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

SOHO/LASCO Available Daily

Only the magnetic field will tell...

(from Chen et al., Low, Gibson)

Our “dark energy problem”

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

Atmospheric backgrounds

Optical backgrounds

0.5m 4.0m

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

SOLAR-C Optics

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

before baffling

First light images

PSPT “blue” disk photometry

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

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