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Video: The time-lapse illustrates the JST250 in action during the tests.

Achieved images of 0.33” PSF with “lucky imaging” techniques

Last September 2015, the JST250 telescope at the Observatorio Astrofísico de Javalambre passed successfully the Provisional Acceptance test review. This milestone represents a big step forward for the completion of the project, only pending the Final Acceptance tests that shall be completed in the next weeks.

Several observing test runs were carried out during the summer to assess on the optical alignment of the system and the image quality at the focal plane. This process was carried out using two fast-readout verification cameras mounted at the Cassegrain rotator, which can be located independently at different positions of the 50 cm diameter large focal plane.

Previously, the sensitivity of the secondary mirror (M2) hexapod movements was calibrated as a function of the main low-order Zernike terms, representative of focus, coma and astigmatism aberrations. This allowed to build a preliminary control law for the M2 hexapod, able to correct for minor opto-mechanical deformations of the system due to thermal expansion and gravity flexure effects. Afterwards, several sequences of thousands of images of very short exposure times, of about 5-10 ms, were acquired during sidereal tracking, allowing to measure both the overall image quality in terms of the PSF (Point Spread Function) during typical tracking times, as well as to apply “lucky imaging” techniques to minimize the effects of atmospheric turbulence on the acquired images.

Despite the fine and final optical alignment is still to be done as part of the Final Acceptance tests, the JST250 has been able to provide images with PSFs as good as of 0.51” under good seeing conditions (<0.7”), demonstrating excellent performances and image quality. Regarding the “lucky imaging” analysis, and keeping the sharpest 0.5% of the individual images, the obtained PSF decreases down to 0.33”, getting very close to the nominal diffraction limit of the telescope plus the error budget for manufacturing, integration and alignment of the system.