On Fri, Dec 30, 2016 at 2:42 AM, Attila Kinali <att...@kinali.ch> wrote: > On Fri, 30 Dec 2016 10:59:03 +0200 > Anders Wallin <anders.e.e.wal...@gmail.com> wrote: > >> out of curiosity, are there any amateur/semi-pro experiments that can >> measure the length of the solar or sidereal day to sub-millisecond >> resolution?
Yes. It is not hard at all to measure the Earth's rotational period, if all you needs is "sub millisecond" It would get harder if you cared about nanoseconds. I worked on an amateur project with some others and while measuring the Earth was not the goal we had to know the Earth's rational period to do the work. The project was about stellar photometry. But I leave that part out..... Basically what we did was mount a camera made out of a small CCD sensor and a 135mm f/2.8 camera lens salvaged from an old 35mm film camera. The camera was fixed to the roof of my garage. (This was THE big cost saving feature: The camera could not move. The mount as fixed at one location in the sky forever, right at the equator) I placed it in one end of a long wood crate and it looked up at the equator through a square hole on the upper end of the box. The box provided some protection from the elements and provided a lot of light shielding. To measure Earth's rotation all you need to do in know exactly when you took an image and to have a GOOD catalog of star locations. Let's say your image captures 200 stars. They are rather blurry and each covers maybe 5 pixels but even so you compute the centroid of each "gaussian blob" and then do a least squares fit of all those centroids to the astrometric catalog. The catalog is "good" to several milliacrseconds and with hundreds of centroids you can figure out were the camera was printed to a few "MAS" (Milli Arc Seconds). We took many images every clear night for several years. Hardware cost today is "not much" and you can use salvaged camera equipment Almost all of the software is available for free. Certainly matching stares to catalog images is. Yes the lens has geometric distortion and the CCD is likely not exactly 90 degrees to the optical axis but the software models this. This is possible because millions of star positions are known to insane levels of accuracy and if they appear in the "wrong" place in your image you can bet the cause is geometric distortion in your camera, especially after seeing the same error in hundreds of images. We used narrow filters to limit the image to just one "color" so the chromatic aberration in the optics i not an issue, filters are cheap. As part of our processing we time-tagged each image and also recored where the optical xis was pointed at. So you'd need a small telescope or big camera lens and a camera that can be triggered by a computer and software. Not really expensive. I'd invest in the best used optics you can and get a monochrome camera. Some people in the past century used transit telescopes to manually measure the time a star crossed a hairline in an eyepiece. Then the next night to observe the same star again. Now you know the length of the day (after you reduce the data) Put you can measure a dozen stars every night and take an average. In concept it is very simple. But today we can measure a tens of thousands of stars per day from a suburban roof top. Almost all other methods of measuring the Earth's rotation do not collect enough data. You need tens or hundreds of thousands of data points. if you want to know the sidereal period to Time Nut standards -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.