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SCUBA-2: The Flat-field Problem

SCUBA-2: The Flat-field Problem. Dennis Kelly (UKATC). Summary. The SCUBA-2 arrays will be Transition Edge Sensors DC-coupled to the data acquisition system. The brightness of the Earth’s atmospheric emission puts severe demands on flat-fielding for simple imaging.

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SCUBA-2: The Flat-field Problem

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  1. SCUBA-2:The Flat-field Problem Dennis Kelly (UKATC)

  2. Summary • The SCUBA-2 arrays will be Transition Edge Sensors DC-coupled to the data acquisition system. • The brightness of the Earth’s atmospheric emission puts severe demands on flat-fielding for simple imaging. • Differential techniques such as DREAM or SCANMAP can avoid this problem. • The arrays are read-out using SQUID amplifiers. These introduce complexities in the setup of the measurement system, and the need to take care to avoid jumps in the measurement zero points.

  3. Sky Time Variations Temperature in K vs time in seconds 0.45 mm SCUBA DC data Signal vs frequency in Hz Quadratic baseline removed

  4. Sky Spatial Noise 1/f11/6 Sky simulation

  5. Photon-Limited S/N

  6. Bolometers Radiation A 0v +V Cold Surface Resistance Temperature

  7. Transition Edge Sensors Resistance Temperature Ptotal = Pinput + Pohmic Pohmic = V2/R I = V/R

  8. TES Response Current Input Power

  9. Response of Different Pixels Current Input Power Two bolometers have different responses to the same input power. Assume the measured relative responses are in error by 5%. In the total power measured, this will mean the deduced power is different by 5% of the signal from the Earth’s atmosphere, which is many orders of magnitude greater than an astronomical signal. If each bolometer is doing a differential measurement, then the 5% difference just turns into a 5% error in photometry.

  10. Changing Pixel Response Two bolometers with Tc 0.12K and 0.13K were modelled, first with the effective bath temperature at 0.06K and then at 0.06001 (ie 10μK higher) 9 . 4 10 9 . 2 10 Error after flatfield 0 9 . 2 10 9 . 4 10 5 5 5 5 5 5 . . . . . . 1 10 1.5 10 2 10 2.5 10 3 10 3.5 10 Bolometer Current The diagram shows that the error in applying the calibration for bath temperature 0.06K when the actual temperature is 0.06001K is one part in 104.

  11. SQUID Ammeter Comp +V +V V0 V0 Magnetic Flux Resistance 0v 0v 0v

  12. bias point A/D range d16 φ = φtrap + φbol - φD/A SQUID Behaviour

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