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

Abstract

Aims. We present a point spread function (PSF) for the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and discuss the effects of its removal on the apparent properties of solar surface phenomena in HMI data. Methods. The PSF was retrieved from observations of Venus in transit by matching it to the convolution of a model of the Venusian disc and solar background with a guess PSF. We described the PSF as the sum of five Gaussian functions, the amplitudes of which vary sinusoidally with azimuth. This relatively complex functional form was required by the data. Observations recorded near in time to the transit of Venus were corrected for instrumental scattered light by the deconvolution with the PSF. We also examined the variation in the shape of the solar aureole in daily data, as an indication of PSF changes over time.Results. Granulation contrast in restored HMI data is greatly enhanced relative to the original data and exhibit reasonable agreement with numerical simulations. Image restoration enhanced the apparent intensity and pixel averaged magnetic field strength of photospheric magnetic features significantly. For small-scale magnetic features, restoration enhanced intensity contrast in the continuum and core of the Fe I 6173 Å line by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For sunspots and pores, the enhancement varied strongly within and between features, being more acute for smaller features. Magnetic features are also rendered smaller, as signal smeared onto the surrounding quiet Sun is recovered. Image restoration increased the apparent amount of magnetic flux above the noise floor by a factor of about 1.2, most of the gain coming from the quiet Sun. Line-of-sight velocity due to granulation and supergranulation is enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc. The shape of the solar aureole varied, with time and between the two CCDs. There are also indications that the PSF varies across the FOV. However, all these variations were found to be relatively small, such that a single PSF can be applied to HMI data from both CCDs, over the period examined without introducing significant error. Conclusions. Restoring HMI observations with the PSF presented here returns a reasonable estimate of the stray light-free intensity contrast. Image restoration affects the measured radiant, magnetic and dynamic properties of solar surface phenomena sufficiently to significantly impact interpretation.

Properties of sunspot umbrae observed in cycle 24

“…Fortunately, the low minimum umbral intensities we found in this work indicate a low degree of stray-light contamination, or at least a very narrow PSF compared with the size of large umbrae. See Yeo et al (2014) for recent results regarding the PSF of HMI.…”

Observables Processing for the Helioseismic and Magnetic Imager Instrument on the Solar Dynamics Observatory

“…Also plotted are two MTFs that are the ideal OTF multiplied by a simple exponential with γ = 2.5 and γ = 4.5. The Yeo et al (2014) MTF is overplotted, as is an MTF determined by the ideal multiplied by an exponential function (Equation (20)). The symbols represent the average of three of the curves observed during ground-based instrument calibration, as reported by Wachter et al (2012).…”

“…Some of these, such as modeling the PSF (or equivalently the MTF) as a sum of simple functions, such as Gaussians (Yeo et al, 2014), exponentials, etc., are inherently unphysical because they indicate sensitivity above the optical Nyquist frequency and do not properly describe the ideal diffraction-limited case (Wedemeyer-Böhm, 2008). Having said that, the use of Gaussians is convenient and is adequate for some purposes.…”

“…A single full-disk intensity image can be deconvolved in less than one second. In contrast, Yeo et al (2014) model the HMI PSF as the sum of five Gaussians that are fit to only the Venus-transit data, and the correction routine runs much more slowly. This new PSF has already been used by Hathaway et al (2015) to forward-model solarconvection spectra and by Krucker et al (2015) to investigate footpoints of off-limb solar flares.…”

Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface

“…Applying a spectral line synthesis code [35] to the MHD snapshots in a process similar to the bolometric intensity calculations just described, we calculated the Stokes parameters of the Fe I 6173Å line (the spectral line HMI observes) corresponding to each point in each bolometric image. The line profiles were processed in such a way as to emulate the spatial resolution, spectral sampling, stray light [36] and noise level [22] of the HMI instrument. We calculated magnetic flux density from the line profiles employing the same algorithm used in the HMI data reduction pipeline [37], yielding the HMI-like magnetogram corresponding to each bolometric image.…”