1. First results of the tests campaign in VISIBLE in VISIBLE for the demonstrator 12 October 2007 SNAP Collaboration Meeting Paris Marie-Hélène Aumeunier C.Cerna, A.Ealet, E.Prieto

2. Demonstrator Objectives Goal Goal: 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: o Source: Halogen lamp + monochromator + optical fiber o Steering mirror: to scan the FoV by step < 1/100 of a pixel o 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 Detector Offner Prism Exit of optical Fiber

3. PLAN 1- Demonstrator PSFs vs simulation 2- Wavelength calibration of emission lines 3- Flux calibration of emission lines and QTH spectrum

4. PSFs Measurement x,y λ λ simulationMeasures x,y λ λ λ =500nm x,y λ 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

5. PSFs Measurement PSF shape WRT λ Wavelength (nm) Spatial FWHM (pixels) Spectral FWHM (pixels) 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)

6. PSFs Measurement Optical Losses y-coord within the slice (slice unit) Efficiency y-coord within the slice (slice unit) Efficiency 900 nm 500 nm Flux losses at the slice edges Wavelength (nm) Optical losses (%)  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 4 slice 2 slice 3 slice 4 slice 2

7. PSFs 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

8. Wavelength Calibration Adjustment of dispersion curves (1) Sources: - emission lines - spatial extended source y1y1 y4y4 y2y2 y3y3 y5y5 λ1λ1 λ2λ2 λ3λ3 λ4λ4 λ5λ5 y-coord (pixels) λ1λ1 λ2λ2 λ3λ3 λ4λ4 λ5λ5 y1y1 y4y4 y2y2 y3y3 y5y5 Spectrum on detector Adjustment of the dispersion curve f x at x-coord given Extract the lines center (barycenter) Halogen lamp + monochromator ∑ images done by scanning the slicer width with the steering mirror

9. Wavelength Calibration Adjustment of dispersion curves (2) Dispersion curves at the slice centerVS SIMULATION Dispersion curves in agreement with simulation at 95 %

10. Wavelength 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 = λ true - λ fitted with y pixel = 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

11. Wavelength Calibration Calibration of emission lines demonstrator simulation 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

12. IN SPEC  Estimation of Silicium line Center Error calibration of Si line < 1 nm using the mean of 5 slices Stars Calibration (simulation)  Error on Redshift from galaxy emission lines HαHα Galaxy spectrum simulated in IR arm at z=1.5

13. Wavelength 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

14. Flux Calibration Objectives Goal: Test calibration procedure able to find the initial flux source better than 1 % < 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

15. Flux 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 Φ ref Source Φ obj at unknown position  χ 2 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

16. Flux Calibration of emission lines Calibration of 300 images (mono λ) at 700 nm No ditheringSpatial dithering  k-theoretical = Φ objet /Φ ref = 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

17. Flux 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

18. Flux 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

19. Flux Calibration of Spectrum 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

20. Flux 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

21. Flux Calibration of QTH spectrum Wavelength (nm) Flux Error (%) SNR  k-theoretical = Φ objet /Φ ref = 1  k adjusted from 2 references images =45 μ=0.44 % σ=2.72% =204 μ=0.15 % σ=0.36 % =204 μ=0.06 % σ=0.50 % Tests conditions zone 1 [450-620 nm] zone 2 [621-830 nm] zone 2 [831-950 nm] Result

22. Flux Calibration of QTH spectrum Wavelength (nm) Flux Error (%) SNR  With spatial dithering =45 μ=0.18 % σ=1.41% =204 μ=0.02 % σ=0.15 % =204 μ=0.05 % σ=0.24 % Result

23. Flux 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 100  Calibration error WRT y  Calibration error WRT spatial dithering Results

24. Flux 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 stars Reference 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

25. Conclusion  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

26. SPARES

27. PSFs Measurement PSF shape WRT y-coord y-coord within the slice (slice unit) Spatial FWHM (pixels) y-coord within the slice (slice unit) Spectral FWHM (pixels)  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 :

28. Steering Mirror Calibration y=3.00078 Flux total = ∑ 5 slices slice 3 slice 2 slice 5 slice 1 slice 4 y 13452 x 1 2345 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

29. Camera 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)

30. Camera 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

31. Pupil Mirror Slit Mirror slice 0 slice -1 slice 1 SLIT 0 SLIT -1SLIT 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 Measurement Principle

32. Straylight 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 5 slice 5 slice 1 slice 5 slice 1 slice 4 3 5 2 3 4 5 1 5 1 5 1 3 Slit 1 Slit 2 Slit 3 Slit 4 Slit 5 PSF on the focal plane (log scale) Flux ratio gives the cross-talk

33. Straylight Measurement Results @ 500 nm 4 K 3/1 =7x10 -4 (95 % CL) 5 3 21 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 K 5/1 =7.7x10 -4 (95 % CL)

34. Straylight Measurement Conclusion  Straylight controlled better than 10 -3  Sraylight measurement limited by the detector noise Wavelength K3 /1 (95 % CL) K3 /5 (95 % CL) 500 nm8x10 -4 9.5x10 -4 700 nm4.8x10 -4 2.5x10 -4