Recently the Nanjing Institute of Astronomical Optics & Technology (NIAOT, CAS) successfully fabricated a 1.35m ultra-thin corrector mirror with high order aspherical terms. This mirror is used in the Zwicky Transient Facility (ZTF), which is a new time-domain survey project of Caltech Optical Observatories (COO) in USA. The mirror is a core component to upgrade the famous 48 inch Schmitt Telescope in Palomar Observatory. After upgrade, its field of view can achieve 47square degree with the help of 16 CCDs (each has the resolution of 6k×6k and the size of 386mm×395mm) in its focal plane, which means it can scan the sky of 3750 square degree with 20.5 stellar magnitude in one hour
The material of this corrector mirror is the fused silica glass. Its diameter is 1.35m,but the thickness is smaller than 15mm,which means the diameter-thickness ratio is larger than 90:1.This feature makes a big challenge to the fabrication, test and support. To satisfy the demands of high surface precision and short fabrication duration, the members of Research Laboratory of Large Aperture Optics Technique in NIAOT did their best to make the quality plan, design the schemes of null test and the mirror support for the fabrication/test, and execute the plan of mirror surface polishing. The mirror was finally completed with the duration less than 9 months after the glass was transported to NIAOT.
The ZTF achieved its first light in Nov.1 2017, and the observation result is good. Prof. Shri Kulkarni, a famous astronomer and the director of COO, sent a letter of thanks to NIAOT, which said: “In mid-2016, COO and ZTF were in danger of a severe schedule slip because of vendor non performance to deliver a large (1.35-meter diameter) aspheric optic. This optic is essential to obtain the best image quality across the large ZTF field. In an excellent example both optician’s skill and organizational cooperation NIAOT was able to deliver the required optic on time, with excellent optical performance. NIAOT cooperated fully with these requests, providing the requested material promptly and clearly. In the end, the lens met both our stringent optical requirements and our ambitions schedule, enabling the timely start of ZTF science observations.” This letter highly praised the abilities of both optics fabrication and the project management of NIAOT. It confirmed that this mirror satisfied the quality and duration demands of ZTF project, so that the observation of ZTF can be started on time.
The success of ZTF corrector mirror shows the fabrication abilities of large aperture and thin aspherical mirrors of NIAOT. It is also a good cooperation example between NIAOT and the top foreigner astronomic research organization.
Packing of the correction mirror for USA
Checking of the correction mirror by ZTF project group
ZTF achieved its first light in Nov.1 2017
