As introduced in the previous post, a various parts for Spectrograph are arriving in France, where we started building up the Spectrograph and verifying its performance.
This time, we are talking about dichroic mirror as one key parts in the spectrograph.
PFS delivers spectra covering such a wide range of wavelengths as from visible to near infrared with a single exposure. This is in fact achieved by splitting the light that goes to each Spectrograph into three separate beams and guiding them to three respective camera units; “visible-blue”, “visible-red” and “near-infrared”. The key optical element splitting the light this way is called “dichroic mirror”. The PFS Spectrograph accommodates two types of dichroic mirrors to make such a three-way split.
Dichroic mirror has a special coating on one surface so that it transmits the incoming collimated beam at longer wavelengths than a specific wavelength, while it reflects the light at shorter wavelengths. The coating process is challenging because of not only the specialty of the coating but also the required high quality in such aspects as the throughput and sharp transition between transmission and reflection which are critical for the instrument performance. Therefore, we have been repeatedly making prototypes, evaluating their performance, and giving feedbacks to the coating design and coating process for optimization, before we start the final production process.
Last month, a prototype of one of the two types of dichroic mirror which reflects the light in the visible-red band and transmits the light in the near-infrared band has been completed. This mirror, like the other parts such as the camera units and fiber slit, has been shipped to France, where it will be integrated first to the mechanical jig and then to the Spectrograph under the validation process.