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astro-informatics / purify

Next-generation radio interferometric imaging.
https://astro-informatics.github.io/purify
GNU General Public License v2.0
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Normalisation of measurement operator using power method #27

Closed Luke-Pratley closed 8 years ago

Luke-Pratley commented 8 years ago

At the start of the conjugate gradient method, the residuals are quite large. However, this depends on the gridding kernel used in the measurement operator. This gives me the impression it is an issue of normalisation. So, we need to normalise A^T * A, probably using the power method.

Luke-Pratley commented 8 years ago

I quickly coded the power method as a function in the measurement operator, and it looks like my idea fixed the problem.

rafael-carrillo commented 8 years ago

I think that’s what we used to do in the example scripts. Basically, we used the normalised visibilities:

y1 = y/sqrt(nu)

and the normalised operator:

Phi_1 = Phi/sqrt(nu)

where nu is the operator norm. This was done only to accelerate the convergence of the CG solver.

On 23 Feb 2016, at 12:53, Luke-Pratley notifications@github.com wrote:

I quickly coded the power method as a function in the measurement operator, and it looks like my idea fixed the problem.

— Reply to this email directly or view it on GitHub https://github.com/astro-informatics/purify/issues/27#issuecomment-187669869.

Luke-Pratley commented 8 years ago

Cool, that sounds like what I have done. And yes, I have done it for the same reason. Good to know that it is consistent.