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Image quality is sensitive to temperature fluctuations on the optical path, even if these are not fully developed turbulence. Thus, it’s crucial to control the thermal environment, be it on a test bench in the laboratory, in instruments (e.g., entrance windows, near electronics), within domes and telescope structures. It is especially crucial where the beam is small (i.e., going through a focus) and the power spectrum of the refractive index can be anything from high frequencies to just tip-tilt.
We have used our optical turbulence sensor AIRFLOW to explore how a DT of a few degrees in the optical path can undo a lot of what an AO system can improve, and we are using our devices to study quantitative ways to minimize the image degradation induced by temperature fluctuations. These may include counterintuitive measures such as fans mixing the air at different temperatures, because mechanical turbulence with no DT doesn’t produce optical turbulence.
By varying the shape of the deformable mirror (DM) to introduce Zernike aberration coefficients through a reasonable range of values, the images produced were read out on the NIRI detector and analyzed for Strehl ratio. Fitting a second-order polynomial to this data set gave an optimized value for each coefficient out to Z49. The Strehl ratio was improved by 6% +/- 2% and the Z5 (45° astigmatism) term showed the only appreciable error contribution to the current NCPA offset of 0.15μm in k-prime (2.12μm). Aside from resulting in a slight improvement in image quality, the technique developed is non-invasive and will be implemented in other instruments and cameras that typically couple with Altair and have outdated or erroneous NCPA files currently. Furthermore, some high spatial-frequency structure in the PSF was found that limited the effect of these corrections, and may be a key component in further investigations towards image quality degradation in Altair + NIRI.
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