July 2, 2000
Keywords: astrometry
Mark IV lenses have rather large field distortion. Errors introduced by this distortion are significantly larger than errors due to centroiding and catalog errors. This requires use of higher order plate constants for transformation from (x, y) image system to (x', y') system projected using the tangent projection.
In addition, it has to be noted that the optical axis clearly does not go anywhere near the CCD sensor center. This is shown in the image below which shows color coded residual sizes for V filter camera images contained on disk 15. Approximate coordinates of optical axis are x=736, y=864 instead of x=1024, y=1024. This amounts rougly to 5 mm offset.
Average absolute value of residuals as a function of distance from the optical axis is given in the next figure. Residuals were averaged in bins of 32 pixels. The curve reveals that rather high order (6) of plate constants is necessary for sufficient correction over the entire field, if the fit is made in (x, y) coordinate system.
The analysis above was made using all 150 s exposure images taken with V filter. Stars used for residual calculation were from the ACT catalog.