4 research outputs found
Is the sky the limit? Performance of the revamped Swedish 1-m Solar Telescope and its blue- and red-beam re-imaging systems
We demonstrate that for data recorded with a solar telescope that uses
adaptive optics and/or post-processing to compensate for many low- and
high-order aberrations, the RMS granulation contrast is directly proportional
to the Strehl ratio calculated from the residual (small-scale) wavefront error.
We demonstrate that the wings of the high-order compensated PSF for SST are
likely to extend to a radius of not more than about 2 arcsec, consistent with
earlier conclusions drawn from straylight compensation of sunspot images. We
report on simultaneous measurements of seeing and solar granulation contrast
averaged over 2 sec time intervals at several wavelengths from 525 nm to 853.6
nm on the red-beam (CRISP beam) and wavelengths from 395 nm to 484 nm on the
blue-beam (CHROMIS beam). These data were recorded with the Swedish 1-m Solar
Telescope (SST) that has been revamped with an 85-electrode adaptive mirror and
a new tip-tilt mirror, both of which were polished to exceptionally high
optical quality. The highest 2-sec average image contrast measured in April
2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630 nm and
853.5 nm with compensation only for the diffraction limited point spread
function of SST is 11.8%, 11.8%, 10.2% and 7.2% respectively. Similarly, the
highest 2-sec contrast measured at 395 nm, 400 nm and 484 nm in May 2016
through 0.37-1.3 nm filters is 16%, 16% and 12.5% respectively. The granulation
contrast observed with SST compares favorably with that of other telescopes.
Simultaneously with the above wideband red-beam data, we also recorded
narrow-band continuum images with the CRISP imaging spectropolarimeter. We find
that contrasts measured with CRISP are entirely consistent with the
corresponding wide-band contrasts, demonstrating that any additional image
degradation by the CRISP etalons and telecentric optical system is marginal or
even insignificant.Comment: In press in Astronomy & Astrophysic
Evaluation of phase-diversity techniques for solar-image restoration
Phase-diversity techniques provide a novel observational method for overcomming the effects of turbulence and instrument-induced aberrations in ground-based astronomy. Two implementations of phase-diversity techniques that differ with regard to noise model, estimator, optimization algorithm, method of regularization, and treatment of edge effects are described. Reconstructions of solar granulation derived by applying these two implementations to common data sets are shown to yield nearly identical images. For both implementations, reconstructions from phase-diverse speckle data (involving multiple realizations of turbulence) are shown to be superior to those derived from conventional phase-diversity data (involving a single realization). Phase-diverse speckle reconstructions are shown to achieve near diffraction-limited resolution and are validated by internal and external consistency tests, including a comparison with a reconstruction using a well-accepted speckle-imaging method