1. Centre of Rotation update on talk from 2003 Y errors in centre of rotation Mike Avison

2. Welcome Skylight Users

3. Results - by brand: Normalised Normalised to 33cm radius (LE collimators) CameraMax Y deviation (mm) Axis(avg)5.0 E Cam 3.1 Hawkeye1.7 Argus4.8(single head) Forte A5.2 Forte B5.2 Skylight2.2

4. Results - by brand: Normalised Normalised to 33cm radius (LE collimators) CameraMax X deviation (mm) Axis(avg)0.8 E Cam 2.3 Hawkeye1.3 Argus0.3(single head) Forte 0.7 Skylight1.4

5. Head Configuration Normally we measure COR errors with heads at 180º Reasonable to assume it may be worse in less balanced configurations (90º, 102º)

6. How? In any head configuration –If projections from each head form a contiguous set, one file is made –If not 2 files are made So acquire round a full 360º of rotation –2 complete sets of 360º data, 2 files Use image algebra to cut and splice a simulated set of data.

7. Results ConfigurationXerrorYerror 180º2.15.4 102º0.65.6 (single head0.72.2)

8. How much correction takes place? Y error is proportional to radius Therefore correction cannot be perfect unless correction factor for each radius Are Philips doing any Y correction for head sag?

9. How much correction takes place? Cam/Head sagº  Predicted*Measured Yerror Y error 1/10.724.12.5 1/20.43 2.51.9 2/10.452.64.3 2/20.100.62.0  As measured with level-meter * radius. tan(sagº) As measured with COR test

10. Y data is aligned by IRC only at start position Start Position head 1 at 180º head 2 at 0º Results in this discontinuity and excessive range in Y error

11. If the means were aligned... mean h1 mean h2 mean h1

12. Effect: Halving COR Y error

13. Conclusions 102º config has similar COR performance to 180º Confirmation: COR error would be about ½ as big if the correction procedure was slightly more sophisticated (e.g. from 5mm to 2.5mm)

14. Conclusions 2 It appears Philips’ correction correction program does do some Y correction but it is probably a constant offset derived with heads at 0º and 180º respectively and at minimum radius Hence –It under corrects at 33cm radius –Discontinuity at 0º and 180º –Performance would be better with offset derived form mean Y values over 360º

15. Acknowledgements Thanks to all the users who provided raw COR data in 2003 and to Alan Britten and Anil Vara who provided the Skylight raw data this year.