EFOSC2ESO Faint Object Spectrograph and Camera
EFOSC2 Focusing Tests 2Crowded Field Test
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|
dsee = measured IQ - DIMM seeing. Positive values means that the telescope seeing is worse than DIMM seeing
|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.|
|Send comments to : ls-spectro
Last modified: Fri Jan 19 05:15:27 SAT 2002