Optical fine tuning of the system during Final Acceptance tests provides images of 0.15” FWHM using “lucky imaging” techniques

The Centro de Estudios de Física del Cosmos de Aragón (CEFCA) has proceeded with the final acceptance of the JST250 telescope installed at the Observatorio Astrofísico de Javalambre. After the provisional acceptance conducted last Autumm, CEFCA has carried out additional tests with the verification camera in order to test proper operation of the telescope, whose excellent results have given the green light for the final aceptance.

During the last months, a fine optical alignment and new “lucky imaging” (LI) testing were performed in different regions of the 50 cm diameter focal plane of the telescope, in order to verify the image quality all over the entire field of view. Moreover, additional stability tests were carried out to measure the sensitivity of the image quality to temperature variations and gravity flexures.

LI techniques consist of conducting series of thousands of consecutive images with very short exposure times, tipically 5—10 ms. This way the individual images are minimally affected by the atmospheric turbulence and only instrumental distortions remain in the images. The best results achieved with such LI tests provide an image quality of 0.15” FWHM, stacking 0.5% of the frames, and a total image quality of 0.29” FWHM under very good seeing conditions (0.39”). These results beat the requirements taking into account the error budget of the telescope design.

In the next weeks, CEFCA will install the first scientific camera of JST250, JPAS-Pathfinder (JPAS-PF), a replica of the JAST80 camera, that will be mounted at the telescope until the arrival of the final camera, JPCam. JPAS-PF will carry out the scientific commisioning of the telescope and it is expected to obtain the first data of scientific quality with some of the J-PAS filters during the next months.

Assessment of the JST250 image quality using lucky-imaging (LI) techniques with the verification camera, in December 2nd 2015. The left panel shows an excellent FWHM of 0.15” as the result of applying LI techniques for a given star, with integration times of 10ms per frame, and aligning and stacking the best 0.5% frames according to their Strehl ratios. The right panel illustrates the combination (no previous alignment) of all the frames acquired during the test, obtaining a fantastic FWHM of 0.29”. The test was performed under excellent seeing conditions, with values as low as 0.39” in that night. This result demonstrates that the JST250+dome system can provide images with PSFs smaller than the seeing value measured by the DIMM monitor.