Centre of Rotationupdate on talk from 2003

Y errors in centre of rotation

Mike Avison

Welcome Skylight Users

Results - by brand: NormalisedNormalised to 33cm radius

(LE collimators)

Camera Max Y deviation (mm)

Axis (avg) 5.0

E Cam 3.1

Hawkeye 1.7

Argus 4.8 (single head)

Forte A 5.2

Forte B 5.2Skylight 2.2

Results - by brand: NormalisedNormalised to 33cm radius

(LE collimators)

Camera Max X deviation (mm)

Axis (avg) 0.8

E Cam 2.3

Hawkeye 1.3

Argus 0.3 (single head)

Forte 0.7

Skylight 1.4

Head Configuration

• Normally we measure COR errors with heads at 180º

• Reasonable to assume it may be worse in less balanced configurations (90º, 102º)

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.

Results

Configuration Xerror Yerror

180º 2.1 5.4

102º 0.6 5.6

(single head 0.7 2.2)

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?

How much correction takes place?

Cam/Head sagº Predicted* Measured

Yerror Y error

1/1 0.72 4.1 2.5

1/2 0.43 2.5 1.9

2/1 0.45 2.6 4.3

2/2 0.10 0.6 2.0As measured with level-meter

* radius . tan(sagº)

As measured with COR test

Y data is aligned by IRC only at start position

Start Position head 1 at 180º head 2 at 0ºResults in this discontinuity and excessive range in Y error

If the means were aligned...

mean h1

mean h2mean h1

Effect: Halving COR Y error

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)

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

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.