The WIYN SAC requested a comparison of DIQ (delivered image
quality) between WTTM (no tip/tilt corrections) and MiniMo. Due to
the overhead involved in setting up this test and detector readout, it
is difficult to achieve the comparison under "identical" conditions.
As example, 15min will lapse between the exposures, overhead of focus included.
Therefore, for the commissioning efforts so far, we have concentrated on
other tasks.
However, we may have a data set that addressees this comparison in part. As I understand the motivation for this comparison, the SAC wished to be demonstrated that WTTM delivered image quality equal to that currently experienced with WTTM; WTTM does not degrade the native DIQ of the telescope.
During June 2003, WIYN cooperated with Lowell Observatory in providing simultaneous data for understanding and calibrating their Differential Image Motion Monitor (DIMM). The two nights were used for WTTM commissioning with tip/tilt on and off imaging and with our nominal DIQ measurement process. I present the data below.
The data obtained proved that WTTM delivered image quality that was as good as 0.32arcsec, and tracked the image quality as delivered by the site and as measured by the DIMM. In particular, the DIMM estimates the site seeing, independent of tip/tilt, i.e., tip/tilt wavefront errors do not in theory effect the DIMM measurement. Therefore, whether the tip/tilt corrections are on or off, the WTTM data should closely track the DIMM data, which it does. Given that the seeing on June 12th was exceptional, one could project that WTTM will deliver superb image quality, with or without tip/tilt corrections, that will maximize what is delivered by the site.
Note: An exceptional night for seeing, one does see the variation in the seeing on very short time scales both as measured by WTTM, but also the Lowell DIMM. Truly stable conditions are hard to come by.