As of the end of August last year, the SALT IQ saga was mostly resolved, but a small amount of astigmatism was still apparent in images obtained in good seeing. With the SAC fixed & a bare CCD unable to contribute aberrations, the remaining problem had to lurk within the primary mirror system. This has been investigated & during the past week, efforts centred on testing the Shack-Hartmann CCAS instrument that's used to align the primary's 91 segments.
The instrument has a light source that's fed, via a beam-splitter, to each of its 2 arms - 1 of which includes a small reference mirror.
& another that incorporates the primary array.
After reflecting off either the reference mirror or the primary, the light passes back through the beam splitter & gets collimated by a camera lens before going through the lenslets.
There are both coarse & fine lenslets that can be placed in the beam to produce either 1 spot per mirror segment (for coarse alignment), or 7 spots per segment (for fine alignment).
The first priority was to test the optical quality of the reference mirror to ensure that it wasn't responsible for introducing astigmatism into the primary during the alignment process. The CCAS instrument had to be dismantled to allow a wavefront camera to be placed at the focus (located down-stream from the beam-splitter).
The lenslets deserved a closer look under a microscope while they were out & about, niftily illuminated from below with a red LED.
In white light, the coarse lenslets appear slightly blue & a faint set of ghosts can be seen in the photo below. The stray satellite spot to the lower right is associated with a damaged lenslet in the outermost ring (beyond the central 91 that matter).
The good news is that the reference mirror's in great shape & can not account for the astigmatism. In fact, the wavefront measurements were in perfect agreement with Darragh's Zemax analysis, which showed that the only aberration that should be present is spherical aberration produced by the beam-splitter (& since both arms are subject to this, it makes no difference).
The next trick was to feed the spots from the perfect reference mirror into the alignment software & pretend that they came from the primary. This ought to have led to a perfect alignment - but It Didn't! So, that points to a software issue that will need to be traced...
Hitesh, Darragh & Francois will thus remains locked up in the CCAS tower until further notice while they figure it out.
In the meantime, Sutherland continues to provide spectacular sunsets to taunt the astronomers!