Monday, November 21, 2011

RSS Fabry Perot Commissioning Status

Report from Ted Williams on the commissioning status of the RSS Fabry-Perot.

1.  The single-etalon modes that are ready for shared-risk science observations are:

650-690 nm

480-510 nm
650-690 nm
820-870 nm

These modes need further work on flat fielding, throughput determination, and stability characterization in order to be considered fully commissioned and ready for routine science observations.  Additional wavelength regions need to be calibrated as resources become available; priorities should include 580-610 nm in both TF and LR, and 480-510 nm, and 820-870 nm in TF.

2. Dual-etalons modes (MR and HR) have a serious reflection problem between the two etalons that introduces a series of ghost images and significantly degrades throughput.  Rectification of this problem is underway, but may take several months to complete.  In the interim, the original MR and HR modes are not available.   There are two alternatives that can be used:

a) Inserting a circular polarizer between the etalons has been demonstrated to eliminate the ghost.   There is more than a factor of 2 throughput loss with the polarizer.  The wavelength range will be restricted to short-wards of approximately 720 nm.  No polarimetry measurements are possible.   We have polarizing material on hand and both MR and HR dual-etalon observations can be carried out subject to the limitations noted above.

b) Single-etalon MR observations can be done.  This non-standard mode is available and has been used successfully for several science programs so far.   In addition to the desired order, the blocking filter will transmit 1 or 2 additional orders.   This will increase the parasitic light, allowing 2 or 3 times as much light from the night sky and from the object's continuum, thus decreasing the expected signal/noise ratio, and possible confusing the spectra because of unwanted spectral lines in the other orders.  Emission line programs are probably ok in this mode, but absorption line programs are probably not.  Observers should use the tables of etalon free spectral range and the blocking filter curves to estimate the effects on their particular program.  Because of the small free spectral range of the HR etalon, single-etalon mode HR is probably not usable.

The calibrated wavelength regions for the etalons are:

480-510 nm
650-690 nm

485-500 nm
650-677 nm

Additional wavelength regions can be calibrated as needs develop and resources permit.  Until the dual-etalon reflection problem is solved, all MR and HR observations must be considered shared-risk science.

3.  Several of the Semester 3 FP proposals seem to be specifying higher spectral resolution than the stated science goals require.  Especially in light of the limitations on the higher resolution modes, PI's should consider carefully the spectral resolution required.   For example, the MR etalon should routinely yield velocity measurements accurate to +/- 10 km/sec.

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