[time-nuts] Re Danjon Astrolabe

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Sep 29 11:02:02 EDT 2006

buehl wrote:
> 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 
> pixel.
> 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
>> Bruce
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Accuracy still won't be much better than1% of the solar diameter or 
about 1 second of time nowhere near the o.1 sec or better hoped for.

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