Image quality

Since aluminisation, there seems to be some issue with the image quality, intermittently showing elongated images. A major difficulty in investigating this is that this is only clearly detectable during good seeing and using the NOTCam high-resolution camera.

Several measurements using the ALFOSC wave-front sensor have been made since the aluminisation of the telescope mirrors in June 2014, but the results are not so clear. It can be seen that the higher order aberrations of tri-coma and quad-astigmatism are consistent in magnitude to before removing the mirrors, though numbers for coma and astigmatism are more variable. Immediately after the aluminisation values for astigmatism were similar to before, but since November have increased to approximately 300nm from $\sim$150nm. On 15 October there was a problem with the air supply to the M1 support system and on 24 October more work on the position of M1 and M2 was done. Coma shows an increase, from typically 50nm to 160nm. If these aberrations cause elongated images as were observed, then you would expect similar angles from both aberrations but we do not see this. For astigmatism the angle is around 50 degrees where as for coma it has no preferred direction varying from 85 degs to -85 degs. It should be noted that it has never been understood that when we get good round images the wave-front sensing has typically given coma amplitudes of $\sim$50nm and astigmatism amplitudes of 150nm.

It has been suspected that the problems were caused by problems with the secondary displacement unit (see above), but several test showed no conclusive relation between the image quality and the deviations of the displacement unit. Original problems with the position of M1 and M2, problems with the M1 support system, and problems with the brushes of one of the azimuth motor probably confused things significantly, but our latest tests now shows that the secondary displacement unit is the problem.