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SALT Flat Field Investigation

SALT Flat Field Investigation. Tad Pryor, Saurabh Jha, Darragh O’Donoghue, Ted Williams November 2012. Introduction. May 2012 Darragh presented his flat field investigation based on sky flat data Disagreement with predictions from models Not understood, potentially serious Why flat field?

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SALT Flat Field Investigation

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  1. SALT Flat Field Investigation Tad Pryor, Saurabh Jha, Darragh O’Donoghue, Ted Williams November 2012

  2. Introduction May 2012 Darragh presented his flat field investigation based on sky flat data Disagreement with predictions from models Not understood, potentially serious Why flat field? SALTICAM photometry (normal and slot mode) RSS Fabry Perot less important for spectroscopy

  3. Observations Leuyten equatorial standard field L107 Observed 2012 July 24 (V), 28 (V), 30 (R) 20-second exposures, 2x2 bin Repeated over ~40 minute track Results for July 30

  4. Field

  5. Stellar Photometry

  6. Stellar Photometry

  7. Residuals

  8. Caveats & Conclusions Model limitations no tracker obscuration, segment reflectivity no independent CCD sensitivity map time consuming with current resources CCD gain variations 7/24: nominal gains seem good 7/28: right CCD gain ~6% greater 7/30: right CCD gain ~3% greater temperature effect? (SCAM subsequent problems?) Accuracy ~5%

  9. SALTICAM Pixel Area Variation Tad Pryor November 1, 2012

  10. Data & Method • R-band image of 47 Tuc (S201111150001); 2×2 • (x, y) and (α, δ) for 1377 stars from David Gilbank • Project (α, δ) to tangent plane (ξ, η) • Fit polynomial transformation (x, y) (ξ, η) • Best-fit linear transformation (6 coefficients) • Fit residuals with terms up to order 3 with “half” cross terms (i.e., x2y but not x3y). • Also fit for shift and rotation between the two chips.

  11. Predicted − Actual Radius for Linear and Full Transformations 14 pixels Radius from (1019, 998) in 2×2 binned pixels

  12. Calculate the pixel area as (dξ/dx)(dη/dy) using the best fitting transformation.

  13. Discussion/Recommendations 5% accuracy: current model corrections OK 2-3% accuracy: improved calsys projector improved models Goal: 1% photometry over field flat field system: dome spot, possible projector operational requirements and commitment software RSS observations and modeling required Darragh & Ted to investigate possibilities and report in 6 months

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