The solar transition region: High resolution FUV-EUV imaging and modeling of unresolved coronal funnels as observed with the multi-spectral solar telescope array II: MSSTA II, the second flight

The Multi-Spectral Solar Telescope Array (MSSTA) is a sounding rocket-borne observatory composed of a set of normal-incidence, multilayer-coated telescopes that obtained selected bandpass spectroheliograms (44A--1550A) of the Solar atmosphere. These images were recorded on specially fabricated XUV and FUV 70mm Kodak film. Rocket launches of this instrument payload took place in 1991 (MSSTA I) and 1994 (MSSTA II) at the White Sands Missile Test Range in New Mexico. Immediately prior to the 1994 launch, visible light focusing tests of each telescope were performed in-situ using a 1951 Standard Air Force High Resolution Test-target to measure optical resolution performance. We determined that the MSSTA II telescopes performed at diffraction limited resolutions down to 0.70 arcsecond at visible wavelengths, and, based on these measurements, we calculate an upper-bound to the focusing errors that incorporate the sum of all uncorrelated system resolution errors that affect resolution performance. Coupling these upper-bound estimates with the in-band diffraction limits, surface scattering errors, and payload pointing jitter, I demonstrate that eleven of nineteen MSSTA II telescopes---having negligible figures of focus errors in comparison to the corresponding visible diffraction limits---performed at sub arcsecond resolution at their operational FUV/EUV/XUV wavelengths during flight. I estimate the in-band performance down to 0.14 +/- 0.08 second of arc. Careful digitizations were then made of selected spectroheliograms, noise-reduced and calibrated to yield energy flux measurements (171 A); and then deconvolved using a blind form of the Richardson-Lucy algorithm to recover the highest angular resolution possible. Using the 1550 A (C IV) and 171 A (Fe IX/X) images, a funnel model that is based on coronal back-heating was constructed and tested against the data. The analysis demonstrates that funnels can account for the quiescent, globally diffuse emission seen in 171 A spectroheliograms, suggesting this emission is mostly of upper transition region origin (T ∼ 650,000 K), and, simultaneously satisfy areal coverage constraints observed at both 171 (60--90%) and 1550 A (≤45%) wavelengths.