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 dataDisagreement with predictions from modelsNot understood, potentially serious

Why flat field?SALTICAM photometry (normal and slot mode)RSS Fabry Perotless important for spectroscopy

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

Field

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)

Stellar Photometry

Residuals

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%

SALTICAM Pixel Area Variation

Tad PryorNovember 1, 2012

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.

14 pixels

Radius from (1019, 998) in 2×2 binned pixels

Predicted − Actual Radius for Linear and Full Transformations

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

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