Triple Product using Calred
Posted: Thu Aug 15, 2013 12:03 am
For a while now I have been trying to decipher the Calred task as I would like to be able to use the triple product to measure the flux for some of my observations.
My understanding is that the triple product has an amplitude that is
A_TP = (Aij * Ajk * Aki)^1/3
Φ_cl = Φij + Φjk - Φki
where Aab is the amplitude of the visibility on baseline a-b, and Φab is the phase of the visibility on this baseline.
These quantities are related to the visibilities via Vab = Aab*exp(iΦab).
I also understand that calred (options=triple) calculates this quantity for each 3-baseline triangle in the array and then returns the mean (or some sort of average).
My problem is that calred (options=triple) will report negative fluxes for sources in the low SNR regeime. I take this to mean that there is no source detected however the negative flux is very troubling to me.
If the Aab are all amplitudes of the visibility they should all be positive definite numbers and there fore A_TP should also be positive definite.
Clearly there is something wrong with the above, yet I cannot figure it out.
A confounding factor is that when I use the task closure to calculate the triple product (closure options=amp) I always get positive numbers. Am I mis-interpreting something or is there something wrong with calred.
As a side note:
What do the columns Con/The/Act represent? There is nothing in the help file or in the calred.for code that explains these columns.
My understanding is that the triple product has an amplitude that is
A_TP = (Aij * Ajk * Aki)^1/3
Φ_cl = Φij + Φjk - Φki
where Aab is the amplitude of the visibility on baseline a-b, and Φab is the phase of the visibility on this baseline.
These quantities are related to the visibilities via Vab = Aab*exp(iΦab).
I also understand that calred (options=triple) calculates this quantity for each 3-baseline triangle in the array and then returns the mean (or some sort of average).
My problem is that calred (options=triple) will report negative fluxes for sources in the low SNR regeime. I take this to mean that there is no source detected however the negative flux is very troubling to me.
If the Aab are all amplitudes of the visibility they should all be positive definite numbers and there fore A_TP should also be positive definite.
Clearly there is something wrong with the above, yet I cannot figure it out.
A confounding factor is that when I use the task closure to calculate the triple product (closure options=amp) I always get positive numbers. Am I mis-interpreting something or is there something wrong with calred.
As a side note:
What do the columns Con/The/Act represent? There is nothing in the help file or in the calred.for code that explains these columns.