ESO Faint Object Spectrograph and Camera
Bias and Dark Current

Bias and dark current frames are taken with the same template - set the exposure time to 0 second for biases and any non-zero exposure for darks. 1. Biases frames are needed for every combination of Binning and Readout Speed used during observations.

2. The first few bias frames after a long pause (the CCD idle for an hour or two) show a "hot" column.  This vanishes subsequently and does not affect  later observations in any manner. One also sees this hot column if for some reason the CCD gets saturated in a big way (during twilight flats for example).

3. CCD #40's bias overscan region is inadequate. The overscan strip from row y=2049 to 2060 gives a level which is higher than the actual bias, is known to vary by up to 10 ADUs during a night (more typically by 3-5 ADUs).  Programmes which need a more accurate determination of the bias should incorporate regular bias measurements during the night. A possibly wider bias strip is under review (Nov 2001).

4. Make sure that the dome is dark when biases and dark current frames are being  taken - there is a low level of light leakage into the instrument which however is not an issue during the night.

5. The best time to take dark current frames is at the end of the night after the telescope has been parked, the dome  closed, and all lights have been switched off.  One can leave the OB running.  However make sure that the exposure sequence will terminate before 8.00 AM (local time) as the telescope staff start working in the dome after that.

6. Typically one needs :

jP2PP Panel with Typical Parameters

The above example shows an OB comprising a sequence of:

The FITS image output by the template is named  EFOSC_Dark.#.fits.

As an example the following image is given, which shows a Bias with normal readout and 2x2 binning. Click on the image to get a larger version:

Parameter Values One does not have to (in fact, should not) change the value of any parameter other than those of:

Send comments to : ls-spectro

Last modified: Sat Apr 09 17:29:07 CLST 2005  

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