RFI Mitigation in AIPS. The New Task UVRFI

1. RFI Mitigation in AIPS.The New Task UVRFI L. Kogan, F. Owen National Radio AstronomyObservatory Socorro, NM USA

2. Visibility expression at the presence of RFIs moving with constant velocities relatively the array Having known the correlator outputs for the times inside of the solution interval and considering the all RFI amplitudes are constant inside of the solution interval, find the value of the source visibility for the solution interval!

3. The Athreya’s presentation of the problem. Concept of circles. Ramana Athreya, APJ, 696, 885-890, May 2009 If RFI includes only one group moving with constant velocity (in particular located at the Earth or one satellite) and its amplitude is constant inside of the solution interval then the trajectory of the correlator output at the complex plane is the circle with radius equaled RFI amplitude and the center coordinates are Re, Im of the source visibility. Fitting the 3 parameters: radius of the circle, and two coordinates of the circle center, the found two coordinates of the circle center are used as the solution for the source visibility without RFI

4. One of the best “circle”

5. The new AIPS task UVRFI UVRFI uses the 2 algorithms: 1. ‘CIRC’ a la Athrea The spiral with 4 unknown parameters: initial radius, linear increment of the radius, 2 coordinates of the center is fitted to the data. The 2 coordinates of the center are used as a solution for the free RFI visibility. 2. ‘CEXP’ The model is represented by the sum of several spectral components with complex amplitude. The simple version of Hogbom clean algorithm (complex Fourier transform, complex solution and complex “PSF) is used to subtract the the complex components. Final solution is the value of the cleaned Fourier transform at zero frequency. The subtraction is not carried out for the component at zero frequency to prevent the subtraction of the signal itself. This rule is canceled for the very high channel amplitude in comparison with other channels.

6. The actual shape of the “circles”. L-band data (given by M. Rupen).

7. The visibility spectrum for one baseline of Rupen’s data (L band)

8. The visibility spectrum for one baseline of Rupen’s data (L band) The averaged output The UVRFI output

9. The image of 3C345 using Rupen’s data (L band) The averaged output The UVRFI output

10. The actual shape of the “circles”. 4-band data (given by B. Cotton).

11. The visibility spectrum for one baseline of Cotton’s data (4 band) The averaged output The UVRFI output

12. The images using the Cotton’s data (4 band) The averaged output The UVRFI output

13. Self averaging of the visibilities in the process of imaging Therefore the effect of self averaging of RFI may not limited by the correlator averaging interval (as Athrea declares) but by the time interval where both U and V can be considered constant!

14. Conclusions The new AIPS task UVRFI uses the two algorithms to mitigate RFI. Modified algorithm of Ramana Athrea fitting the spiral into the curve RE/IM of the visibility. Subtracting the complex exponents representing RFI, using a simple CLEAN algorithm at the Fourier plane The second algorithm showed better result for the two data I used and allows to mitigate more than one sources of RFI (ground, satellites) The mitigation of the RFI is not so impressive because the RFI of the data are used are too far from the circle model. In some cases, the RFI may be self averaged during the imaging