[time-nuts] inexpensive, black box, GPS or NTP based TTL time capture?

Rob Seaman seaman at noao.edu
Wed Oct 18 13:33:28 EDT 2017

Attila Kinali wrote:
> On Wed, 18 Oct 2017 08:47:58 -0600
> Rob Seaman <seaman at noao.edu <https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>> wrote:
> > I’m at the annual planetary sciences meeting (in Provo this year) and several 
> > groups have expressed interest in duplicating our setup (details of FO 
> > converters, Schmitt triggers, etc, omitted) in a “cheap black box” to quote
> > one fellow. Lots of people contribute productively to NEO observations, 
> > including amateurs and small teams with little funding. Improving their 
> > timekeeping would help keep rocks from falling on you and your family.
> What is this black box supposed to do? Just provide a PPS? IRIG-B?
> Or does it need to have time-stamping capabilities? If so, how many
> channels?
> What are your time precision/accuracy requirements?
> What how cheap is "cheap"? What is the volume?

Thanks for the quick reply. I should have included the subject line in the message:

	"inexpensive, black box, GPS or NTP based TTL time capture”

Telescope domes are filled with equipment, in particular the camera shutter, that can be instrumented to issue a pulse suitable for hardware time capture. We use Meinberg IRIG PCIe cards to trap these and read the timestamps using their library routines under Linux. “Black box” to the person I was talking to meant no IRIG in the dome, no Linux, and no PCIe slot, but rather a self-contained unit that syncs to GPS. When last he implemented such a feature at another telescope he didn’t even have time capture, but rather the device issued a trigger at a specified moment, so the timestamp and the shutter opening were inverted. He also seems to prefer the timestamps be issued on a serial connection, not via library.

Unlike laboratory instrumentation, a telescope environment needs to be both very automated and very forgiving. Money may also be constrained. However, telescopes are also often very flexible and I am willing - no, eager - to consider completely different arrangements of equipment.

So, reliable timestamping of a TTL input is the ulitmate goal. One channel would be sufficient, but multi-channel would not invalidate an option. PPS or IRIG-B (DCLS IEEE-1344) are not required, but might form an intermediate part of the solution. Reference could be GNSS or possibly NTP.

Precision varies, but milliseconds down to microseconds. Accuracy should match the precision, meaning UTC accurate to same. Extra credit for multiple timescale support.

Cheap is what I want to know. I see the previous thread on BB-black and could imagine a solution using the real time capabilities of that for a few hundred bucks, but these are not experimenters, per se. That’s why they want a black box. Volume is one to several, but could imagine a bulk order if savings were significant. Hundreds of dollars might be the price point.


Rob Seaman
University of Arizona

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