EUROPEAN SOUTHERN OBSERVATORY
Implementation of a polarimetry mode for CES
An initial feasibility and scientific interest study
||2003-04-19 first draft, tdall|
This document is intent to initiate a discussion concerning the implementation of a spectropolarimetric mode for ESO's highest resolution spectrograph, the CES on the 3p6, a configuration we for the moment will name CES-Pol. CES-Pol would be the only high resolution spectropolarimeter in the southern hemisphere.
After some rounds of discussion and some technical feasibility tests, another document shall be prepared giving final recommendations of the future of CES-Pol.
The prime reason for implementing a polarimetric mode for CES is the very high resolution. There are currently no other high resolution spectroplolarimetry instruments on the southern hemisphere, so CES-Pol would be a unique facility.
In order to be truly useful, CES-Pol will have to be able to measure all polarisations; both linear and circular. Ideally the two linear components and the two circular components should be measured simultaneously, which means adding an extra fiber to the CES.
However, there might be problems fitting two spectra on the CCD, when using the high-resolution image slicer. Thus, in this case a new CCD might be needed.
Another option is to maintain the current configuration with only one fiber, and measure the polarisations sequentially. This does not compromise the science objectives, although it requires more exposure time and more careful calibrations.
The advantage of this scenario is that it will likely be much cheaper and simpler to implement. However, a careful analysis of the impact on the science must be performed. This is where the readers of this document can contribute with their opinions.
With CES integrated in the HCFA during normal operation, there are several possibilities for the installation of the polarising elements.
There are several issues to be discussed. In case of 1. above, the available space in the HCFA has to be asessed, and whether to use the existing carriage for the iodine-cell. In case of 2. above, the additional overhead in changing adapter will have to be evaluated, and the technical feasibility will have to be addressed. The same is true for point 3. above.
The possible addition of one more fiber to CES is currently just an idea. It may turn out to be technically unfeasible due to the design of the CES.
The exact nature of the polarisers to be used is not clear at the moment. Several designs exist, but a closer examination should await decisions on where to install the polarisers and whether CES-Pol will be with one or two fibers.