A major cleaning of RSS optics took place in September/October 2014 during which optical
coupling fluids in the collimator were replaced as well. Early pre-installation laser measurements
and relative standard star measurements post-installation indicated approximately 30 to 40%
increase in RSS throughput. In January 2015 we were able to verify absolute RSS throughput
values on-sky using so called burst-mode observations: the primary mirror is purposefully misaligned
to have each 1-metre segment produce an individual image of a standard star. An example on the
right shows the imaging field of RSS: the pattern of points left of the centre is actually the spectrophotometric standard EG 21. Photometry is performed on each individual point and since the size of each mirror is now constant, absolute efficiencies of the system can be derived (assuming many other variables such as extinction, primary mirror efficiency, filter transmissions, etc. are known).
These results confirm the significant throughput improvement. RSS efficiency
increase by a likely 50%, and possibly even more in the bluest wavelengths. The Figure
on the left shows the overall Telescope+RSS throughput development since 2011 until now.
There had been a steady decrease since 2011, with the red points showing the pre-fix
September 2014 values. The green points show the current situation.
Telescope throughput has been measured as well (with some caveats) and assuming it was
constant from September 2014 until now the figure on the left shows the RSS Optics only
efficiency. While the very reddest wavelength points may be artificially a little “too” high,
the overall trend is very encouraging.