[time-nuts] USB Low Cost GPS Timing Receiver

Chris Albertson albertson.chris at gmail.com
Mon Nov 29 03:34:46 UTC 2010


I'm new on this list but This is something I know a little about.

Pointing a telescope acuratly is more then just knowing the time.  The
air refracts, or bends the line of sight depending on the angle from
zenith and no matter how strong the structure of the telescope sags
under it's own weight.   I'd not bring this up except that I know
there are people here concerned about very small errors.

The solution is a closed loop system.  You measure the location of a
star on the image plain periodically and compute it's motion and then
send small correction signals to the motors driving the scope to drive
the star's apparent velocity to zero.  In general if things are set up
well you can measure a star's location to about 1/10th of a pixel
because the star is blurred and you can fit a function.  There is more
to it but you get the point, you only need to know the time well
enough to center an object in the field of view.  Typically 15 arc
seconds of very good.  For that you need to know the time to just
better than one second.  I can get that without owning a GPS receiver
using NTP servers on the Internet.

For other uses you might need much better time.  For example high
speed photometry (occupations) knowing current time to 10 ms would be
good   Some people will record audio from WWV simultaneous with the
optical data.

If you already have NTP as 1 ms youhave what you need already

On Sun, Nov 28, 2010 at 6:37 PM, Ralph Smith <ralph at ralphsmith.org> wrote:
> OK, after dropping the amount of coin on the mount, scope, and camera, (my guess in the neighborhood of $40,000 US) a few hundred bucks for a timing solution shouldn't be too bad. Would an external NTP box, perhaps a Soekris Net4501 driven by a Thunderbolt or another GPS receiver with PPS work? My Windows boxes typically track my Soekris NTP server within a few ms, my Macs to about 1 ms, and my FreeBSD boxes to about 30 microseconds.
>
> Ralph
>
> On Nov 28, 2010, at 7:50 PM, Brooke Clarke wrote:
>
>> Hi Hal:
>>
>> FOV is 0.2 min x 0.3 min @0.47arcsec/pixel when using 1x1 binning.  The seeing doesn't often justify 1x1 binning.
>>
>> Mount is the Paramount ME.
>> http://www.bisque.com/help/paramountme/performance_specifications.htm
>> There's a software package called TPoint where you manually point to known stars and that data is used to fit a model of the common mount errors and so correct them.  For example if the RA and DEC axis are not at exactly 90 degrees.  After doing many dozens of stars this correction (and the PEC correction) allow making images without a guide star for say 10 minutes, something that's impossible on a Meade or Celestron type setup.
>>
>> The scope is a Planewave 17":
>> http://www.planewave.com/index.php?page=1&id0=0&id=1
>>
>> Camera is SBIG  STL-11000M
>> http://www.sbig.com/sbwhtmls/online.htm
>>
>> Have Fun,
>>
>> Brooke Clarke
>> http://www.PRC68.com
>>
>>
>> Hal Murray wrote:
>>> brooke at pacific.net  said:
>>>
>>>> The telescope has around arc second pointing capability so I think he  needs
>>>> a hundredth of a second or slightly better.
>>>>
>>> That doesn't sound right.  What's the field of view of the telescope?
>>>
>>>
>>>
>>>> I've found some NMEA software that will set a PC clock, but it expects  the
>>>> data on a COM port.  Don't know how it might work with USB data.
>>>>
>>> I don't use Windows, but most likely it will magically show up as COMx when
>>> you plug it in.
>>>
>>>
>>>
>>>>> It is not clear whether the message is fixed of if it will reply
>>>>> to queries.
>>>>>
>>> Normally, NMEA devices send a clump of "sentences" each second.  Usually, you
>>> can turn off the ones you don't want.
>>>
>>>
>>>
>>>
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-- 
=====
Chris Albertson
Redondo Beach, California



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