Astronomy 920 Commissioning of the Prime Focus Imaging Spectrograph

1
Astronomy 920Commissioning of the Prime Focus
Imaging Spectrograph

• Meeting 2
• Intro VPH Grating Spectroscopy
• Ken Nordsieck

2
Optics Layout
3
VPH Gratings
4
Throughput
5
NIR Upgrade Path

• NIR upgrade path preserved by collimator allowing
  full 320 nm 1.7 µ wavelength range support
  structure

6
Assembly and Integration
7
Slitmask Cutter

• Commercial laser
• MOS custom slits cut in carbon-fiber
• Longslit stainless steel, tilted and polished
  for viewing by SALTICAM/ slitviewer optics
• PI planning tool produces slitmask file for
  inclusion in proposal

8
Grating Optics

• Grating Equation
• Resolution
• Why Large Telescope Spectrographs are Big
• VPH Gratings

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Grating Equation and Resolution

• m ? / ? sin a sin ß 2 sin a (Littrow)
• d? / dß (?/m) cos ß
• angular size of monochromatic slit
• ?a ?? Dtel / Dcol
• ?ß ?a cos a / cos ß
• ?? (slit) (?/m) cos a ?? Dtel / Dcol
• at Littrow
• R ?/?? (slit) 2 (Dcol / Dtel) tan a / ??

Dtel dia of primary Dcol dia of collimated
beam ? spacing of grating grooves ? angle in
sky a angle of incidence ß angle of
diffraction m order of diffraction
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Resolution

• R ?/?? (slit) 2 (Dcol / Dtel) tan a / ??
• Does not depend on
• diffraction order (!)
• grating groove density (!)
• For fixed seeing, improve only by
• going to higher grating angle (eg Echelle)
• increasing the size of your collimated beam ()
• Note for small telescopes, also worry about
  grating diffraction limit Rdif Dgrat /?
• For SALT/ PFIS Dcol 150 mm, Dtel / Dcol
  73.3
• R 5640 for 1 arcsec slit, 45 deg grating angle

11
VPH Gratings

• Created by exposing holograph material (DCG) to
  interference pattern from laser
• Index of refraction of DCG is modulated in space
• Large inexpensive custom design efficient at
  high groove density

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PFIS VPH

• 1300 gr/ mm VPH (? 769 nm)

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Tunable Blaze

• Peak efficiency is at Littrow gt Blaze angle
  changes with a
• If increase ß as tilt grating, can use grating at
  large range of central wavelengths gt change
  camera angle a ß