EFOSC2

ESO Faint Object Spectrograph and Camera

EFOSC2 Focusing Tests

Technical time 17/1/2002

Aim: To test the new Focus Prism template written by Emilio Barrios, and to compare its performance with the other focusing methods (Focus Sequence and Focus Wedge).

Telescope pointed to spectrophotometric standard LTT1788 near the zenith. Three sets of tests were made:

The focus is started from 50 encoder units below the best value
The focus is started from 50 encoder untis above the best value
The focus is started from around the best value.

The DIMM seeing and the serrurier temperature were noted down during each focus procedure. Furthermore the seeing is measured from the best focused image. The results are shown below:

Best encoder values obtained using different focusing methods:

Focus Sequence	dsee	Focus Wedge	dsee	Focus Prism	dsee	Method	Temp
1933	0.06"	1951	0.24"	1936	-0.23"	Started from 50 Encoder units below	12.1 -- 11.8
1939	-0.30"	1953	0.34"	1947	-0.15"	Started from 50 Encoder units above	11.7
1941 (see graph below)	-0.06"	1949	-0.09"	1943	-0.15"	Started near best focus	11.6 -- 11.5
1938	-0.04"					Last test.	11.5
mean dsee=	-0.09"	mean dsee=	0.16"	mean dsee=	-0.18"
mean=1938		mean=1951		mean=1942

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

Note that both Focus Sequence and Focus Prism use 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 13 units, and doesn't matter if we start the focusing from above or below.
The Focus Prism also gives systematically higher values than the Focus Sequence, but only by an average of 4 units, which is not significant.
The Focus Sequence gives a seeing slightly better (10%) than the DIMM seeing.
The Focus Wedge gives a seeing slightly worse (15%) than the DIMM seeing.
The Focus Prism on average gives a seeing about 20% better than the DIMM seeing.

Below shows the plot from the focus sequence in the 3rd set of test:

It can be seen that:

Both the value given by the Focus Sequence (1941) and the Focus Prism (1943) are very close to the best focus value (1944). Both give the smallest FWHM.
The Focus Wedge value (1949) gives a slightly higher (3%) FWHM.

Preliminary conclusions

The Focus Wedge doesn't give the best focus, but a systematically higher value. This artificially increases the seeing by about 3--15%.
Both the Focus Sequence and the Focus Prism give good focus values. It seems the Focus Prism maybe slightly better than the Focus Sequence (maybe because the Focus Sequence program doesn't calculate the best value?), and may possibly improve the seeing by about 10%.

Send comments to : ls-spectro

Last modified: Fri Jan 18 03:15:27 SAT 2002