Astronomy 920 Commissioning of the Prime Focus Imaging Spectrograph

1
Astronomy 920Commissioning of the Prime Focus
Imaging Spectrograph

• Meeting 6
• Fabry-Perot
• Ken Nordsieck

2
Bandpass

• By Mode
• Imaging effective width of filter
• Grating Spectroscopy ?? correspond to slit
• Fabry-Perot ?? of etalon
• For sky, use arcsec2 corresponding to resolution
  element
• Imaging, Fabry-Perot ?O area of seeing disk
• Grating Spectroscopy ?O (2 rs) (2 r0)
• To calculate readout noise, need number of bins
  corresponding to resolution element
• n ?O/(bxby / (7.8 pix/arcsec)2)
• RON RON/bin sqrt(n readouts)
• Done!

3
Etalon Basics

• Etalon adjustable interference filter.
  destructive interference except at wavelengths
• m? d cos ?
• m order d programmable gap
• Plates must be flat, parallel to ?/m, typically
  1/50-1/100 wave
• The more reflective the coating, the more bounces
  before exiting f finesse
• distance between orders d? d/m free spectral
  range
• resolution ?? d? / f d/mf
• m limited by free spectral range, f by practical
  coatings

4
Finesse and Free Spectral Range
PFIS etalons wanted 30, got 20
d?
5
PFIS etalons

• With constant finesse, higher gap has higher
  resolution (larger order to get same wavelength)
• Cant make one etalon do everything, since cannot
  practically scan more than about 6 microns (10
  waves)
• PFIS has 3 etalons, used in 4 modes
• MR and HR always used with LR, for order blocking

6
Order Blocking
Interference Filter
HR etalon
LR etalon

• PFIS has 40 interference filters, 150 Ang
  wide, 4300 9000 Ang
• HR, MR used in double etalon mode scanned
  together with LR

7
Fabry-Perot Efficiency

• FP efficiency at spec in red, slightly below in
  blue

8
Fabry-Perot Resolution
9
Scanning

• PFIS control system allows arbitrary sets of
  discrete steps (equal exposures)
• linear scan for line profiles
• dithering onto continuum for absorption lines
• Off axis, must take account of wavelength
  gradient due to cos ?
• center to edge gradient
• ?(r)/ ?0 1/ cos ? (1- .00365(r/4)2 )
  (24 Ang at Ha)
• Inside bullseye, wavelength changed by less
  than resolution
• D(bullseye) 1.4 (10,000/R)1/2
• - if object larger than bullseye, extend scan to
  cover same wavelengths
• - if object not round, pays to move telescope and
  rescan, instead of extending scan

10
Programs

• FP best for programs requiring good spatial
  coverage, but few spectral resolution elements
• Velocity mapping
• line flux maps