Scientific Results

New technique to measure the cavity defects of Fabry-Perot interferometers

Year: 2019

Authors: Greco V., Sordini A., Cauzzi G., Reardon K., Cavallini F.

Autors Affiliation: CNR-Istituto Nazionale di Ottica
National Solar Observatory, Boulder, USA
INAF-Osservatorio Astrofisico di Arcetri

Abstract: Context. Several astronomical instruments, for both nighttime and solar use, rely on tunable Fabry–Perot interferometers (FPIs).
Knowing the exact shape of the etalons’ cavity is crucial for assessing the overall instrumental transmission profile and its possible
variations during the tuning process.
Aims. We aim to define and test a technique to accurately measure the cavity defects of air-spaced FPIs, including distortions due to
the spectral tuning process that are typical of astronomical observations.We further aim to develop a correction technique to maintain
the shape of the cavity as constant as possible during the spectral scan. These are necessary steps to optimize the spectral transmission
profile of a two-dimensional spectrograph (polarimeter) using one or more FPIs in series, and to ensure that the spectral transmission
profile remains constant during typical observing conditions.
Methods. We devised a generalization of the techniques developed for the so-called phase-shifting interferometry to the case of FPI.
This measuring technique is applicable to any given FPI that can be tuned via changing the cavity spacing (z-axis), and can be used
for any etalon regardless of the coating’ reflectivity. The major strength of our method is the ability to fully characterize the cavity
during a spectral scan, allowing for the determination of scan-dependent modifications of the plates. We have applied the measuring
technique to three 50mm diameter interferometers, with cavity gaps ranging between 600 m and 3 mm, coated for use in the visible
range.
Results. The technique developed in this paper allows us to accurately and reliably measure the cavity defects of air-spaced FPIs, and
of their evolution during the entire spectral scan. Our main, and unexpected, result is that the relative tilt between the two FPI plates
varies significantly during the spectral scan, and can dominate the cavity defects; in particular, we observe that the tilt component at
the extremes of the scan is sensibly larger than that at the center of the scan. Exploiting the capability of the electronic controllers to
set the reference plane at any given spectral step, we then develop a correction technique that allows the minimization of the tilt during
a complete spectral scan. The correction remains highly stable over long periods, well beyond the typical duration of astronomical
observations.

Journal/Review: ASTRONOMY & ASTROPHYSICS

Volume: 626      Pages from: A43 – 1  to: A43 – 14

KeyWords: Fabry-Perot interferometers, Interferometry.
DOI: hhttps://doi.org/10.1051/0004-6361/201935302

English