Image quality of EFOSC2/NTT

All information below relates to EFOSC2 at the 3.6m. It will be updated once new values are measured for EFOSC2 at the NTT.

Currently there are some occasional problems with poor image quality that appear to be related to the movement of the rotator and associated temperature changes. The update to this page remains pending while these are investigated.

Seeing

The best seeing that one may expect with EFOSC2/3.6m is about 0.65 arcsec and more generally good seeing is 0.75 - 0.8 arcsec. The typical seeing is about 1.0 arcsec and usually about 0.2 - 0.4 arcsec worse than the DIMM values.

PSF Distortion

The Focal Reducing optics of EFOSC2 considerably distort the PSF and this distortion varies across the CCD. The best (most circular) PSF occurs close to the centre of the CCD.

CCD Flexure

The flexure of CCD #40 as a function of telescope pointing was determined by measuring spectral line shifts at various pointing positions relative to the zenith. The shift varies between -0.5 and +0.15 pixels over the altitude and azimuth accessible to the telescope. In general, the flexure is worst when pointing north.

Improving the Image Quality

The seeing, distortion and flexure cannot be improved upon. However there is an additional component of distortion introduced by the telescope optics which can be eliminated.

Focus:   One should always focus the telescope every hour or so even when observing the same field. In general any shift in Z of the telescope by 15-20 degrees on the sky should be accompanied by a focus procedure.

Zenith Distance:   The PSF size and distortion worsen when one observes at large hour angles (> 4 hr ) or far to the north. So it is best to observe sources as close to the zenith as possible. It may be noted that this telescope has an equatorial mount and hence observing close to the zenith is not a problem.

Mirror Stress:  It has also been noticed that pointing the telescope to extreme positions (> 4 hr zenith distance) results in bad image quality even after the telescope is brought back close to the zenith. This persistence of distortion is believed to be due to unrelieved stresses on the telescope primary. It can usually be corrected by  executing a standard sequence of telescope pointings, known to the telescope operators, which requires 10-15 minutes. The observer may request the telescope operator to carry out this procedure if desired.