1. Astronomy 920 Commissioning of the Prime Focus Imaging Spectrograph
Meeting 6
Fabry-Perot
Ken Nordsieck
Oct 14, 2005
Ast 920 Meeting 6

2. Bandpass
By Mode:
Imaging: effective width of filter
Grating Spectroscopy: Δλ correspond to slit
Fabry-Perot: Δλ of etalon
For sky, use arcsec2 corresponding to resolution element
Imaging, Fabry-Perot: ΔΩ = area of seeing disk
Grating Spectroscopy: ΔΩ = (2 rs) * (2 r0)
To calculate readout noise, need number of bins corresponding to resolution element
n = ΔΩ/(bx*by / (7.8 pix/arcsec)2)
RON = RON/bin * sqrt(n * #readouts)
Done!
Oct 14, 2005
Ast 920 Meeting 6

3. Etalon Basics
Etalon: adjustable interference filter. destructive interference except at wavelengths:
mλ = d cos θ
m = order; d = programmable gap
Plates must be flat, parallel to λ/m, typically 1/50-1/100 wave
The more reflective the coating, the more bounces before exiting: “f = “finesse”
distance between orders: δλ = d/m “free spectral range”
resolution: Δλ = δλ / f = d/mf
m limited by free spectral range, f by practical coatings
Oct 14, 2005
Ast 920 Meeting 6

4. Finesse and Free Spectral Range
PFIS etalons: wanted 30, got 20 δλ
Oct 14, 2005
Ast 920 Meeting 6

5. PFIS etalons
With constant finesse, higher gap has higher resolution (larger order to get same wavelength)
Can’t 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
Mode TF LR MR HR
Gap 4 10 26
127
Resolution
250
700
1400
8000
Bullseye
>8’
5.3’
3.7’
1.6’
Oct 14, 2005
Ast 920 Meeting 6

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
Oct 14, 2005
Ast 920 Meeting 6

7. Fabry-Perot Efficiency
FP efficiency at spec in red, slightly below in blue
Oct 14, 2005
Ast 920 Meeting 6

8. Fabry-Perot Resolution
Oct 14, 2005
Ast 920 Meeting 6

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 θ
center to edge gradient:
λ(r)/ λ0 = 1/ cos θ ~ ( *(r/4’)2 ) (24 Ang at Hα)
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
Oct 14, 2005
Ast 920 Meeting 6

10. Programs
FP best for programs requiring good spatial coverage, but few spectral resolution elements
Velocity mapping
line flux maps
Oct 14, 2005
Ast 920 Meeting 6