[time-nuts] Timed photography (was: Re: A silly question ...)

Tom Van Baak tvb at LeapSecond.com
Mon Oct 1 06:09:38 EDT 2018


> Tom - I love the idea of taking (a series of) accurately timed photograph(s), but have no idea
> how to go about it - can you please explain how you did it for your mains-driven clock?

Hi Peter,

During that year I collected lots of mains timing data. There are many ways to do that; in my case I measured the precise time of every 60th zero-crossing against my house standard [0]. From that you can make any number of graphs of accumulated mains time error over a day, or month, or year. Or make histogram plots of mains frequency distribution. Or ADEV plots, etc. And while those plots are all technically accurate, they don't emotionally convey the live jitter and wander that ones sees in a mains sinewave. So that's when I came up with the goal to make a short time-lapse movie showing how changes in mains frequency affect a typical wall clock.

The method was rather crude. I used a tripod and webcam so that each frame showed both the mains-drive 12h analog clock in the center and a cesium-driven 24h digital clock in the lower left. The window shade in the background indicated if it was day or night (IIRC that was accidental but it was a nice touch). To capture an image exactly every 15 minutes, I wrote a PC script that included a precise timer and URL fetch. It was probably good to 0.1 second of UTC. I then used an ancient MS program called GIFAnimator.exe to assemble the individual images into the 1/4 second per frame movie.

You could do the same today using a mix of RPi, NTP, cron, wget/curl, etc. Or there's probably some cool Python library that does webcam time lapse movies in one line of code. In my case I used an old Windows XP laptop; the precise timer was alarm.exe [1], the web fetch tool was url.exe [2], and instead of NTP the PC jam syncs [3] to NIST every 5 minutes. I wouldn't recommend using these fragile play tools, but they're out there in my tools directory if you want to have a look.

/tvb

[0] By "house standard" I mean whatever your best 10 MHz / 1PPS source is at home; Rb, Cs, GPSDO, etc.
[1] http://leapsecond.com/tools/alarm.c (alarm.exe)
[2] http://leapsecond.com/tools/url.c (url.exe)
[3] http://leapsecond.com/tools/daytime.c (daytime.exe)



----- Original Message ----- 
From: Peter Vince 
To: Tom Van Baak ; time-nuts at lists.febo.com 
Sent: Monday, October 01, 2018 2:14 AM
Subject: Timed photography (was: Re: [time-nuts] A silly question ...)


Tom - I love the idea of taking (a series of) accurately timed photograph(s), but have no idea how to go about it - can you please explain how you did it for your mains-driven clock?


   Peter (London)


On Mon, 1 Oct 2018 at 09:39, Tom Van Baak <tvb at leapsecond.com> wrote:
...
> For hourly photos consider using a webcam or smart phone & time-lapse photo app.
>
> Here's an example of measuring time drift in mains with a photo every 15 minutes. Watch how the red seconds hand drifts by up to 4 seconds during the day: http://leapsecond.com/pages/tec/mains-clock-ani.gif




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