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 > EFFECTIVE SOLUTIONS > > > > 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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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 >> >> _______________________________________________ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> -------------------------------------------------------- >> Any opinions expressed in this email are those of the individual and not >> necessarily Genetix Ltd (Genetix) or any company associated with it. 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Genetix will not be liable for >> direct, special, indirect or consequential damages as a result of any >> virus being passed on, or arising from alteration of the contents of this >> message by a third party. >> >> _______________________________________________ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> > > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > 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. Bruce
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