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:

TF
650-690 nm

LR
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:

MR
480-510 nm
650-690 nm

HR
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.

NGC 5364

The beautiful face on spiral galaxy NGC 5364 is shown above in a false-colour optical image. The extensive spiral arm structure is clearly visible, winding around the central galaxy bulge. The bright coloured spots within the spiral arms indicate the positions of HII regions which are areas of large (some can be several hundred light years across!) clouds of ionized gas associated with intense star formation. The galaxy is located in the Virgo cluster around 70 million light years from Earth (redshift of 0.004) from us.

NGC 5364 was observed using the RSS on SALT in Fabry-Perot mode on 11th May 2011. A succession of 60s images scanning across the H-alpha (6563A) emission line were taken and three of those images were combined to produce the colour picture above. The colours shown provide a sense of the rotation of the spiral arms. Blue colours show regions that are moving towards us (the peak of the line emission is blueshifted relative to its rest wavelength) and red colours show regions that are moving away from us (the emission is redshifted).

Foreground stars are visible in white in the image, and if you look carefully you can spot the Fabry-Perot ghost of the bright star on the bottom left of the image. The ghost is situated at the top right of the image. As a consequence of not auto-guiding during these observations, the position of the ghost is slightly different in each individual image frame used to produce the resultant colour image and therefore the three constituent colours are clearly separated in the final picture.

NGC 6781

NGC 6781 (top) is a planetary nebula (PN) in the constellation of Aquila. The main ring measures approximately 2 arcminutes in diameter and is about 1 kpc distant. The true 3D geometry of a PN cannot usually be grasped from images alone and studies have indeed shown that NGC 6781 is a toroidal shell (bottom, Schwarz & Monteiro 2006) typical of so-called bipolar nebulae. If you look carefully you can make out numerous filaments of dust which are responsible for the patchy reddening observed in this nebula. NGC 6781 was observed on 10 May 2011 with RSS in Fabry-Perot mode which scanned through the emission lines Halpha 6563 (green), [NII] 6583 (including HeI 6678, red) and [SII] 6730 (blue). The ionising central star is best shown at bluer wavelengths (see here).

Credit: Observers were Ted Williams, David Buckley, Brent Miszalski (processing and write-up), John Chisholm and Nicola Loaring.