Recent Problems

This page contains information about recently discovered problems and new instrument artefacts, especially those that are not yet described in the User Manual.

In a few cases problems have occured with order identification in settings with 346nm central wavelength: the orders found by the pipeline in the order defintion flat and during extraction/wavelength calibration of the science frame may be different. This is not recognised by the pipeline and the lines in the final wavelength-calibrated spectrum appear shifted by one order. The example given below shows the overlap between an observation with 346nm and 437nm central wavelength where the red spectrum (346nm) is clearly shifted. The problem may be resolved by choosing a different order definition flat.

The UVES CCDs are described here.

The trap mentioned in the User Manual at column x=1609 of the red upper (MIT-LL) CCD has changed its features since 2003-09-19. It consists of two columns, one column has pixel values around 1000, the other around 380. These values are now almost independent of the exposure level so that the columns are very prominent in, for example, bias frames. Previously, the columns could be seen only in well exposed frames like flat fields. Depending on the signal of the science target, the effect of these traps may be a broad emission or absorption feature in the extracted spectrum. An example for a setting with 580nm central wavelength is shown below.