UVES master FLATs: dependence of structures on slit width and slit function

Example: Y derivatives for setting 580d1_1x1

Y-derivative frames: The first raw frame of a stack has been shifted in Y (slit direction) by 1 px and subtracted. The result frames (blue: one CCD, red: two CCDs U and L) show any structure in Y direction enhanced. Interorder space is masked to not contribute to the resulting std dev.

The Y structure consists of gain fluctuations, slit function and fringing. This example has been chosen to show no fringing and to illustrate the amplitude of the slit function. The slit function is most pronounced for the narrowest slit, 0.3".

Orientation of all raw frames is rotated by 90 degrees.

derivY Slit width: 0.3" 0.4" 0.5" 0.8" 1.0"

Origin of slit function. The slit function describes the slope of efficiency along the slit coordinate (vertical in the displays). Its variations are caused by imperfections of slit jaw geometry, dust grains and reflections. Its impact is completely removed by dividing the flat field into the science data. However, if strong variations occur along the slit, one might wish to avoid positioning the object close to a region with lower transmission. Also for objects with extended structure the precise knowledge of the slit function is useful.

steps towards slit function: rotate by 90 degrees collapse 10 rows cut one order only

Slope of slit function. The measured UVES slit function (ps version here) has been obtained from master flats taken in settings 580d1_1x1 with 0.3 or 0.4 arcs slit width. The signal in one order has been collapsed within 10 columns. The inclination of the orders has been neglected.There are three measurements taken between 2001-03 and 2002-01, plus one more on 2002-02-10. The slit function has been normalized to one. It is given here along the slit coordinate Y (vertical axis in the displays above). The region with flux is marked by the two broken lines.

The function has one sharp featurecentred at Y=7, with about 87% amplitude. This feature shows up clearly in the derivative plots close to the lower edge. There are three more faint features less than 3% deep. The central and upper region of the slit has no detected features at all.

The slit function has been very stable (both in amplitude and overall characteristics) between 2001-03 and 2002-01. A sudden change in shape in 2002-02 (top curve) is most likely due to an intervention by the end of January 2002.