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Application of a Charge Transfer Model to Space Telescope Data

www.stecf.org/poa. Application of a Charge Transfer Model to Space Telescope Data. Paul Bristow Dec’03. The STIS Calibration Enhancement Project.

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Application of a Charge Transfer Model to Space Telescope Data

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  1. www.stecf.org/poa Application of a Charge Transfer Model to Space Telescope Data Paul Bristow Dec’03

  2. The STIS Calibration Enhancement Project • A comprehensive empirical calibration pipeline already exists for STIS. We aim to improve those components which benefit from physically motivated corrections. • Current work includes: • Wavelength Calibration • Calibration lamp line list - new lab measurements at NIST • Optical Model • Optimal Spectral Extraction • Detector Model

  3. STIS CTE trails grow with time:

  4. CCD Model: Concept • Forward Simulation: • Start with • 2D Charge distribution on chip • Distribution of bulk traps • Shift the charge under each electrode as during readout • Calculate capture and emission of charge in each shift • Take into Account: • Status of bulk traps • Dark current • Trap capture and emission timescales • Chip clocking frequency, architecture, gain etc.

  5. Differenceimage Differenceimage Correctedimage Correctedimage Simulationoutput Correction: Rawdata >> SIMULATION = - - = Rawdata Rawdata • Further iterations (usually not necessary): >> SIMULATION

  6. Cleaning CTE Trails

  7. CCD Model: Implementation • Follows a prescription outlined in Philbrick 2001 (Thanks to Rob for correspondence)

  8. CCD Model: Implementation • Types: P-V (Si-E), O-V (Si-A), V-V • Timescales: • Emission - essentially from lab data • Capture - always smaller than clock period (for STIS at least) • Trap density: • NIEL appropriate to HST orbit • Linear scaling with time

  9. Application to STIS • BIAS offset and pattern must be removed from raw data before input to CTI model • Divide by GAIN • Negative value pixels corrected (unphysical) • Reference dark files also suffer from CTE and must be corrected • Reference bias files contain hot columns which are actually corrected by the CTI model => bias files must be modified

  10. Comparison to Empirical Corrections for Imaging Data • Empirical calibration used as a test of physical model • On average we should expect agreement • This comparison can also help to calibrate the physical model • Good general agreement (especially for objects to which the empirical calibration applies)

  11. Special Cases: Images

  12. Trap Distribution: • Mean (over chip) density fixed by NIEL and time on orbit. • Uniform versus arbitrary clustering • Mapping traps • Ground testing no good, traps only begin to appear on orbit. • Pocket pumping • On board Fe55 (Chandra) • Dark current 

  13. “Initial” Trap Status • Possibilities: • All empty/All full • Function of charge collected during exposure (collecting phase electrodes)  • Long timescale traps • Turns out not to be critical

  14. Notch/Mini-channel • The STIS CCD was designed to have a mini-channel • There seems to be no pre-launch data which characterises it. • Represent channel with 2 parameters: nmc (<nt) and Nsat-mc (<<Nsat) • CCD readout model produces better results with Nsat-mc=0 

  15. Quantisation and Noise • There’s no such thing as half an electron! • Original implementation of readout model used integer, i.e. quantised, representation of charge. • e.g. nex-INT=Poisson(nex-FLOAT) i.e. random • Current version uses floating point representation (fractional e-), i.e. deterministic  • Similar results (for CTI effect on fluxes, charge trails etc), However….

  16. Quantisation and Noise (cont.) • Quantised model predicts that the readout process increases the background  background as a function of row • More transfers, more random events => seems reasonable! • Not observed! • For this reason the deterministic version has been adopted

  17. Exporting to other Detectors (WFPC2) • Initial attempts to apply to WFPC2 require large, arbitrary changes to trap normalisation to get reasonable results. • Difficult to find a normalisation that fits a range of data. • Differences (STIS to WFPC2): • WFPC2 is front illuminated • Charge/volume relation may be different? • Different mix of trap timescales? • Clocking, chip size, gain, time on orbit etc. should be easily accounted for.

  18. www.stecf.org/poa Conclusions • We have developed a model of the STIS CCD readout and used it to correct real data • The result for STIS provide a substantial calibration enhancement for CTI effected data • The model is generic and portable to other CCD detectors • There remain aspects which, while not critical to STIS calibration, could be better understood and implemented.

  19. Comparison to Empirical Corrections for Spectroscopic Data • Same principle as for imaging data • Once again, good general agreement

  20. Special Cases: Spectroscopy

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