The purpose of the new "ctestis" task is to correct signal levels of point-like sources in photometry tables measured from STIS CCD images for charge loss due to imperfect Charge Transfer Efficiency (CTE). The algorithm used to correct for CTE-induced signal loss is the one published in Goudfrooij, Bohlin, Maíz-Apellániz, & Kimble (2006, PASP, Vol. 848, 1455-1473). Tests to STIS data taken between March 1997 and August 2004 show that the values of CTE loss derived using this algorithm should be accurate to about 3% rms, leading to a correction whose uncertainty is typically significantly lower than the Poisson noise associated with a point source detection. No significant differences in CTE loss were found for different aperture sizes, although this has been verified only for a limited range of aperture sizes (2, 3, and 5 pixel radii).
The algorithm was derived from measurements of point sources in a relatively sparse field (the outskirts of a Galactic globular cluster), as detailed in the PASP paper mentioned above. Hence, results returned by the application of this algorithm to imaging photometry in highly crowded fields should be used with caution. While the CTE loss in crowded fields is likely less severe than in sparse fields, application of this task still yields important information on the potential signal loss due to CTE effects.
This task is included in STSDAS release v3.6. Details on the use of the new "ctestis" task are provided as part of the STSDAS release in the form of a help file.