Wide field telescope using spherical mirrors Jim Burge and Roger Angel University of Arizona Tucson, AZ Jim Roger arizona.edu

Drivers for WFMOST ( Wide Field Multiple Object Spectroscopy Telescope) Large aperture + wide field of view –Large apertures (>100 m 2 ) and long integration required to obtain high resolution spectra –Large field of view required for many simultaneous observations Cost cannot be astronomical –Use spherical mirrors Optimize for Integral Field Spectroscopy

Why use spherical mirrors +They cost less Easier to manufacture and test Lower operating costs (all segments are the same) +They provide a wide field of view By symmetry, sphere has same curvature everywhere When the light “walks” on sphere for different field points – sphere is exactly the same -Spherical mirrors cause “spherical aberration” This degrades images Corrective optics must be used to compensate for this problem

Classical Schmidt design Spherical primary mirror concentric about the stop Aspheric corrector plate coincident with aperture stop Spherical focal surface concentric about the stop halfway between corrector and mirror

LAMOST Reflective Schmidt Design 4- m clear aperture 5° FOV, 2 arcsec fibers (Wang et al. Applied Optics )

Arecibo 300 m fixed spherical dish (built into the mountain in Puerto Rico) Receiver and correction optics are moved to ‘steer’ line of sight

Hobby Eberly, SALT Telescope rotated in AZ Corrector moved for EL pointing

ESO’s OWL Telescope (OverWhelmingly Large) 100 m spherical primary 5 mirror corrector Fully steerable

It’s time for a new Paradigm! Build on ideas from Meinels Use a large steerable telescope with spherical mirrors Keep wide field of view, accept spherical aberration SA Use SA correctors, each with its own small field of view Populate the spherical ‘focal plane’ of the telescope with correctors, each with its image plane.

Layout of telescope Primary mirror 34-m diameter concave sphere Image on spherical surface Secondary 30 m entrance pupil at the common centers of curvature (There is nothing physically there. This is the image of a stop that is actually located in the correction optics) Primary, secondary mirrors and focal surface are all concentric 15-m diameter convex sphere

Image plane correctors Light from telescope Two fields shown, separated by 5 arcmin Imagers for local corrected field Spherical aberration correctors ‘Image’ surface for telescope 7-m diameter sphere Correctors with local imagers are placed on the spherical surface according to the science of the night “Image” formed here 74 mm (2 arc min) diameter

The correctors The 2-mirror telescope creates images with spherical aberration Rather than to correct the entire field of view at once, we use individual correctors that work only over a small field of view. Numerous correctors are used to simultaneously collect light from many regions at once We have developed simple two-mirror correctors that provide excellent correction.

Correctors Light from 2-mirror telescope. Each image has 2 arcmin spherical aberration, but no field aberrations Image at f/4 Sine condition enforced M3 : 120 mm diameter Aspheric relay mirror M4 : 120 mm diameter Provides aperture stop Aspheric corrector mirror

The magic of optics Tangled up rays in Beautiful images out!

Excellent Correction! 60 um spot radius 0.2 arcsec image diameter at edge of field Y direction max field at 2 arcsec X direction max field at 8 arcsec Image quality improves for larger corrector mirrors

Integral Field Spectroscopy Optical corrector 2 x 10 arcsec field 0.25 arcsec sampling 8 x 40 lenslets (150 µm pitch) each coupled to a fiber Each unit subtends 3 x 6 arcmin, but sees 2 x 10 arcsec Fiber bundles feed spectrograph 140 units, 50,000 fibers! At focus, lenslet array couples to fibers

Operation of IFU Each lenslet feeds a fiber Fibers are lined up to become “slit” for spectrograph

Adaptive Optics M4 is the system aperture, Conjugate to plane 40 meters above telescope Perfect place for a deformable mirror! AO WFS and imager DM Look under natural guide star ‘lampposts’ over small corrected field an arcsecond field with 3 mas resolution Again, numerous units can operate simultaneously