Posted: March 21 2018
I am attempting to calculate the parameter covariance matrix from a Fisher matrix generated by finite difference differentiation of the TT Cls generated by CAMB; however, I seem to be running into a problem with the accuracy of the final digit outputted into the text file. Namely, my derivative power spectrum seems to have a blocky structure, particularly near the peaks, which is consistent with CAMB generated Cls that are not sufficiently accurate for the step size of my derivative. I have tried a larger step size, and that problem goes away; however, the calculated covariance matrix becomes very sensitive to changes in the step size indicating that it is not very accurate. What accuracy settings should I edit in the params.ini file to increase the accuracy of the outputted Cls? Do I also need to edit CAMB?