Calibration Access and Data Handbook

Generation from In-Orbit Data

Analysis of in-orbit data closely follow the CSL data analysis. Stars with known astrometric positions are matched with their image centroids. Objects from different observations can be used.
The photometric calibration fields are here especially useful, as they contain a list of objects with accurately known position. The measured position, the expected position and the offset between the measured and expected positions are calculated and written into a file. An equivalent of the IDL-routine DISTORT_FORM.PRO can be used to analysis the data:

• select catalogued positions for different observations/pointings (e.g. positions of objects in photometric sequence)
• preselect fit input sample on brightness range and position reliability
• fit data with polynomial
• calculate residuals
• sort residuals on position
• average residual within resolution element of CCF file
• force residual of V-filter at center to zero
• polynomial fit to averaged residuals

The new distortion map was generated from in-flight observations of the G153 field. The U filter was used and the positions of 813 stars were measured. The old map was based on a LMC field with only 230 sources. Since the main source of distortion is the detector fiber taper, this map is applied to all filters, including two OM grisms. The platescale is a natural by-product of this analysis. In orbit the platescale was found to be 0.4765 arcsec/subpixel for the V-filter.

The distortion map for the grisms, which describes the displacement of the zero orders in the grism image with respect to the direct image was measured in the same way. Observations of the Sco X-1 field in revs. 402 and 688 using the V filter and both grisms have been used to measure the positions of the sources and their respective zero orders. Then a map has been computed following the same method described for the geometric distortion.