As described in the WTTM Users manual, the autofocus sensor is incorporated as part of the tipt/tilt error sensor. Errors in focus are derived from astigmatic aberrations produced by a converging beam of light through a tilted glass element and compensating optic designed to produce a specific level of astigmatism and remove the coma and chromatic aberration; astigmatism, coma and chromatic aberrations are introduced as the light passes through the beam splitter, 10mm thickness and at a 45deg incidence angle. Errors in focus can then be sensed by measuring the ratio of light flux in the four quadrants of the tip/tilt error sensor.
The key to this is that the aberrations as delivered by the telescope are controlled. Specifically, if astigmatism or coma are significant, but stable, the focus sensing scheme is valid, but if otherwise unstable, aberrations will be introduce onto the four quad-cells and the flux ratios will be changed. In otherwords, any reference with the focus sensor that one establishes as "best focus" will be offset by the change in delivered telescope aberrations. Note: aberrations introduced by the atmosphere that vary at 1Hz or faster, are averaged out by integrating the focus error signal over a time period that the user chooses, generally 30sec.
Chuck Claver aurgued at a very early state in the lifetime of WTTM that the telescope aberrations were changing in an unacceptable manner, a statement that I stubbornly refused to accept. The key is that WTTM is monitoring the changing optics on an almost continual bases, whereas my experience with wavefronts extended over a few hours on any given night and at sampling frequencies that of order 7-10minutes. Therefore, WTTM provided the first opportunity of the telescope variations. Note also that Bruce Bohannon, in a commissioning report published in 1996, indicated that astigmatism and coma varied, or it's scatter, was of order of 0.2microns. The report noted that the scatter was the error in measurement, not the actual changing of the telescope optics. Indeed, we may have been over-optimistic in the optics stability.
The conclusion then is that the WTTM Focus Error Sensor is only as reliable as the telescope optics. The following graphic show one the problem. There are obvious offsets in the focus signal as measured by the WTTM Focus Sensor. The data sets for the Kron-I and Johnson-R filters are as follows:
As one sees from the graph, in particular the I-band data, there is a sudden shift in the focus sensor signal. The only apparent reason was a shift of the telescope coma as verified by wavefronts which bracketed the experiment for I. The change, coma by about 0.2microns. On a good note, one can see the potential of the focus sensor in the R-band data where a reasonable linear response. NOTE: Best focus in R does not correspond to a 0signal of the focus sensor. This is due to the fact that the two focal planes, focus sensor and the R filter shifted focus of the CCD are coincident.
- Establish telescope guiding with the WIYN IAS Guide Probe.
- Establish best focus on the WTTM CCD for the filter in question.
- Reference this "best focus" to the output of the WIYN IAS AutoFocus sensor as baseline.
- Adjust the telescope focus by small amounts and record the averaged WTTM focus sensor signal.
- Between each telescope defocus, return to the CCD best focus and verify with the WIYN IAS Autofocus sensor.
- Continue gathering datapoints, occasionally verifying CCD bestfocus via a CCD exposure.
