The fit parameters are 1. the emissivity of the test mass and 2. the emissivity of the inner shield. Turning on absolute_sigma=True, the covariance matrix is:

[[4.40872002e-07 1.98911860e-07]
 [1.98911860e-07 1.50052351e-07]].

Interpreting this, the standard deviation  of the parameters is:

e_testmass (painted with Aquadag): 0.00066398.

e_innershield (rough Al): 0.00038737.

As discussed in today's meeting, these values much lower than expected. I'll look more into how scipy.curve_fit calculates these values, and will use the fitting script discussed to better quantify the error.