1. Tips and Tricks for analysing CDS data Dr Peter Young SOHO/CDS Project Scientist CCLRC Rutherford Appleton Laboratory Dr Peter Young, CCLRC/RAL

2. Overview • Photon statistics • Calibration uncertainty • Cosmic rays • NIS1-NIS2 alignment • Burn-in correction • SOHO roll Dr Peter Young, CCLRC/RAL

3. Photon statistics • vds_calib yields photons/s/pixel • use Bill’s expression to yield fractional noise (fN) • √(2N + n)/N • proceed with calibration to yield intensity units • apply fNto yield error on intensity Dr Peter Young, CCLRC/RAL

4. Calibration uncertainties • For large counts, calibration uncertainties dominate over photon statistic errors • Difference between relative and absolute calibration uncertainties (e.g., line ratios vs. DEM) • Relative uncertainty is λ dependent and varies from 12 to 29% • For absolute uncertainty, additional contribution of 13% • See Sect. 3.3 of Lanzafame et al. (2005, A&A, 432, 1063) for more details Dr Peter Young, CCLRC/RAL

5. Cosmic Ray Removal • Three routines available in software • cds_new_spike • cds_clean_exp • cds_clean • Make sure to set $CDS_EXTERNAL for routines to run quickly Dr Peter Young, CCLRC/RAL

6. Fill or not to fill? • The cosmic ray routines allow two options for dealing with CRs • fill with median value of surrounding pixels • flag as missing data • The latter is best option for preserving statistics Dr Peter Young, CCLRC/RAL

7. NIS1-NIS2 alignment • NIS1 and NIS2 images are not spatially coincident IDL> gt_nis_alignment, qlds, x, y • Pre-loss: X~1”, Y~2” • Post-loss: X~1”, Y~3-7” • Depends on Y-location! Dr Peter Young, CCLRC/RAL

8. Burn-in Correction • Narrow slit burn-in is not fully corrected for in the CDS software • Affects both line width and intensity IDL> result=get_width_corr(’22-sep-2005’,629.7) • Currently 20% for O V λ629.7 Dr Peter Young, CCLRC/RAL

9. SOHO Roll • To find out SOHO roll angle on a particular date: IDL> angle=get_soho_roll(’22-sep-2005’) • To make CDS images appear in the usual format, use IDL> data=gt_windata(a,0,/invert) Dr Peter Young, CCLRC/RAL

10. Dr Peter Young, CCLRC/RAL

11. User Routines • Do you have IDL routines for your CDS analysis that you’d like to share? • Are there any new IDL routines that you’d like to see? • Routines can be added to CDS branch of Solarsoft • Requirements: • provide header in standard format • check that obvious errors are caught Dr Peter Young, CCLRC/RAL