[time-nuts] Re Danjon Astrolabe

buehl buehl at superlink.net
Fri Sep 29 10:22:01 EDT 2006

Hi all:

Robert's "pinching an idea from the early - - " brings to mind what we did 
in the early days of video/ TV.  Hope this inspires some useful ideas.

To measure spot size on CRT, a small slit was placed against the TV or 
oscilloscope screen and a photo sensor measured the light intensity coming 
through the slit.  As the circular spot entered the slit, the change in 
light intensity increased; and as it left the slit light intensity 
decreased.  If the slit was wide, the photo detector output has a sine wave 
rising edge, a flat top, and a sine wave falling edge.  If the slit was 
very narrow, there is no flat top.

If we use the slit to watch the sun, we have the equivelent of a pinhole 
camera with only one direction of focus.  Making a structure with 2 
photodetectors, or 3 as TVB suggests, and carefully place such that one is 
on each edge of the image.  As one outputs a rising edge, the other outputs 
a falling edge.  In theory, when properly positioned, both photodetectors 
will have equal output when the sun image is exactly between 
photodetectors.  This is also the point of greatet rate of change.  This is 
equal to the inverse of the "fine wire gnomon" mentioned by TVB.

Simple detection would be using a comparitor or zero crossing 
detector.  You get one output as the sun approaches the center, and 
switches as the sun image crosses center.  Any offset or inacruacy would be 
identical day to day, so the time interval is repeatable.  This should be 
very simple to build.

As a computerized approach, each output could be converted with an A to D 
and mathematically analyzed.  The ultimate refinement would be a linear 
array, such as that used in a scanner.  If you can get the image to pass 
over a linear array with 4,000 pixels within 3 seconds, this would allow 
calculating time verses position at the rate of less than 1 millisecond per 

Accuracy could be increased by making the distance from the slit to the 
detector greater (increasing the size of the pinhole camera image and 
increasing the rate of travel across the sensor), or making the slit 
smaller, or making the detector diameter smaller.   Using a piece of 
fiberoptics connected to photodiode makes the detection diamater equal to 
the fiber.  I have had good results attaching one end of cheap plastic 
fiber to tip of photodiode with clear glue or epoxy.  Cheap plastic fiber 
like that used in decorative lamps, or experimentor type sold by Radio 
Shack, will also pass IR for short distances.

One source of inacuracy is when a sloud only shades one edge of the sun.  I 
suspect the IR image of the sun is more precise than the visible image, 
because defraction (tildel effect) is less when passing through clouds.

Since a slit provides only one direction of movement, proper angular 
positioning of the slit would minimise changes in the suns orbit from day 
to day, week to week.

Would like to hear comments from those amoung you having greater expertise 
in designing such a device.

Tom Buehl

At 03:27 AM 9/29/2006, you wrote:
>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.
> > SNIP
>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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