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NIKA2+ hardware adaptation to focuses differences

Historical context

In the first runs of NIKA2 in 2015 ref, we noticed that the 2mm beam was severely degraded and concluded that the dichroic #1 was severely distorted. After years using the dichroic #2, which was flat but had a problem of misplaced transition in the 1 mm band, and an unsuccessful try with the dichroic #3 which had a distortion problem similar to dichroic #1 ref, we finally came back to dichroic #1 for the NIKA2+ upgrade in March 2025 ref. This time the dichroic #1 was mounted differently than in the early runs 10 years before and than the dichroic #3 in 2017. The choice of mounting the dichroic #1 membrane in sandwich between 2 aluminum rings maintained in the copper mount with a copper flange screwed on the mount with a torque of 0.5 N*m, result from a series of tests conducted in IRAM lab these last years [[ref], which goal was to understand the cause of the distortion and find a way to suppress it or at least minimize it down to an acceptable value.<<br>> From the lab tests we were expecting a residual distortion in NIKA2 at cold temperature between 0 and 1 mm amplitude. The first order effect of the distortion is a displacement of the image surface in the cryostat, that can be compensated thanks to a displacement of the KID array. Beyond 1 mm distortion some second order aberration effect start to be more and more significant and degrade the beam even at the best focus.<<br>> A collection of copper rings between 1 and 10 mm thickness was fabricated, to be used as possible shims to be placed between the KID array holder and its mount on the cryostat, in order to shift the array position along the optical axis.

For the March 2025 intervention we changed a number of elements in the NIKA2 cryostat ref, but decided to not change the position of the arrays since we didn't knew exactly what would be residual distortion of dichroic #1 with its new mount in the NIKA2 cryostat environment.<<br>> During one year of using NIKA2 on sky, between run ref and run ref, we monitored the focus differences between the arrays. All these values are gathered in a document showing the evolution of the focus values and differences in a few plots ref. We noticed a constant focus offset between Array 1 and Array 3 (possibly due to a small residual distortion of the polarizer or a small difference in optical path length), and changing focus offset between Array 2 and the other arrays, passing from a small value with consequence on the beam to an unacceptable value where the 2 mm beam quality is very severely affected when the focus is set for the 1 mm Arrays. Somme Zemax simulations allows visualizing this effect with plots quantifying this effect ref. After each full warmup of the cryostat, the focus difference between Ar2 and the others at the following cool down was a bit more degraded, up to reaching an apparent asymptote at the last warmup in January 2026.

The residual distortion of the dichroic, its evolution with warmups and the possible measures to mitigate is were discussed at several NIKA2 meetings, in particular the following ones ref. From these discussions and given the various other non optimal elements in NIKA2+, in particular the new Array 3 not performing as well as the new Array 1, we ended up with the following plan for this intervention in March 2026: *

Preliminary work before March 10th, 2026

Participants and key Hardware

Participants:

Key Hardware: *

March 10th, Tuesday