SIMPLEImaging and Mosaicking PipeLinE

Wei-Hao Wang (王 為豪)
Academia Sinica Institute of Astronomy and Astrophysics

Outline✦✦

A brief introduction to SIMPLEReduction flowchart for near-IR mosaiccamera images✦

More on distortion correction✦

Current SIMPLE distributions

What is SIMPLE?
✦✦✦SIMPLE =SIMPLE Imaging and Mosaicking PipeLinE
An IDL-based data reduction package.
More optimized for optical and near-IR
extragalactic deep imaging.

✦Can process data from large mosaic cameras.

✦

Data reduction is quite automatic.

Showcase Images

M13 at H band, by UH2.2m and ULBCAM (HAWAII2 × 4)
Reduced by SIMPLE v.-1.0

M17 at J band, by UH2.2m and ULBCAM
A showcase image for ULBCAM on Physics Today Magazine (2004).
Reduced by SIMPLE v.-1.0 and assembled in Photoshop.
RGB color coded by 2MASS JHK low-resolution images.

SWIRE Lockman Hole at H band, by UKIRT and WFCAM (HAWAII2 × 4)
Initial reduction by UKIRT pipeline. Mosaicked by SIMPLE v.1.0

Image size = 124002 pixels = 53.82 arcmin2

Groth Strip at NB921 by Subaru SuprimeCamMosaicked with SIMPLE v.1.0 by L. Trouille (UW-Madison)

Image size = 18000 × 16000 pixels = 0.74 deg2

GOODS-N at Ks by Subaru MOIRCS (HAWAII2 × 2)
Image size = 10400×7280 pixels = 19.8×13.8 arcmin2
Max integration time = 22 hr
Reduced by SIMPLE-MOIRCS v.1.0
Barger, Cowie, & Wang (2008)
Wang, Barger, & Cowie (2009)

GOODS-N at Ks by CFHT WIRCAM (HAWAII2 × 4)
Image size = 65302 pixels = 32.72 arcmin2
Integration time = 39 hr
Reduced by SIMPLE-WIRCAM v.0.9
Wang, Cowie, & Barger (2007)
Barger, Cowie, & Wang (2009)

Full data release: Wang et al. (2009, in preparation)

SIMPLE Reduction of theWIRCam GOODS-N Images✦✦

RMS astrometric offset relative to the HST ACS catalog is 0”.
03 (both RA and Dec) on high S/N sources.
Photometry (calibrated with 2MASS) is uniform in a ~0°.2
scale within 1.3%.

The SIMPLE Reduction
Dithered Raw Exposures• Median Stack to Derive Flat Field
Flattened Images• Detect and Mask Objects

BasicReduction

Object Masked Raw Images• Flat Fielding• Background Subtraction• Spatial σ Filter for Cosmic RayRemoval• Removal of Instrumental Features

GeometricCorrection• Median Stack to Derive Flat Field
Second Flat Field

The SIMPLE Reduction
Reduced Single Exposures• Detect Objects in Each Image• Derive Image Offsets with Overlapping Objects• Bootstrap Distortion Functions
Geometric Transform A• Warp Images to Correct for Distortion• Shift and Stack the Distortion Corrected Images

GeometricCorrection

A Deep Stacked Image• Detect Objects• Compare Detected Objects and the Astrometric
Catalog to Determine Absolute Astrometry

Geometric Transform B⊗A• Warp the Images

Image Stacking andPhotometric Calibration

Geometric Transform B• Warp and Project the Image to the Desired WCS

AstrometricCatalog

The SIMPLE Reduction
Distortion Corrected and WCS
Projected Single Exposures• Detect Objects in Each Image• Derive and Correct for Photometric
Offsets with Overlapping Objects

Photometrically Matched Single Exposures

Image Stacking andPhotometric Calibration• Align the Images• Temporal σ Filter for Cosmic Ray Removal• Mean Stack the Images
A Deep Wide-Field Mosaic• Photmetric Calibration with a Standard
Star or with a Phometric Catalog

Final Image

PhotometricCatalog

SIMPLE Distortion CorrectionThe optical distortion functions:x′ = F(x,y), y′ = G(x,y),x, y = object position in the undistorted framex′, y′ = object position in the distorted frame
The displacement of a star in two dithered images:∆x′ ≈ ∂xF ∆x + ∂yF ∆y∆y′ ≈ ∂xG ∆x + ∂yG ∆y.
∆x, ∆y = dither offset of the telescope (same for all stars)∆x′, ∆y′ = displacements of star in the images (different for all stars)
With N stars and M dithered images, we can solve for ∂xF, ∂yF, ∂xG, ∂yG
with a system of 2×N×(M-1) linear equations.
The solutions are the 1st-order derivatives of the distortion functions.
Based on the method described by Anderson & King (2003) for WFPC2.

SIMPLE Distortion Correction
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✦

✦✦

Only requires a few (>5) dithered images and >10 compact
objects in the images to obtain good solutions. Usually much
more are available.
No need for any external information.
Can handle time-dependent distortion (e.g., that caused bytelescope flexure).
Quality of the image registration is not limited by the quality ofthe astrometric catalog.
Still need an external catalog for projection and absoluteastrometry.

Current SIMPLE Distributions✦✦✦✦✦✦Best optimized for extragalactic deep field observations.
Basic tools are available for general processing:flat-fielding, background subtraction, distortion
related functions, image warping, mosaicking, etc.
Two highly optimized automatic pipelines for Subaru
MOIRCS and CFHT WIRCam are available.
An optimized version for SuprimeCam is considered, andwill be released if implemented.
http://www.asiaa.sinica.edu.tw/~whwang/idl/SIMPLE/
We would like to hire a postdoc to work on SIMPLE stuffs.
Please contact me if you are interested.