2Bhk flat with an area of 877 sq. ft available at Mahishbathan,Salt Lake,Sector-v.
SALT Flat Field Investigation
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Transcript of SALT Flat Field Investigation
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SALT Flat Field Investigation
Tad Pryor, Saurabh Jha, Darragh O’Donoghue, Ted Williams
November 2012
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Introduction
May 2012 Darragh presented his flat field investigation based on sky flat dataDisagreement with predictions from modelsNot understood, potentially serious
Why flat field?SALTICAM photometry (normal and slot mode)RSS Fabry Perotless important for spectroscopy
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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
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Field
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Stellar Photometry
-5.6
-5.5
-5.4
-5.3
-5.2
-5.1
-5
July 30 2012
611611 model612611 model
Image Sequence Number
m(in
st) -
m(s
td)
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Stellar Photometry
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Residuals
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Caveats & Conclusions
Model limitationsno tracker obscuration, segment reflectivityno independent CCD sensitivity maptime consuming with current resources
CCD gain variations7/24: nominal gains seem good7/28: right CCD gain ~6% greater7/30: right CCD gain ~3% greatertemperature effect? (SCAM subsequent problems?)
Accuracy ~5%
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SALTICAM Pixel Area Variation
Tad PryorNovember 1, 2012
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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.
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14 pixels
Radius from (1019, 998) in 2×2 binned pixels
Predicted − Actual Radius for Linear and Full Transformations
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Calculate the pixel area as (dξ/dx)(dη/dy) using the best fitting transformation.
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Discussion/Recommendations
5% accuracy: current model corrections OK2-3% accuracy:
improved calsys projectorimproved models
Goal: 1% photometry over fieldflat field system: dome spot, possible projectoroperational requirements and commitmentsoftware
RSS observations and modeling required
Darragh & Ted to investigate possibilities and report in 6 months