[mmaimcal] Matching of frequency profiles for ALMA and ACA correlators

[rlaing at eso.org]

On Mon, 20 Jun 2005, Darrel Emerson wrote:

> Are at least some of the following true?
> 
> 1. If we're prepared to give up a factor of ~2 in ACA correlator frequency
> resolution, then there's no problem, because we can reweight terms
> of the ACA FX correlator in order to equalize the frequency response with
> the main ALMA array sinc response.  Especially true since we'll usually
> want to grade responses (Hanning or whatever) to reduce frequency sidelobes.

This is correct, as I understand things.

> 
> 2. Doesn't the ACA correlator support a greater number of channels
> than the main ALMA array anyway (I'm not at all sure if it does or not)?
> If so, then combined with (1) above that's the end of the story.

Yes, a larger number of channels is potentially supported, but the maximum
resolution is the same.

> 
> 3. How often is the ultimate in ACA frequency response really going to be
> needed?  Could it be that in most cases where extreme frequency resolution
> and maximum number of channels are needed, the angular extent of the
> source is such that the combination of ACA and main ALMA may not
> be so important?  In other words, is there any tendency for spectrally
> narrow emission not to be spatially very extended (the reverse of the Orion
> CO spectrum.)  I don't know if this is true.
> 
>   My feeling, which may be probably completely wrong, is that it'll be
> sufficiently rare that we run into a significant  imaging problem 
> because of this,
> since in most cases we can sacrifice FX resolution to equalize the frequency
> responses, that it's not even worth the effort of making simulations to 
> quantify
> things further.

That is my feeling too, and Stephane's argument reinforces it.

Regards

Robert