A tilted interference filter in a converging beam

Abstract

Context. Narrow-band interference filters can be tuned toward shorter wavelengths by tilting them from the perpendicular to the optical axis. This can be used as a cheap alternative to real tunable filters, such as Fabry-Pérot interferometers and Lyot filters. At the Swedish 1-meter Solar Telescope, such a setup is used to scan through the blue wing of the Ca ii H line. Because the filter is mounted in a converging beam, the incident angle varies over the pupil, which causes a variation of the transmission over the pupil, different for each wavelength within the passband. This causes broadening of the filter transmission profile and degradation of the image quality. Aims. We want to characterize the properties of our filter, at normal incidence as well as at different tilt angles. Knowing the broadened profile is important for the interpretation of the solar images. Compensating the images for the degrading effects will improve the resolution and remove one source of image contrast degradation. In particular, we need to solve the latter problem for images that are also compensated for blurring caused by atmospheric turbulence. Methods. We simulate the process of image formation through a tilted interference filter in order to understand the effects. We test the hypothesis that they are separable from the effects of wavefront aberrations for the purpose of image deconvolution. We measure the filter transmission profile and the degrading PSF from calibration data. Results. We find that the filter transmission profile differs significantly from the specifications. We demonstrate how to compensate for the image-degrading effects. Because the filter tilt effects indeed appear to be separable from wavefront aberrations in a useful way, this can be done in a final deconvolution, after standard image restoration with Multi-Frame Blind Deconvolution/Phase Diversity based methods. We illustrate the technique with real data.

Ca II H sunspot tomography from the photosphere to the chromosphere

“…17, which is lower than the other blue data because of the effect of the pupil apodisation on the point spread function (PSF) at large filter-tilt angles (Löfdahl et al 2011;Henriques 2012). The spatial resolution of the red-beam data is about 0.…”

“…This will be referred to as the fixedwing filter (FW). The fourth camera was placed behind a narrowband interference filter mounted on a rotation stage with central wavelength just red-wise of the core of Ca ii H. The rotation stage allows tilting of the filter with respect to the optical axis, which shifts the centre of the passband towards the blue, effectively scanning the Ca ii H blue wing as described by Löfdahl et al (2011).…”

Three-dimensional temperature mapping of solar photospheric fine structure using Ca ii H filtergrams

“…2) computed from the tilt angle as in Löfdahl et al (2011), and I s i (λ) is the synthetic spectrum.…”

High-resolution observations of the solar photosphere, chromosphere, and transition region

“…Both CRISP and CHROMIS are dual Fabry-Pérot filtergraph systems based on the design by Scharmer (2006) and are capable of fast wavelength sampling of spectral lines. Before the installation of CHROMIS in September 2016, the blue beam was equipped with a number of interference filters, including a full width at half maximum (FWHM) of 10 Å wide filter for photospheric imaging at 3954 Å, and an FWHM = 1 Å wide filter centered on the Ca ii H line core at λ = 3968 Å (see Löfdahl et al 2011). The CRISP instrument has a pair of liquid crystals that together with a polarising beam splitter allow measurements of circular and linear polarisation in for example the photospheric Fe i 6173 Å, Fe i 6301 Å, and Fe i 6302 Å lines, and the chromospheric Ca ii 8542 Å line.…”