Transcript The SOLARC Off-axis Coronagraph

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
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
Q
dy
e
Third Order Transverse :
coma
  y 2 / f
astig.
  y 2
 d
blur
f
For small angles, Q, blur is astigmatic and only weakly
dependent on off-axis distance. SOLARC is diffraction limited
over 15 arcmin field
SOLAR-C Optics
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….
Starting optical performance
...After 2 iterations
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 @ 1micron over 15’fov
10.4 deg tilt angle
M1: 0.5m F/3.7
before baffling
First light images
PSPT “blue” disk photometry
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
Green-line coronal photometry
SOLARC measured sky
Measured Evans photometer sky
Integrated Kolmogorov+dust
Integrated Kolmogorov + solar disk
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