Revisiting the ALMA Band 1 Optics Design

In this paper, we revisit the optics design for the ALMA Band 1 cartridge as presented previously by M. Carter and report on progress made towards that end. Since the layout of the ALMA cartridges is not optimised for the lowest frequency band, certain design trade-offs must be made; most importantly the use of a re-focusing lens is required to avoid blocking other bands and the ALMA calibration device assembly. Furthermore, we are motivated to analyse the optics design because close to half of the receiver noise budget is consumed by the optics, mostly due to truncation, reflection, and dielectric loss of the lens and infrared filters. Any small improvement in the optics is worthwhile as its contribution is cascaded through the receiver. Also of significance, the antenna and cryostat layout has changed since the original reports and that related to Band 1 must be clarified and updated.