[time-nuts] The forbidden question

[jimlux]

On 6/6/19 2:17 PM, Michael Wouters wrote:
> My radio astronomer colleagues tell me that there is about a 20 minute
> limit to VLBI observing runs because of atmospheric instability so this
> limits improvements to be had from better clocks. My recollection is that a
> maser is still sufficient out to 100 GHz. There is a paper about this that
> I will dig out later.

I would think it depends on the frequency of observation. The DSN does 
 >8 hour tracks on a regular basis for ranging and that's with a overall 
measurement uncertainty on the order of 1E-15 at tau=10,000 seconds. The 
station is much better.  But that's at 8 or 32 GHz.



> 
> Cheers
> Michael
> 
> On Thu, 6 Jun 2019 at 12:01 pm, Joseph B. Fitzgerald <
> jfitzgerald at alum.wpi.edu> wrote:
> 
>>
>> Regarding Dana's remarks on VLBI, consider the recent black hole image
>> released by the Event Horizon Telescope.    Measurements were taken at 230
>> GHz, and they would like to begin measurements at 345 GHz.    Hydrogen
>> masers were used at each telescope.    I am no expert, but I strongly
>> suspect that a better clock would result in longer/better observations.
>> The H maser only runs at 1.42 GHz ... the astrophysicist wizards are
>> proposing to do their measurements at about 250 times higher in frequency!
>>
>>
>> -Joe
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