[ ESO ] La Silla


La Silla Science Operations: NTT

EMMI coronograph

A coronographic plate can be inserted in the red arm, in the RILD mode. It is used to blot out bright stars which could give problems during long integrations. It can be used in red imaging and for slitless (grism) spectroscopy. The plate is mounted in the starplate wheel, also used for mounting the slit plates in the grism mode.

The coronograph consists of a transparent plate which has 6 circular dots of varying size. The telescope is pointed such that the object to be blotted out falls behind one of the dots. Note that the peak of the PSF is blotted out, but not the wings. The main advantage in imaging is that long(er) integrations can be used without saturating the CCD. With slitless spectroscopy, a spectrum of a faint source on the same row as a bright source can be taken.

The following 'blots' are present:

Size ('')
Approx. Pos. on CCD
2025 - 1733
2347 - 1737
2667 - 1741
2021 - 2247
2343 - 2251
2663 - 2253

The precise position depends on how it is mounted and should be determined by the astronomer during the afternoon using a dome flat image. The field of view is limited by the mounting of the plate, and is reduced to 4.1 x 7.7 '.

Some of the dots (1.9'' and 4.9'') fall fairly close to the edge. It may be necessary for the observer to change the orientation of the field to keep the interesting part of the field in the field of view, by rotating EMMI. This can be done during the acquisition of the field. Pointing and centering of the object is done using the RILDAT02 acquisition template. The x,y position of the desired blot is measured in pixels and then introduce in the template. The acquisition template is then taking an image of the field with the right angle if needed, the stars is then selected on the image displayed on the RTD. The right offset of the telescope is then calculated and applied.

The throughput was measured by taking a FF image with and without the plate, which gave 96%. The wavelength dependence has not been measured. The plate has some defects (=dirt), but overall is in good condition. Non-deteriorated images of 0.9 arcsec were obtained during a test night, the same as without the plate.
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