Post on 13-Jan-2016
First results of the tests campaignFirst results of the tests campaign in VISIBLE in VISIBLE
for the demonstratorfor the demonstrator
12 October 2007SNAP Collaboration MeetingParis
Marie-Hélène Aumeunier
C.Cerna, A.Ealet, E.Prieto
Demonstrator ObjectivesDemonstrator Objectives
GoalGoal: Validate the performances slicer concept and test the calibration procedure
Straylight Measurement controlled at 10-3
Wavelength Calibration at the nanometer level
Flux Calibration better than 1 %
The demonstrator optical bench:The demonstrator optical bench:
o Source: Halogen lamp + monochromator + optical fibero Steering mirror: to scan the FoV by step < 1/100 of a pixelo Demonstrator: the same characteristics as the SNAP spectrograph in board (low spectral resolution + under sampled IR)o Detectors: Camera Apogee in visible
Rockwell HgCdTe in IR
Steering Mirror
Slicer Imager
DetectorOffner
PrismExit of optical
Fiber
PLANPLAN
1- Demonstrator PSFs vs simulation
2- Wavelength calibration of emission lines
3- Flux calibration of emission lines and QTH spectrum
PSFs MeasurementPSFs Measurement
x,y λ x,y λ
simulation Measures
x,yλλ
λ=500nm
x,yλ
Double verification:
Measure of PSF shape
Measure the optical losses
λ=900nm
PSF within the central slice when the point source hits the slicer
center
At the input simulation, we need:- λ (provided by monochromator)- (x,y) into the slice (given by steering mirror)- position of the initial pixel on the detector plane (fitted manually)
Objectives
The demonstrator: an opportunity to test the simulation with real data
Check the alignment of the complete instrument at ≠ (x,y,λ)
Check the diffraction effect by the slice edge
Method
PSFs MeasurementPSFs Measurement PSF shape WRT λ
Wavelength (nm)
Sp
ati
al FW
HM
(p
ixels
)
Sp
ectr
al FW
HM
(p
ixels
)
grating 1
grating 2
Lines simulated as Dirac
In the spatial direction: maximum ∆FWHM sim / exp ~ ½ pixels at the best focus
In the spectral direction: Emission lines have a broad width Gap for spectral FWHM at 600 nm due to the change of
monochromator’s grating
Ideal focus+ Perfect detector
Best focus adjusted
Wavelength (nm)
PSFs MeasurementPSFs Measurement Optical Losses
y-coord within the slice (slice unit)
Effi
cie
ncy
y-coord within the slice (slice unit)
Effi
cie
ncy
900 nm
500 nmFlux losses at the slice edges
Wavelength (nm)
Op
tical lo
sses (
%)
At λ>600 nm : optical losses predicted with a precision better than 2 %
At λ < 600 nm: the highest flux losses the PSF is finer, the diffraction by the slice edges are more important
Current Work: implement in simulation « dead area » between 2 slices responsible of higher optical losses
slice 3 slice 4slice 2
slice 3 slice 4slice 2
PSFs MeasurementPSFs Measurement
Good agreement of data with the simulation:
No severe default detected from one slice to another:
PSF shape homogenous along the slicer width
Optical losses at the slice edges checked better than 2
% for λ > 600nm
Conclusion
Wavelength CalibrationWavelength Calibration Adjustment of dispersion curves(1)
Sources:
- emission lines
- spatial extended source
y1
y4
y2
y3
y5
λ1
λ2
λ3
λ4
λ5
pixelx yf
y-coord (pixels)
λ1
λ2
λ3
λ4
λ5
y1 y4y2 y3 y5
Spectrum on detector
Adjustment of the dispersion curve fx at x-coord given
Extract the lines center
(barycenter)
Halogen lamp +
monochromator
∑ images done by scanning the slicer width with the steering
mirror
Wavelength CalibrationWavelength Calibration Adjustment of dispersion curves(2)
Dispersion curves at the slice center VS SIMULATION
Dispersion curves in agreement with simulation at 95 %
Wavelength CalibrationWavelength Calibration Calibration Procedure of punctual emission lines
1. Compute the wavelength using the dispersion curves of each slice
2. Correct slit effect for punctual sources
3. Calibration error = λtrue - λfitted
pixelxfitted yf with ypixel = center of lines on the detector
Slit Effect (definition)
When the point source scans the slicer width (i.e spectrograph slit), the lines center moves also on the detector
Causes calibration error between the real and fitted λ
Visible pixel
λ Dispersion
Correction of slit effect
Mean of 5 slices: average the spectrum of each slice weighted by the flux into the slice
Spatial dithering: average the images done when the point source position is shifted of a random value (Normal distribution of RMS 1/5 slices)
Procedure steps
Wavelength CalibrationWavelength Calibration Calibration of emission linesdem
onst
rato
rsi
mula
tion
613 nm
612 nm
835 nm
823 nm
405 nm
485 nm
Data in agreement with the result predicted by simulation Slit effect corrected with mean of 5 slices No need of spatial dithering Offset (1 nm) for the high wavelength current work
IN SPEC
14.4Si nm 1.44Si µm
Estimation of Silicium line Center
Error calibration of Si line < 1 nm using the mean of 5 slices
Stars Calibration (simulation) Stars Calibration (simulation)
Error on Redshift from galaxy emission lines
Hα
0.87H
syst nm
1.5H H fittedz z
0.002 5 /1000z
Galaxy spectrum simulated in IR arm at z=1.5
Wavelength CalibrationWavelength Calibration Conclusion
Wavelength calibration possible with classical procedure (despite of low spectral resolution + undersampled)
Calibration error < 1 nm using the information within the 5 slices (mean of 5 slices)
Perspectives: Fly calibration
find adequate calibration lamp: it is difficult to use blended lines
Flux CalibrationFlux Calibration Objectives
Goal:
Test calibration procedure able to find the initial flux source better than 1 %
%12222 otherncalibratioSTATflux
< 0.33 %Which means ?
Calibrate all effects that damages the source flux better than 1/3 %:
Optical losses: diffraction, coating,… Detector: quantum efficiency, pixel response (fringing for
CCD, intra-pixel sensitivity variation, CTE for IR detector, etc)
Difficult ! AND the PSF is under-sampled in the IR range and so sensitive to intra-pixel variation
Flux CalibrationFlux Calibration Method
Telescope Focal Plane (slicer)
1/10 of a pixel
slice 1
slice 0
slice -1
A library of reference images to characterize the spectrograph response for any (x,y,λ)
Done from reference source (point source) with the flux known better than 1 %
Reference images
Reference source Φref
Source Φobj at unknown position
ref
objk
χ2 minimization per pixel
Linear Interpolation of flux per slice m
A method to calibrate the object flux at unknown position from the library
Find the « nearest » reference images
Interpolate the object image from the selected images to deduce the ratio k
Flux Calibration of emission linesFlux Calibration of emission lines
Calibration of 300 images (monoλ) at 700 nm
No dithering Spatial dithering
k-theoretical = Φobjet/Φref = 1
k adjusted from 2 references images
Mean error= 0.22 %Std of mean = 0.39 %
Mean error= 0.17 %Std of mean = 0.14%
Results at 450, 500, 700 and 900 nm
At 700 nm, flux error << 1% (95% CL)
Dispersion error improved with spatial dithering (4 images)
Precision of k-theoritical to improve for best measurement
(95% CL)
Result
Flux Calibration of spectrumFlux Calibration of spectrum Method
First step: Select the reference images
λ
x,y
slice 2 slice 3 slice 4
Mask @ 700 nm
Extract a region of image associated to a narrow band of spectrum
λ
slice 2 slice 3 slice 4
x,y
Flux Calibration of spectrumFlux Calibration of spectrum Method
First step: Select the reference images
Extract a region of image associated to a narrow band of spectrum
λ
x,y
slice 2 slice 3 slice 4
Minimization method applied to
“monochromatic” image
Flux Calibration of SpectrumFlux Calibration of Spectrum
j
iij
First step: Select the reference images
1 –Minimization per pixels
2 –Minimization on ratio of slice flux
Sensitive to the position within the slice
Object Point
Reference Points
Selected Points
1/20 of a slice = 20 arc sec
1/20 of a slice
one slice
the same minimization as the one developed for the mission line
Method
Flux calibration of spectrumFlux calibration of spectrum
Second step: Find the ratio k(λ, ∆λ) between the reference spectra and the spectrum to calibrate
Method
Linear interpolation of the flux captured in each slice by narrow band
Flux Calibration of QTH spectrumFlux Calibration of QTH spectrum
Wavelength (nm)
Flu
x E
rror
(%)
SN
R
k-theoretical = Φobjet/Φref = 1
k adjusted from 2 references images
<S/N>=45μ=0.44 %σ=2.72%
<S/N>=204μ=0.15 %σ=0.36 %
<S/N>=204μ=0.06 %σ=0.50 %
Tests conditions zone 1[450-620 nm]
zone 2[621-830 nm]
zone 2[831-950 nm]
Result
Flux Calibration of QTH spectrumFlux Calibration of QTH spectrum
Wavelength (nm)
Flu
x E
rror
(%)
SN
R
With spatial dithering <S/N>=45μ=0.18 %σ=1.41%
<S/N>=204μ=0.02 %σ=0.15 %
<S/N>=204μ=0.05 %σ=0.24 %
Result
Flux Calibration of QTH spectrumFlux Calibration of QTH spectrum
95 % CL
Calibration error WRT SNR
No degradation when the point
source moves within the slice 4 images dithered enough to
improve calibration Error Flux < 1% when SNR > 100
Calibration error WRT y
Calibration error WRT spatial dithering
Results
Flux CalibrationFlux Calibration
Slicer technology helps the flux calibration
Move the point source of a 1/ 10 of a slice
Conclusion
Fly calibration
Even moving by small steps in the FoV (1/10 pixel), the detector image changes completely :
The PSF is cut differently by the slicer The sampling of each PSF’s part is also different
• Adapt the method to calibrate secondary stars from fundamental starsReference library made with artificial lamps at ground
made with fundemantal stars in fly
Intensity spatial variation do not the same
Take into account the sky background and the variation of detector gain map
ConclusionConclusion
INFRARED campaign in coming Adapt calibration procedure in fly : find adquate calibration source,
Good agreement between PSFs measurements and simulation (shape and optical losses) Straylight in specification (<10-3) at 500 and 700 nm
Visible campaign Report:
Good results for wavelength calibration of emission lines: correction of slit effect works (mean of 5 slices) Accurate flux calibration (1 %) for emission lines + spectrum @ k=1
The optical performances of slicer are checked in visible
The calibration procedures have been tested and validated
Perspectives
SPARESSPARES
PSFs MeasurementPSFs Measurement PSF shape WRT y-coord
y-coord within the slice (slice unit)
Sp
ati
al FW
HM
(p
ixels
)
y-coord within the slice (slice unit)
Sp
ectr
al FW
HM
(p
ixels
)
Complete PSF
Uniform along the slice width
No diffraction effect
PSF cut by the slice
Sensitive to the position within the slice
Diffraction by the slice edge: PSF widening
PSF projected along the spatial direction :
PSF projected along the spectral direction :
Steering Mirror CalibrationSteering Mirror Calibration
y=3.00078
Flux total = ∑ 5 slices
slice 3slice 2 slice 5slice 1 slice 4
y1 3 4 52
x1 2 3 4 5
Method:- scan the slicer width with monochromatic
point source by step of 1/10 of a pixel- guess the slice edges around the curves
intersection of flux per slice
Goal: calibrate the relative position along the
slice width given by the steering mirror
Steering Calibration at 700 nm
Result:
Precision < 1/10 of slice width
Camera CalibrationCamera Calibration Fringing (1)
Definition: sensitivity variation of pixel as a function of λ due to interference among the incident and reflected beam within the CCD layers
Consequence
Input spectrum: Halogen lamp Spectrum on detector
Wavelength (nm)
(semi-log scale)
Wavelength (nm)
(semi-log scale)
Camera CalibrationCamera Calibration Fringing (2)
Fringing characterization:
- Fit the QTH spectrum on detector
- Compute the response per pixel i and per λ
∆λ/pixel < 5 nm 5 < ∆λ/pixel < 10 nm
Result:
Fringing responsible of flux
variations: up to 10 % for λ<700 nm up to 5 % for λ> 700 nm
Low spectral resolution smoothes the
flux variations at high wavelength
Pupil Mirror
Slit Mirror
slice 0slice -1
slice 1
SLIT 0SLIT -1 SLIT 1
CROSS-
TALK
Only the slice 0 well aligned with the optical axis is lighted on the pseudo-slit 0
In case of straylight, the light coming from the slices may be also seen by the pseudo-slit 0
Ghost Images (CROSS-TALK )
slicer
Straylight
Straylight MeasurementStraylight Measurement Principle
Straylight MeasurementStraylight Measurement Experimental method
slice 1
slice 2
slice 3
slice 4
slice 5
PSF [slicer] PSF [spectrograph]
PSF at the spectrograph entrance saved into the slice
3
PSF into the slicer plane
Method: Sum N images of punctual monochromatic source to extract possible light coming from the slice 3 onto the slits 1 and 5
1
3
5slice 5
slice 1
slice 5
slice 1
3/3
1/31/3 F
FK
3/3
5/35/3 F
FK
slice 1
slice 43
5
2 3 4 5
1
5
1
5
1
3Slit 1Slit 2
Slit 3Slit 4
Slit 5PSF on the focal plane (log scale)
Flux ratio gives the cross-talk
Straylight MeasurementStraylight Measurement Results @ 500 nm
4
K3/1=7x10-4
(95 % CL)
5321
3
3
3
3
slit 1
slit 2
slit 3
slit 4
slit 5 As predicted by simulation, the PSF parts at the edges of the slit 2 and 4 spreads out of the slice image because of the spectrograph PSF convolution
K5/1=7.7x10-4
(95 % CL)
Straylight MeasurementStraylight Measurement Conclusion
Straylight controlled better than 10-3
Sraylight measurement limited by the detector noise
WavelengthK3 /1
(95 % CL)K3 /5
(95 % CL)
500 nm 8x10-4 9.5x10-4
700 nm 4.8x10-4 2.5x10-4