The IQ Fix

The SALT Image Quality (IQ) team along with their engineering and astronomer colleagues are delighted to announce that the mammoth "IQ fix up" engineering work has been completed and the first test results are extremely positive!

During the period April 2009 to August 2010, SALT was out of action whilst under-going major testing and repair work to fix the "IQ problem"; a problem with the quality of the astronomical images obtained, unfortunately exhibited since first light. So what was this "IQ problem" and how was it fixed?

The most serious contributor to the SALT IQ problem was the focus gradient exhibited across the field of view, which meant that only a small portion of the 8 arc min field could be kept in good focus at any given time.  By adjusting the telescope's focus, different parts of the field could be optimised, but always at the expense of other regions. This was not sufficient for scientific observations which require sharp focus over the whole field.

Additionally, star images were sometimes doubled in regions that were out of focus and there was severe astigmatism (elongation) in the images.  Finally, all of these problems varied from night to night rather randomly, YUK!

So, after approximately three years of on sky testing, the IQ team determined that that cause of the problem lay in the Spherical Aberration Corrector (SAC) which is mounted on the top of the telescope and has four mirrors inside designed to correct for the effects of SALT's spherical primary mirror. Spherical mirrors exhibit an aberration called rather unimaginatively "spherical aberration" where light rays are focused at slightly different points depending on how far from the optical axis they are. A spherical mirror therefore does not produce a perfect focal point and additional corrective optics are needed to produce perfectly sharp images.

It was determined that there must be some mis-alignment in these supposedly corrective mirrors causing the poor quality images, and the cause of this mis-alignment was eventually pinned down to the mounting between the SAC and the structure (called the non-rotating structure) it is mounted to on the top of the telescope.

In April 2009 the SAC was removed from the top of the telescope and placed into the clean room ready for extensive testing and repair. The relative alignments of all the mirrors were checked and adjusted as necessary. Additionally, the surfaces profiles of all the mirrors were checked (and in some cases even given a spring clean!) to make sure all was in order. This work took 16 months to complete in total! A new collar to mount the SAC to the non-rotating structure was also designed and built during this period to ensure that the now perfectly aligned SAC would stay that way once it was remounted on the top of the telescope. After all, the old mounting had caused all the problems in the first place!

On 10th August 2010 the SAC was now ready for action, newly aligned, cleaned and complete with its new mounting collar. It took engineers all day to hoist the SAC back in place back at the top of the telescope and during the following few weeks we would find out if the 16 months of tests and repairs had helped!

.... And the answer is....

YES! The latest images from August 2010 show that the focus gradient is gone! A small amount of astigmatism still remains in the images, which we now know has to reside within the primary mirror (since we've fully re-aligned the SAC & tested the surface figures of its mirrors and found them all to be in great shape), however this should be easily corrected.

In an official report the IQ team leader Darragh O'Donoghue stated:-

• The SAC is working properly. 

• SALT’s IQ has been hugely improved compared to before the SAC repair.

• The telescope is scientifically usable with almost round images and no focus gradient across its field of view.

• Prospects for addressing the remaining issues and optimizing the image quality are excellent.

• The IQ issue is no longer a “show stopper”.

What wonderful news! The SALT astronomy operations team is now busy with on sky testing of the telescope and re-commissioning exercises and we envisage being back into normal operational mode in early 2011.

For more details about the IQ fix, take a look at Lisa Crause's IQ Fixup blog.