Table of Contents
Analysis of available X-ray data reveals that all fiducial light data may be improved by removing offsets caused by changes in the periscope alignment of the fiducial transfer system. Changes in periscope alignment within an observation are directly correlated with changes in periscope gradients. These gradients are available in engineering telemetry, and a simple linear combination of these telemetered gradients provides an acceptable model of the periscope offsets.
This ECR describes a new CALDB HDU, appended to the existing PCAD CALALIGN product, that contains the calibration constants (coefficients) that describe the linear combination of gradients required to apply the periscope correction.
Each CALALIGN file has a new HDU called "PERIFIDCORR" appended. The HDU contains 4 columns and 1 row. The columns are as follows:
| Column name | Unit | Type | Description |
|---|---|---|---|
| peri_y_oobagrd3 | deg/degC | Real4 | Y-Angle Coefficient on gradient OOBAGRD3 |
| peri_y_oobagrd6 | deg/degC | Real4 | Y-Angle Coefficient on gradient OOBAGRD6 |
| peri_z_oobagrd3 | deg/degC | Real4 | Z-Angle Coefficient on gradient OOBAGRD3 |
| peri_z_oobagrd6 | deg/degC | Real4 | Z-Angle Coefficient on gradient OOBAGRD6 |
The HDU values are as follows:
| peri_y_oobagrd3 | peri_y_oobagrd6 | peri_z_oobagrd3 | peri_z_oobagrd6 |
|---|---|---|---|
| 6.98145650e-04 | -1.67009240e-03 | 9.51578351e-05 | -2.79084775e-03 |
The values of the coefficients were derived using data and scripts in the directory:
/proj/sot/ska/analysis/periscope_tilt
See the Calibration Method section of the periscope correction pipeline ECR for an explanation of the method. 16 long observations with bright point-like sources and the largest within-observation changes in periscope gradients were used to fit the coefficients, and these coefficients were checked against a larger set of similar observations (175) unconstrained by changes in periscope gradients.
The simultaneous fit over the 16 observations used the same model for the periscope offset as implemented in this new aspect pipeline code:
dy = peri_y_oobagrd3 * OOBAGRD3 + peri_y_oobagrd6 * OOBAGRD6
dz = peri_z_oobagrd3 * OOBAGRD3 + peri_z_oobagrd6 * OOBAGRD6
See ECR: Add FTS Periscope Correction to Aspect Pipeline for implementation details.