The SOLARC Offaxis Coronagraph

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The SOLARC Off-axis Coronagraph
J.R. Kuhn, R. Coulter, H. Lin, D. Mickey

• Why a ground-based solar coronagraph
• SOLARC design and performance
• Future prospects

NASA/SRT, UH/IfA, NSF/ATST
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SOHO/LASCO Available Daily
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Only the magnetic field will tell...
(from Chen et al., Low, Gibson)
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Our dark energy problem
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Empirical encouragement

• 3.9m Eclipse

Off-band
On-band
SiIX?
Judge et al. claim spectroscopic detection (ApJ,
in press)
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Why an IR (reflecting) off-axis coronagraph?

• Zeeman magnetic sensitivity
• Lower scattered sky background
• Lower scattered instrument optics background
• Lowered scattered dust background

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Scattering sources

• Atmosphere
• seeing
• aerosols
• atomic molecular scattering
• Telescope
• diffraction
• mirror roughness
• mirror dust

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Atmospheric backgrounds
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Optical backgrounds
4.0m
0.5m
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SOLARC

• Reflecting - broadband, IR
• Off-axis - unobscured, low scattering
• All major National solar telescope facilities are
  off-axis or unobscured (Dunn, Evans, McMath)

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Off-axis telescope myths

• Aberrations are worse than conventional
  telescopes
• They cant be aligned
• Large off-axis mirrors arent manufacturable

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Aberrations

• This is not an asymmetric optical system, it is a
  decentered system
• The full aperture is not illuminated

Q
dy
e
f
For small angles, Q, blur is astigmatic and only
weakly dependent on off-axis distance. SOLARC is
diffraction limited over 15 arcmin field
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SOLAR-C Optics
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Linear Alignment Algorithms
Coma and astigmatism easily separated
Linear algorithm minizes cos(2theta) and
sin(2theta) spot distributions (and rms spot
size) while varying M2 tilt and decenter.
...After 2 iterations
Starting optical performance
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SOLAR-C
Gregorian focus 8m f.l.
M2
F/20, efl 8m, prim-sec 1.7m 0.5m, 1.5m fl
primary 55mm, secondary l/10 p-v figure diff.
Limited _at_ 1micron over 15fov 10.4 deg tilt angle
M1 0.5m F/3.7
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before baffling
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(No Transcript)
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First light images
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PSPT blue disk photometry
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Measured secondary PSF
Over 5 orders of magnitude no mirror or other
spurious scatter terms detected
l 656 nm
Short exposure images nearly diffraction limited
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Green-line coronal photometry
SOLARC measured sky
Measured Evans photometer sky
Integrated Kolmogorovdust
Integrated Kolmogorov solar disk
l 530 nm
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Summary

• The SOLARC off-axis coronagraph on Haleakala is
  operational
• Optical fabrication and alignment issues are no
  more difficult than for a conventional telescope
• Post-focus instrumentation includes
• Mid-IR imager
• Near and mid-IR spectrograph