As one might expect, it is difficult to seperate out all influences
of flexure for such an optical system as WTTM. Our attempt to least
characterize the result of flexure is presented here.
Using the pinhole mask, which presents 0.2arcsec point sources onto the CCD, we locked the WTTM tip/tilt guide function onto a point source nearest to CCD center. The position of point source was then measured at varying NIR (Nazymth Instrument Rotator) angles. The results are shown below.
The Rotator angle is allowed to rotate +/- 180d for 0d. WTTM resides on the side of this port, staring into the center. Therefore, rotation of the port will change the gravity vector that acts on the instrument dewar, WTTM error sensor, optical housing or WTTM can, and the WTTM pickoff mirror which feeds the light from the telescope into WTTM. All of these are, at some level, influenced by the change in rotation. A point of simplification, because the point source is generated by a pinhole mask place at the entrance to the optical housing of WTTM, the instrument entrance, we can eliminate the WTTM pickoff mirror from this analysis.
From the flexure map, we can make the simple analysis:
- The change in X/Y tip/tilt (abscissa), measured here in arcsec, is the the accumulated corrective signal to the mirror. This is most likely a result of point source movement on the error sensor which would be an accumulative result from flexure of the WTTM optical housing, and WTTM error sensor.
- The change in focal plane position (ordinate) of the point source is influenced by mechanical flexure of the CCD dewar within it's mount, and to a smaller degree, errors introduced by the WTTM error sensor flexure.
- For typical objects, on object for ~1hr, one will not be changing the rotator angle by 180d, but more of order 25d therefore, flexure should not be more than 0.1arcsec effect.
