The SOLARC Off-axis Coronagraph • Why a ground-based solar coronagraph • SOLARC design and performance • Future prospects J.R. Kuhn, R. Coulter, H. Lin, D. Mickey NASA/SRT, UH/IfA, NSF/ATST
Dec 21, 2015
The SOLARC Off-axis Coronagraph
• Why a ground-based solar coronagraph
• SOLARC design and performance
• Future prospects
J.R. Kuhn, R. Coulter, H. Lin, D. Mickey
NASA/SRT, UH/IfA, NSF/ATST
Empirical encouragement
• 3.9m Eclipse
Off-band On-band
SiIX?
Judge et al. claim spectroscopic detection (ApJ, in press)
Why an IR (reflecting) off-axis coronagraph?
• Zeeman magnetic sensitivity
• Lower scattered sky background
• Lower scattered instrument optics background
• Lowered scattered dust background
Scattering sources
• Atmosphere– “seeing”– aerosols– atomic molecular scattering
• Telescope– diffraction– mirror roughness– mirror dust
SOLARC
• Reflecting - broadband, IR• Off-axis - unobscured, low scattering
• All major National solar telescope facilities are off-axis or unobscured (Dunn, Evans, McMath)
Off-axis telescope “myths”
• “Aberrations are worse than conventional telescopes”
• “They can’t be aligned”
• “Large off-axis mirrors aren’t manufacturable”
Aberrations
• This is not an asymmetric optical system, it is a “decentered” system
• The full aperture is not illuminated
d blur
y astig.
f / y coma
:TransverseOrder Third
2
2
dy
Q
f
e
For small angles, Q, blur is astigmatic and only weaklydependent on off-axis distance. SOLARC is diffraction limitedover 15 arcmin field
Linear Alignment Algorithms
Linear algorithm minizes cos(2theta) and sin(2theta) spot distributions (and rms spot size) while varying M2 tilt and decenter….
Coma and astigmatism easily separated:
Starting optical performance ...After 2 iterations
SOLAR-C
M1: 0.5m F/3.7
M2
Gregorian focus8m f.l.
F/20, efl 8m, prim-sec 1.7m0.5m, 1.5m fl primary55mm, secondaryl/10 p-v figurediff. Limited @ 1micron over 15’fov10.4 deg tilt angle
Measured secondary PSF
Over 5 orders of magnitudeno mirror or other spuriousscatter terms detected
Short exposure imagesnearly diffraction limited
l = 656 nm
Green-line coronal photometry
Measured Evans photometer sky
Integrated Kolmogorov + solar disk
Integrated Kolmogorov+dust
SOLARC measured sky
l = 530 nm