ESPaDOnS
examples of frames

Examples of flat field frames

Below are some example flat field frames (close-up views) corresponding to different instrument configurations. One can notice in particular that:

two sets of interleaving orders are present in the left image (polarimetric configuration) while only one set of orders shows up on the other two images ('object only' spectroscopic configuration);
the number of slices per spectrum is different for the different setups, each spectrum being divided into 3, 4 and 6 thin stripes on the left, middle and right image respectively, with the two side stripes being slightly weaker than the middle ones.

polarimetric configuration (2 fibres and 3 slices per fibre)	'object only' spectroscopic configuration (1 fibre and 4 slices per fibre)	'object only' spectroscopic configuration (1 fibre and 6 slices per fibre)

Note that the second image corresponds to a configuration not offered for observations as it gives no advantage over the official 'object only' spectroscopic configuration (1 fibre, 6 slices per fibre) depicted in the third image. It is only displayed here as an illustration of how ESPaDOnS behaves.

Examples of Th/Ar and Fabry-Perot frames

These are now example comparison frames (close-up views) of different types (Th/Ar and Fabry-Perot) and for 2 different instrument configurations (polarimetric and object only' spectroscopic configurations). In addition to what was noted on the flat field frames above, one can observe here that:

thorium lines are very numerous and rather narrow in the vertical (ie grating dispersion) direction, with a full width at half maximum of order 1.5 pxl (the exact value being 1.7 and 1.4 pxl for the left and right image respectively);
although wider than the thorium lines, the Fabry-Perot interference features are well defined and very regular, offering a very interesting alternative to Th/Ar lamps for tasks such as estimating the spectrograph slit tilt and shape.

Th/Ar frame, polarimetric configuration	Fabry-Perot frame, polarimetric configuration	Th/Ar frame, 'object only' spectroscopic configuration

The spectral resolutions associated to the left and right image are equal to 69,000 and 81,000 respectively.

Examples of polarimetric frames

The example presented below illustrates the ability of ESPaDOnS to diagnose polarised light. Among the very extensive tests carried out, the one included below depicts ESPaDOnS response to fully polarised light with north-south (Q=1) linear polarisation. To estimate the amount of polarisation for a given polarisation state, four subexposures are successively taken with the half-wave Fresnel rhombs set to 4 different configurations (configurations 1 and 4, and configurations 2 and 3 being roughly equivalent by pairs). When estimating linear polarisation along north/south and east west axes (Stokes parameter Q), the rhombs are successively rotated to position q1, q2, q3 and q4. On the frames included below, one can notice that:

the right beam disappears almost completely when rhombs are set to position q1 (middle image) compared to an image obtained with unpolarised illumination (left image); a similar result is obtained with rhombs set to position q4;
the situation is reversed, with the right beam now almost completely extinguished, when rhombs are set to position q2 (right image); a similar result is obtained with rhombs set to position q3.

Unpolarised illumination	Fully polarised illumination (Q=1), with rhombs set to position q1	Fully polarised illumination (Q=1), with rhombs set to position q2

Note that in both cases, the extinguished beam still shows up at a very weak intensity level (of less than 1% that of the main beam). This is due to slight residual chromatic inaccuracies in the properties of Fresnel rhombs.



© Jean-François Donati, last update May 26 2004