Last update 2019 March 20

Accurate timing of observations is an absolute necessity. In FITS format the time from the PC clock is automatically inserted in to the FITS header.

For video observations of occultations or some other critical time-dependant phenomena, the time is inserted onto the frame from a GPS Video Time Inserter. Good PC clock time can also be achieved using planetary CCD cameras in SER recording mode using GPS disciplined NTP. These methods are accurate to 0.03s or better

There are a number of ways of obtaining accurate time signals in order to set the PC clock;
         – radio controlled clocks
         – the internet
         – Global Positioning System receiver

Observatory set-up

Richard Miles took some time to come up with an arrangement that worked. See his diagram. Bear in mind that technology has now moved on. Parallel port CCD cameras are a thing of the past, and all are USB2 and data can be saved to large disks or memory card. Apart from components, the configuration is more-or-less unchanged for FITS imaging.

With the addition of a video camera for occultation work, the GPS unit now inserts UT into the video stream before it is recorded. Then by using an analogue-to-digital converter on a USB port, the analogue video with frame time stamps (to 0.001s) is recorded to a drive with software such as Virtual Dub, without using NTP

The purposes of the various pieces of equipment are;
Skysensor 2000    Alignment and go-to (using RA and Dec coordinates of object to be observed/imaged)
GPS receiver         Provides correct time to PC clock.
CD-R drive            Storage of images for later processing

Imaging and tracking is performed using Astroart and Megastar software. All equipment is mains powered – batteries tend to die quickly especially in cold weather.

For asteroid imaging orbital elements of the object(s) to be imaged are input in to Megastar and the tracks generated and displayed for the time of observation. The track is compared with the MPC ephemeris obtained at the same time as the elements and the CCD image compared with the displayed Megastar field. This should ensure that I am in the right place at the right time – so far it has worked well.

A similar process is adopted for asteroidal occultations. In this case the field of view is being monitored in real-time. For asteroids and stars of similar brightness the star and asteroid will appear as a close “double” reducing in separation. The occultation coordinator uses SkyMap pro and EQASCOM with a SkyWatcher EQ6 Any ASCOM compatible software could be used. Examples include CdC and C2A. It is advantageous to check the software web page to see if the asteroid orbit files are obtainable.

PC time keeping
PC clock

I initially relied on the PC clock to provide correct time (automatically inserted in to the FITS header). However this tended to loose about 10 secs or more per image so, during an imaging session, became somewhat inaccurate. Please be aware that a number of CCD cameras do ‘freeze’ the PC clock during image download so continual correction of this clock is necessary for accurate astrometry.

The most recent operating systems (e.g W7 to 10) don’t suffer from this sort of time freezing, and USB3 cameras are becoming the norm. Never-the-less the PC clock still needs to be regulated to achieve UT accuracy in the 10ms range that may be needed for some observations. W10 is least effected by system interrupts.

Global Positioning System

One day a dormant brain cell went ‘ping’ and the idea of using a GPS receiver was born. Initial conversations with the supplier were both encouraging and discouraging. Yes – it would work via a serial – USB converter, and it did. No – you couldn’t automatically update the PC clock with it. Back to the WWW – a search turned up several pieces of software which would do just that. My final choice was a Garmin Etrex Venture receiver with a serial – USB converter and Tac32 software from CNS Systems. You still have to juggle things a bit with the PC clock to take account of BST but it all seems to be working quite well. You also need to allow a little time between images, I allow 10 secs, for the GPS receiver to update the PC clock. Test suggest that, using the GPS 1pps signal, the accuracy, at best, is 0.1secs.

A GPS controlled PC clock is now common to many users (2019) with accuracy of 0.03s or better. More adept observers are using RasPi kits to control the clock (referred to as “disciplining”) through the network port. There is also a commercial GPS solution via USB. This is the TimeBox designed by Dr. Cesar Valencia Gallardo.


Internet timing

A number of packages are listed below. The best accuracy one can expect from such software is probably of the order of +/- 0.03 secs.


Chronos is a freeware program that synchronises the PC clock with about 15 time servers. ‘Chronos time’ seems to be consistently about 0.5 to 0.6 secs behind UTC. Chronos is one of the more user-friendly internet timing applications.

This program may have improved, but a similar software is available from Meinberg


Dimension4 is a freeware program but, unlike Chronos, it synchs to just one time server. You select a time server by clicking on the Server however you appear to get no indication whether or not the server is off-line if the server is offline. Actual usage, for astrometric timing, shows Dimension4 to be accurate to, or slightly better than, 0.1 secs. If you look in the history file the corrections will indicate if they are applied or not.


AboutTime appears to use the same protocol as Dimension4 for getting time from a server. Its accuracy appears to be the same as Dimension4 – but there is no separate time display to actually check the clock accuracy..

AboutTime has been retired, as it is not compatible with 64 bit Widows and newer operating systems that have build-in time synchronisation.

Reporting Time:

International Occultation Timing Association (IOTA)

All occultation timings will be reported to IOTA, via the BAA which will keep copies.

IOTA accepts all observations. Visual, video, CCD and driftscan are the common methods. IOTA and the BAA-ARPS can recommend how the timings are derived and reported. Asteroid occultation timings are published on web pages and used for professional study. For UK and Europe, see: www.euraster.net.

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