An observatory for the next generation of ultra-high energy cosmic rays (UHECRs) should be expanded for clarifying the origin and nature of UHECRs. In order to realize a huge UHECR observatory, we are developing Cosmic Ray Air Fluorescence Fresnel lens Telescope (CRAFFT), which is a low-cost fluorescence telescope. We tested a performance of prototype CRAFFT at Telescope Array (TA) site, and succeeded to detect UHECR air showers synchronized with TA detectors in 2017. We are currently developing a reconstruction method based on waveform fitting with sufficient analysis accuracy, and optimizing the detector configuration to improve its performance with low cost. Progress in detector optimization, reconstruction method, and calibrations such as uniformity of PMT sensitivity are discussed.