EFOSC2

ESO Faint Object Spectrograph and Camera

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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

Note that Focus Prism uses 1x1 binning. Focus Wedge uses 2x2 binning.
The encoder values given by the Focus Wedge is always higher than that given by the Focus Sequence, by an average of 5 units.
Focusing with the Focus Wedge was fairly easy even in a crowded field, and there wasn't any problem in finding suitable stars.
Focusing with the Focus Prism is very hard in a crowded field, and over the entire field we could only find 3 stars which are suitable. Quite a lot of time was lost in the process.

Preliminary conclusions

As with tests conducted on the previous night, the Focus Prism seems to give a better dsee, although the IQ from both methods are the same and stable. Probably the DIMM seeing is not very representative of the actual conditons at the telescope.
Focusing with the Wedge performs much better in a crowded field. The usefulness of the Focus Prism in a crowded field is questionable.

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