Abstract
Using the 2016 Mercury transit of the Sun, we characterize on orbit spatial point spread functions (PSFs) for the Near- (NUV) and Far- (FUV) Ultra-Violet spectrograph channels of NASA’s Interface Region Imaging Spectrograph (IRIS). A semi-blind Richardson–Lucy deconvolution method is used to estimate PSFs for each channel. Corresponding estimates of Modulation Transfer Functions (MTFs) indicate resolution of 2.47 cycles/arcsec in the NUV channel near 2796 Å and 2.55 cycles/arcsec near 2814 Å. In the short ({approx},1336~mathring{mathrm{A}}) and long ({approx},1394~mathring{mathrm{A}}) wavelength FUV channels, our MTFs show pixel-limited resolution (3.0 cycles/arcsec). The PSF estimates perform well under deconvolution, removing or significantly reducing instrument artifacts in the Mercury transit spectra. The usefulness of the PSFs is demonstrated in a case study of an isolated explosive event. PSF estimates and deconvolution routines are provided through a SolarSoft module.
Highlights
Images from a telescope are often well approximated as a convolution of the real scene with the instrument point spread function (PSF, e.g. Hecht, 1987)
Poduval et al (2013) characterized the diffuse component of the point spread functions (PSFs) for the Atmospheric Imaging Assembly (AIA) EUV telescopes on board the Solar Dynamics Observatory (SDO) spacecraft using a lunar occultation of the Sun
We take the mean as the best PSF estimate for the two near ultraviolet (NUV) windows, since it preserves normalization while reducing the noise in the wings of the PSFs
Summary
Images from a telescope are often well approximated as a convolution of the real scene with the instrument point spread function (PSF, e.g. Hecht, 1987). The Sun is an extended, high-contrast, and (often) finely structured source in extremeultraviolet (EUV) wavelengths (Walker et al, 1988; Golub et al, 1990; Antolin and Rouppe van der Voort, 2012) Observations of such scenes may be subtly smoothed (or blurred) by an instrument PSF, reducing contrast between adjacent bright and dark features. DeForest, Martens, and Wills-Davey (2009) constrained semi-empirical PSFs and measured the effect of stray light in the Transition Region And Coronal Explorer (TRACE) extreme-ultraviolet (EUV) telescope using Venus transit observations. Poduval et al (2013) characterized the diffuse component of the PSFs for the Atmospheric Imaging Assembly (AIA) EUV telescopes on board the Solar Dynamics Observatory (SDO) spacecraft using a lunar occultation of the Sun. We use the May 2016 Mercury transit to characterize on orbit PSFs for the IRIS SG. The PSFs determined here are distributed via SolarSoft (SSW, Freeland and Handy, 1998) with a deconvolution routine, iris_sg_deconvolve.pro, suitable for direct application to IRIS SG Level 2 data
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