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Flat-fielding

Flat-fields are required to correct for the pixel-to-pixel sensitivity variations of the CCD. Sky-flats taken after sunset or before sunrise are recommended. To obtain optimal results, flat-fields should be taken during each observing night. Go to the main camera section:

Setup of the main camera: Choose a filter and the binning. Recall that you need flats for any combination of these settings used during observations.

Switch off tracking: This is recommended to avoid remaining star light to be recorded always by the same section of the CCD. If remaining stars drift through the field, affected pixels can be removed later by median filtering.

Choose appropriate exposure time: Adjust the exposure time to obtain an intensity level of several ten thousand counts (but below saturation ($\approx$60000 counts)!).

Make a sequence of flat-field exposures: Select exposing/save in the misc pull-down menu. Record at least 11 exposures to obtain reasonable statistics. You may adjust the varying intensity level due to the changing sky brightness by changing the exposure time on-line. The new exposure time setting gets active as soon as the next exposure is started.

The master flat-field is obtained during the reduction process by scaling and averaging the sequence of flat-field exposures. The quality of the master flat-field increases with the number of flat-field exposures.


next up previous contents
Next: Guiding the telescope Up: Calibration exposures Previous: Dark current calibration   Contents
Dieter Engels
2003-01-15