[time-nuts] USB Low Cost GPS Timing Receiver

[Bruce Griffiths]

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?
>
>    
Worst case (declination = 0), 1 arc second of pointing is equivalent to 
about 1/15 (~67ms) second of time.
The telescope field of view isn't really relevant (as long as its 
greater than 1 arcsec) if one needs to maintain 1 arc second pointing.
In this case a timing accuracy of 10ms isn't overly conservative.
  If one merely needs to ensure that the telescope can acquire the 
object of interest so that it is well within the field of view of an 
eyepiece, then 1 arc second pointing isnt necessary.
However if one needs to place the object of interest directly on a 
spectrometer slit or fibre then a somewhat lower pointing error is 
desirable.

To achieve 1 arc second pointing accuracy usually requires correcting 
for such effects as
Tube flexure
The orthogonality error between axes
drive train eccentricities
The value of dUT1.
etc

One also requires high resolution encoders (or equivalent on each axis).
Alternative a stellar compass system like those used to gage the 
orientation of spacecraft can be employed.
Despite what Meade would have one believe the application of such 
devices to telescope pointing did not originate with them.


>    
>> 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.
>
>
>    

Bruce