Last updated 2008 July 9
Accurate timing of observations is an absolute necessity. One (my) way of doing this is to save images in FITS format and have the time, from the PC clock, automatically inserted in to the FITS header.
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
2.0 Observatory set-up
It has taken me some time to come up with an arrangement that seems to work. Since I haven’t described my set-up elsewhere I will do it here.
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.
3.0 PC time keeping
3.1 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.
3.2 Global Positioning System
One day a dormant brain cell went ‘ping’ and the idea of using areceiver 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 receiver with a serial – USB converter and 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.
3.3 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.
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.
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. The Windows clock display is within 0.1 secs for about 3 out of every 4 secs, and the better part of a second late for the 4th second. An advantage over Dimension4 is that AboutTime goes through a list of servers until it can synch with one - whereas in Dimension4 you apparently need to have selected a currently operational server.
Net Time appears to be quite accurate. One test showed only an 0.08 second difference from a GPS system.
While searching the net for the web sites of the software listed above I came across The Horology Source. This lists many other programs for timekeeping and related links.