ESO Faint Object Spectrograph and Camera
EFOSC2 Focusing Tests 2
Crowded Field Test

Technical time 18/1/2002

Aim: To test how the 3 focusing metthods (Sequence, Wedge & Prism) fair in very crowded fields.

Method: Telescope pointed to the centre of LMC. Two sets of tests were made, each with a Focus Wedge and then a Focus Prism. The DIMM seeing and the serrurier temperature were noted down during each focus procedure. After each focus procedure, an R band image with 2x2 binning is taken, and the Image Quality (IQ) is measured. The results are shown below:

Focus Seqence was tested in the beginning, but failed dramatically (gives an offset of -60 encoder units), so not continued.

Best encoder values obtained using different focusing methods:

Focus Sequence dsee Focus Wedge dsee Focus Prism dsee Method Temp
Failed! 1948 0.42 (IQ=1.35") 1944 0.21 (IQ=1.16") Started near best focus 11.3
1949 0.39 (IQ=1.16") 1943 0.09 (IQ=1.16") Started near best focus 11.3
mean dsee= 0.41 mean dsee= 0.15
mean=1949 mean=1944

dsee = measured IQ - DIMM seeing. Positive values means that the telescope seeing is worse than DIMM seeing

Preliminary conclusions
Below shows images from the 3 focusing procedures:
Focus image with the Focus Sequence. The program gives a wrong focus offset after this.
Image from Focus Wedge. It is easy to find suitable stars.
Image from Focus Prism. The splitting of an image into 5 results weaken the flux of the stars, and increase the number of faint background stars. It was only possible to find 3 stars suitable for focusing, after a long search.

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Last modified: Fri Jan 19 05:15:27 SAT 2002

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