[time-nuts] Re Danjon Astrolabe

Robert Atkinson robert.atkinson at genetix.com
Fri Sep 29 03:27:50 EDT 2006

How about pinching an idea from the early radar and missile technologies
- Conical Scanning. Basically you offset the detector of feed antenna
from the point of focus and then rotate it. If the signal is off centre
you get sinusoidal modulation of the signal. The phase of the modulation
tells the antenna steering what direction to move to get back on target.
A practical solution is to angle and spin the secondary mirror of a
reflecting (e.g. cassegrain) telescope. A index sensor gives you your
phase reference. 

Robert G8RPI.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Dr Bruce Griffiths
Sent: 29 September 2006 01:09
To: Tom Van Baak; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Re Danjon Astrolabe

Tom Van Baak wrote:
>> The scheme probably needs three photocells to be sure that the one
>> in the middle is darker than the others. Might be able to mask it
>> with a slit and use a fine wire gnomon, in a coarse/fine servo.
>> Could use a variable frequency motor and precision reduction, like
>> a phonograph turntable only much slower.
Whilst the resolution may be good, the accuracy of an open loop 
microstepped stepper motor isn't that great.
Its usually worse than when not using microstepping.
Variations in friction torque on the motor will also dramatically affect

its positioning accuracy.
A high resolution position encoder mounted on the sundial base is 
essential if you need to accurately determine its direction.
Servomotors with encoder feedback will achieve a much higher performance

than a stepper motor.
If gears or rollers are used then backlash in gears or microcreep in 
rollers will reduce the positioning accuracy.
The sundial base bearing runout can also affect positioning accuracy.

The equivalent time error is not likely to be much  smaller than a few 
seconds at best


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