This task generates the rotation measure image from stokes Q and U measurements at several different frequencies. You are required to specify the name of at least one image with a polarization axis containing stokes Q and U planes and with a frequency axis containing more than two pixels. The frequencies do not have to be equally spaced (ie the frequency coordinate can be a tabular coordinate). It will work out the position angle images for you. You may also specify multiple image names, in which case these images will first be concatenated along the spectral axis using ia.imageconcat. The requirements are that for all images, the axis order must be the same and the number of pixels along each axis must be identical, except for the spectral axis which may differ in length between images. The spectral axis need not be contiguous from one image to another. See also the po.fourierrotationmeasure tool method for a new Fourier-based approach.

Rotation measure algorithms that work robustly are few. The main problem is in trying to account for the n-pi ambiguity (Leahy et al. 1986 [1]) 

The algorithm that this task uses is that of Leahy et al. in see Appendix A.1. But as in all these algorithms, the basic process is that for each spatial pixel, the position angle vs frequency data is fit to determine the rotation measure and the position angle at zero wavelength (and associated errors). An image containing the number of n-pi turns that were added to the data at each spatial pixel and for which the best fit was found can be written. The reduced chi-squared image for the fits can also be written.

Note that no assessment of curvature (i.e. deviation from the simple linear position angle - lambda^2 functional form) is made.

Any combination of output images can be written.

The parameter sigma gives the thermal noise in Stokes Q and U. By default it is determined automatically using the image data. But if it proves to be inaccurate (maybe not many signal-free pixels), it may be specified. This is used for calculating the error in the position angles (via propagation of Gaussian errors).

The maxpaerr parameter specifies the maximum allowable error in the position angle that is acceptable. The default is an infinite value. From the standard propagation of errors, the error in the linearly polarized position angle is determined from the Stokes Q and U images (at each directional pixel for each frequency). If the position angle error for any pixel exceeds the specified value, the position angle at that pixel is omitted from the fit. The process generates an error for the fit and this is used to compute the errors in the output images.

Note that maxpaerr is not used to mask pixels in the output images.

The rmfg parameter is used to specify a foreground RM value. For example, you may know the mean RM in some direction out of the Galaxy, then including this can improve the algorithm by reducing ambiguity.

The parameter rmmax specifies the maximum absolute RM value that should be solved for. This is quite an important parameter. The default value, 0, indicates no ambiguity handling will be used. So some apriori information should be supplied; this is the basic problem with rotation measure algorithms.


1. Leahy et al. 1986, Astronomy & Astrophysics, 156, 234 (ADS)