it circumference.

However, since this aberration changes much faster than
the annual aberration, it has indeed to be compensated for,
since it will change significantly during an exposure time.

This is a very important compensation in the HST, because it is
in low orbit so the aberration due to it's movement around the Earth
is (more than) twice as big in amplitude, and change very fast,
and the exposure times are often very long. So the telescope
has to swing back and forth with an amplitude of ~7" during 96 minutes.
This is achieved by a feedback control system steering reaction wheels.
The HST is locked on the target with a precision of 0.01".

However, since the minor-axis in the aberration ellipse depend on
the angle from the orbital plane, the aberration of the stars
within the field will be (slightly) different. This is a problem
since all the stars can't be kept at the same pixel all the time.
A primary target is chosen, and an error of a couple of pixels
has to be accepted for other stars far out in the field.

I will guess a similar technique was used in Hipparcos.
But maybe not.
Does Jerry know?

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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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
.

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