Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

摘要

We present a PSF for SDO/HMI and discuss the effects of its removal on theapparent properties of solar surface phenomena in HMI data. The PSF wasretrieved from observations of Venus in transit by matching it to theconvolution of a model of the venusian disc and solar background with a guessPSF. Observations recorded near in time to the transit of Venus were correctedfor instrumental scattered light by the deconvolution with the PSF. Granulationcontrast in restored HMI data is greatly enhanced relative to the original dataand exhibit reasonable agreement with numerical simulations. Image restorationenhanced the apparent intensity and pixel averaged magnetic field strength ofphotospheric magnetic features significantly. For small-scale magneticfeatures, restoration enhanced intensity contrast in the continuum and core ofthe Fe I 617.3 nm line by a factor of 1.3, and the magnetogram signal by afactor of 1.7. For sunspots and pores, the enhancement varied strongly withinand between features, being more acute for smaller features. Magnetic featuresare also rendered smaller, as signal smeared onto the surrounding quiet Sun isrecovered. Image restoration increased the apparent amount of magnetic fluxabove the noise floor by a factor of about 1.2, most of the gain coming fromthe quiet Sun. Line-of-sight velocity due to granulation and supergranulationis enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc.There are indications that the PSF varies across the FOV, with time and betweenthe two CCDs. However, all these variations were found to be relatively small,such that the derived PSF can be applied to HMI data from both CCDs, over theperiod examined without introducing significant error. Image restorationaffects the measured radiant, magnetic and dynamic properties of solar surfacephenomena sufficiently to significantly impact interpretation.