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1Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP
Initial Data Reduction for the FPP Spectro-Polarimeter
October, 2004
Bruce W. Lites
303 497 1517
2Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP FPP Spectro-Polarimeter Data
OBJECTIVE:The objective of the initial processing is to prepare the FPP-SP data
in a form suitable for scientific data analysis.
PROPOSED METHOD:I propose to adapt extensive data reduction tools developed for the
Diffraction-Limited Spectro-Polarimeter (DLSP) at the National Solar Observatory/Dunn Solar Telescope to the FPP-SP.
– DLSP is an instrument that evolved from the Solar-B Concept Model Spectro-Polarimeter
– DLSP codes written in general way to be easily adapted to other spectro-polarimeters
– Software both in IDL and FORTRAN (for speed of reduction)
3Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP
Steps for FPP-SP Initial Data Reduction
Prior to analysis of science data:
•Prepare the dark and flat field correction images
•Prepare the polarization calibration matrix
For science data, the steps in order are:
•Dark and flat correct
•Apply polarization calibration
•Remove spectral “skew”
•Merge the two polarization beams
•Fringe removal
•Correct for spectral curvature
•Compensate for residual I→Q,U,V crosstalk
4Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Illustration of the Reduction Process
I illustrate this data reduction scheme with the procedure as adapted to data from the new spectro-polarimeter at the Swedish Solar Telescope (SST).
These data from 1 April 2006 demonstrate the correction “end-to-end” resulting in fully calibrated data.
5Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Dark and Flat Field generation
•Dark measurements from ground-based instruments are simple – just block the beam to the spectrograph.
•FPP-SP darks are problematic, no shutter.
•Flat field images: for FPP-SP must average many independent images of quiet granulation near disk center.
•rms contrast of granulation expected to be ~15%
•To achieve flats accurate to 0.5% rms, need ~900 independent measurements of granulation
•Many coarse maps of quiet Sun required!
•Flat procedure for spectra: obtain an average spectral profile from the average of flat images, then divide the spectrum by this profile to obtain the flat image. The multiplicative flat-field correction is the inverse of the flat image.
6Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Multiplicative Flat Field Corrections
Dark 0 Flat 0 Dark 1 Flat 1
Sample dark, multiplicative flat field images for the FPP-SP obtained in sun tests on 13 June 2005.
•Fewer flat images were needed than on orbit because the seeing was bad
•Dark images are very uniform
•Multiplicative flat images show little trace of spectral lines
•Flat images scaled ±10%
Corrected image = (Raw-Dark)xMflat
7Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Flat Field Correction
Dark-corrected Flat Field Data Multiplicative Flat Field Correction
8Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Dark Corrected Data
I
U
Q
V
9Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Dark/Flat Field Corrected Data
I
U
Q
V
•Opposite Q,U,V signatures in two orthogonal polarization image pairs
10Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP FPP-SP Calibration Matrices X-1
CCDSIDE0
CCDSIDE1
Original Smoothed ResidualSpectral ROI 112-224
11Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP
FPP-SP Variation of X over Slit Scan Range
•Error Bars: polarization matrix requirement
•Slit scan position -225 taken at low light level, so discarded
12Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Polarization Calibrated Data
I
U
Q
V
•Same Q,U,V signatures in two orthogonal polarization pairs
•Symmetric Q,U
•Antisymmetric V
•Opposite seeing crosstalk in Q,U,V pairs
13Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Skew Corrected Data
I
U
Q
V
14Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Merged Data
I UQ V
Seeing crosstalk eliminated
15Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST Spectral Curvature Removed
I UQ V
16Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP SST High Sensitivity Q,U,V
I UQ V
Q,U,V Grey Scale: ±0.5% Ic
17Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP
SST Residual I→Q,U,V Crosstalk Removed
I UQ V
18Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Other FPP-SP Reduction Issues
•Slit Scan Vignetting
•Variation of SP throughput exists as a function of slit scan position. Also a 2-D variation vs. slit scan position (x) and distance along the slit length (y)???
•Polarized Spectral Fringes
•Known to exist in the polarization calibration matrices
•Smoothed over in the representation of the polarization calibration matrix as a function of (λ,y).
•Refinement of the Calibration Matrix
•It is possible to use solar observations of a sunspot umbra to refine the polarization calibration matrix. This will be difficult in view of variations of the matrix in (λ,x,y).
19Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP
19 August 2004 NAOJ SP intensity vs. scan mirror position before pre-slit repair. FPP on OBU with solar feed.
Scan Mirror Step Number Scan Mirror Step Number
26 May 2005 NAOJ SP intensity vs. scan mirror position after pre-slit repair. FPP on optical bench. Solar feed with telescope simulator.
Slit Scan Vignetting
20Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Slit Scan Vignetting
•Additional measurements: Careful observational study on-orbit of flat field observations taken over full range of slit scan positions
•Analysis: Derive variation of intensity of these flat field observations as corrected by a flat field derived at the center of the scan range. Derive the normalization factor as a function of (x,y): ASP(x,y)
•Corrections: Apply the normalization function ASP(x,y) to all FPP-SP map data. Applies equally to Stokes I,Q,U,V
21Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP FPP-SP Polarized Spectral Fringes
•Spectral fringes are apparent at the few x 10-3 level (or less) in the calibration matrices
•These fringes are not represented in the interpolated, smoothed representations of the calibration matrices
•Will they show up in the final data on orbit?
CCDSIDE1, Spectral ROI 0-112
22Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Polarized Spectral Fringes
An example of fringe removal from DLSP spectral data. One must examine final calibrated data from space to look for residual fringes of concern.
Before Fringe Correction After Fringe Correction
23Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Reduction Code Strategy
•Preliminary analyses done in IDL
•Calculation of flat field corrections
•Calculation of vignetting corrections
•Preparation of polarization response matrix corrections
These corrections are determined only occasionally
24Lites
FPP-SP Initial ReductSOT #17 Meeting, NAOJ,
April. 2006
Solar-BFPP Reduction Code Strategy
•Routine Map corrections done with FORTRAN code spawned from IDL control routine
•Application of dark/flat corrections
•Polarization calibration
•Skew removal
•Merging orthogonal polarization images
•Spectral curvature removal
•Residual I→ Q,U,V crosstalk correction
•Vignetting correction
•Fringe removal?
