The Stacker

1. The Stacker Gordon Hurford, Ed Schmahl, Richard Schwartz 18-April-2005

2. What is the Stacker? • Current imaging algorithms based on time-binned event lists • Time bins must be very short (~1-100ms) to preserve modulation • Few events per bin (statistics, display, Forward Fit issues) • Large number of bins (long integrations impractical) • Stacker is a form of superimposed epoch analysis • Compresses data from an arbitrarily long interval into the equivalent of a 1-rotation integration • Almost no loss of imaging information

3. How does the Stacker help? • Makes long integrations feasible But… solar rotation ~ 10 arcsec/hour at disk center • Some improvement to image quality • Improved chi^2 • Improved Forward Fit performance • Helps with background and flare-variability issues • Improvements in imaging speed • fewer time bins to fit • reuse stacked data (future) • Opens the way to visibilities

4. How does the Stacker work? (1) • Pointing changes • Modulated light curve is different in each rotation  Cannot just stack data with rotation period • Count rate in each time bin depends on: • Source geometry and location • Grid transmission and modulation amplitude • The occurancetime of the bin is not relevant. • Roll angle and phase (relative to map center) is relevant  Substitute roll angle / phase bins for time bins

5. How does the Stacker work? (2) • Stacker associates each time bin with a roll / phase bin • Accumulates counts and live time in each phase bin • Calculates average grid transmission and modulation amplitude for each roll / phase bin • Converts populated roll/phase bins back to equivalent time bins  Existing mapping algorithms can be used as is

6. Typical Modulation Profiles

7. Mapping Time Bins to Roll/Phase Bins

8. One Rotation

9. Multiple Rotations

10. Roll and Phase Bins

11. Populated Roll / Phase Bins Subcollimators 1-9 23 July 2002 12-25 keV 80-second integration Counts ____________________ livetime*gridtran*modamp c 23 July

12. Profiles in Roll Bins Subcollimator 5 25 March 2002 12-25 keV 80-second integration Counts/phase bin c 23 July

13. Stacked Modulation Profile Grid 8 7680 time bins 288 roll / phase bins

14. Comparison of Unstacked vs Stacked PIXON Maps Unstacked Stacked

15. Comparison of Unstacked vs Stacked PIXON Fits

16. Using the Stacker (1) • Default is not to use the stacker • No advantage if integration time is only a few rotations • Invoke with switch, /use_phz_stacker • Number of phase bins • Reduces s/n if too small. • Default=12 (99% efficient) • Obj -> set, phz_n_phase_bins = nnn

17. Using the Stacker (2) • Number of roll bins • Reduces s/n near edge of fov if too small • Minimum value is determined by (max source offset) / (angular pitch) • Default: Number of roll bins is calculated automatically assuming source offset = 60 arcsec or image_dim * pixel_size/2 • To set source offset explicitly, phz_n_roll_bins_control = 0 phz_radius = nnn (arcsec) • To define number of roll bins explicitly, phz_n_roll_bins_control = [n1,n2,,,,n9] or n • Should the number of roll bins be even or odd? Even = conservative choice.

18. Status of the Stacker • Basic capability is in ssw in the atest subdirectory • Not yet systematically tested with all algorithms/options • No known bugs • To be implemented: • Better handling of variable flux & background • Features to support saving / retrieving / combining stacked counts from different intervals • Correction for solar rotation