[mmaimcal] Re: [alma-sw-ssr] question about frequency/band switching speeds and rates

[R. Lucas]

Darrel:

This is what we have in the OTF interferometry use case:

""2. The scheme currenly envisioned for phase tracking is the following 
(D. Emerson):

- We apply, on one of the LO's, the phase and fringe frequency needed to 
track the center of the antenna beam (taken at the center of the 
integration period),

- an additional phase, constant during the integration, as needed to 
obtain a continuous phase at the beginning of the integration.

- this phase value is kept and later subtracted from that of 
visibilities obtained during that integration. This scheme should not 
unneededly restrain the  final data rate.""

I think this resulted of a discussion with Larry and yourself at the 
time. We always phase-track a fixed position in the sky, corresponding 
to the center of the primary beam at mid-integration, but make sure 
there is no phase jump at the end of each integration.

Cheers

Robert


Darrel Emerson wrote:
> I believe Robert Lucas is working on the details, but I don't think
> we want to track the phase centers as such as they move across
> the sky with the primary beam in OTF interferometry.
> If you do that, you end up with artificially high fringe rates that
> are too high to sample - Larry pointed this out.  However, if
> you keep the phase center more or less fixed, but track the fringe
> frequency rather than its absolute phase, that's a more slowly
> changing quantity across the sky, & it becomes manageable.
> Of course you do need to track the delay center with the primary
> beam, but not necessarily the phase center.
> 
>   There was some discussion on this issue a couple of years back.

I think that was rather a couple of couples of years back.


> 
>              Cheers,
>                    Darrel.
> 
> Bryan Butler wrote:
> 
>>
>>
>> On 2/3/05 22:45, Bryan Butler wrote:
>>
>>>
>>> . OTF mosaic cell phase-center saltation: rate=100 usec; speed=10
>>>   usec.
>>>   [for the rate, i've assumed slewing at 1 deg/sec, and 950 GHz,
>>>   with 2 samples per FWHM; speed is a guess - it will depend on
>>>   how fast the correlator can reset delays]
>>
>>
>>
>> OK - got this one totally wrong.  slew rate for OTF interferometry is 
>> 3'/sec, not 1d/sec, which corresponds to 15 msec required 
>> reconfiguration speed if the phase centers are changed discretely as 
>> the antennas slew across the source...  but it's not clear this can be 
>> accomplished if the correlator phase center can only be updated on 48 
>> msec boundaries?
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-- 
Robert LUCAS,            Institut de Radioastronomie Millimetrique
300 rue de la Piscine,  F-38406 St Martin d'Heres Cedex   (FRANCE)
Tel +33 (0)4 76 82 49 42                  Fax +33 (0)4 76 51 59 38
E-mail: mailto:lucas at iram.fr                http://iram.fr/~lucas/