[Comm2011] VST: scattered light and other issues

[Konrad Kuijken]

Dear all,

A short report from my stint watching the VST last week:

- I was there for six nights. The first 3.5 nights were taken up with an INAF-VST visitor-mode GTO catchup run: conditions were quite good but don't blame me for the lack of KiDS data ;-) The final two nights were mediocre, unfortunately, with clouds, wind, moon, poor seeing...

- There are still issues with the telescope secondary mirror, which is not quite stable - regularly it ends up in a tilted configuration which requires manual adjustment. There are also some doubts about the reliability of M1 control at ESO.
- ESO engineering division is making a plan of action, in which the first priority is to 'gain control' of M1 and M2. The hope is that once this is achieved (may require intervention in the M2 hexapod, as well as a good look at the hardware and software of M1) the other operational inefficiencies (slow field acquisition and image analysis convergence, failure of automatic guide star acquisitions, image quality issues due to M2 tilt) may disappear. This would be major progress in bringing the VST to smooth and more efficient operations.

- In parallel two ESO optical experts (Bernard Delabre and Andrew Rakich) arrived on Paranal to spend a week looking into stray light issues during the full moon period. We made great progress in identifying a number of sources:
 - the baffle of M2 appears to be smaller than called for in the design, which means that sky light can enter directly into the 'chimney' around the central hole in M1.
 - even with the baffles as designed, there is a light path from sources about 12-15 degrees away from the pointing center directly into the chimney. These light rays do not strike the detector directly, but they can reflect off the inner edge of the second corrector lens. Ray tracing shows that this gives patterns very similar to the large whisps that we often see. (In parallel I had discovered that these whisps only occur when there is a bright source in this angular range - the reason that this did not seem to be repeatable is that in several cases the culprit was Mars, a well-known bright object not in bright star catalogues...)
 - the part of M1 that is inside the central chimney (and hence not visible to direct sky light) is reflective, and reflects bright parts of the telesocpe and dome around.

Andrew and Bernard are doing experiments with cardboard extensions to the baffles to check these theories and look for additional issues. But it looks as if some dominant sources of scattered light are fairly easy to remove.
A report will come in due course.

Also, the reflections near the edge of the OmegaCAM field, which show up as perpendicular spikes from stars some 5arcmin outside the field of view, look like they are a diffraction effect caused by a sharp edge higher up - most likely one of the masks inside the dewar. Also the corner reflections look quite similar to initial ray tracings, and are again probably a diffractive effect.

Progress!

Koen