Keck Secondary Focus Shift Test

 

In an effort to discover the opto-mechanical hardware that may be causing an intermittent telescope focus shift, we are using an HP distance measuring interferometer to compile the relative distance between the primary mirror environment and the secondary mirror environment as the telescope slews in elevation. The HP laser is mounted on the tertiary tower just below the primary mirror. A retro-reflector is mounted near the secondary mirror socket and is mechanically coupled to the secondary support structure. The following charts show the primary to secondary  relative distance as the telescope was continually slewed between 75 and 35 degrees in elevation. A focus shift on the order of 150 microns secondary piston was not observed during the January 13, 1998 open loop test. It is believed that  large changes in elevation over a short time may be a factor in causing the focus shift.

 



 

NOTE: In the compiled slews chart the progressive drift downward is an indication of increasing shear between the laser source and the retro-reflector. Looking at the first 4 slews we can expect the telescope secondary focus to be repeatable to within + / - 5 microns. Thus, if a 150 micron focus shift were to occur and the HP laser system was attached to the mechanical components causing the shift, the physical path difference would easily be detected. Future plans involve using the HP interferometer while the telescope is focussed on stars at high and low elevations. Doing this will close the loop and possibly identify where the problem is occurring on the telescope structure.

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