Between the last Commissioning/Science Verification run (which ended January 30th) and the start of operations in P81 (Apr-Sep 2008), the entrance window to the instrument cryostat suffered a significant degradation and operations began in less than optimum conditions. The problem was resolved during an intervention conducted 2008-08-20 -- 2008-08-24.

All data acquired between 2008-04-01 and 2008-08-20 are affected by this problem, though to what degree generally depends on the circumstances of the observations, though often in an apparently random way...

During the Aug intervention it was discovered that the entrance window degradation was caused by a greasy film. The greasy film is believed to have been deposited during the 3 month period between the end of Commissioning/Science Verification and the start of observations when, for technical reasons, HAWK-I was kept under vacuum but at room temperature, instaed of it's usual cryogenically cooled state.

The degradation caused TWO effects to be evident in reduced SCIENCE data:

• Pupil-Ghosts: This was the first noticed, and most noticable, effect of the degradation, hence the name of this web-page. This problem tends in general to be worse when the pupil angle changes quickly between individual DITs. However there are plenty of exceptions to this rule, i.e. data sets with small changes in pupil angle between each DIT where the effect is none the less strong.
• Sudden Lateral shifts: Randomly, there appear suddenly between two DITs 'sudden latteral shifts' in the alignment between entrance window and detectors, thus changing the pattern of the degradation on the detectors and thus affecting the sky subtratction for the science. This effect could affects both data which show strong pupil ghost effects as well as data with weak or even non-existant pupil ghost effect.

Example data from April 2008 where the Pupil Ghost pattern and the effects of a 'sudden lateral shift' are clearly present [AB = HAWKI.2008-04-29T02:47:25.267_tpl.ab]:

RAW frames	RAW frame N minus RAW frame 1, N=[2,TPL.NEXP]
Product frame