High Spectral HR images

VTT - Double-stacked etalons in collimated beam. Finesse of 46 for HR etalon and 36 for LR etalon.

Source : K.G Puschmann et al, The new Göttingen Fabry-Pérot spectrometer for two-dimensional observation of the Sun. A&S 451, 2006.

SST - CRISP SpectroPolarimeter - Double-stacked (high-resolution and low-resolution) etalons in telecentric beam. FHWM = 0.06 A.

Source : Pereira et al, The apparence of spicules in high resolution observations of Ca H and Ha. Astrophysical Journal, 2016.

Very-high resolution spectrum of the solar limb - NSO/SPO 16-inch coronagraph

The 0.8 A central part of the Ha line corresponds to the highest level of the chromosphere visible in the Ha line.
Any etalon having part of its transmission curve beyond this central window of 0.8 A (i.e. Ha ± 0.4 A) will pass light from the lower levels of the chomosphere, and possibly of the photosphere.

For comparison:
- In a collimated beam, the bandpass at 10% peak transmission of a double-stack Lunt 40 is 1.0 A, and the bandpass at 10% peak transmission of a 0.3 A etalon (such as the one of the Heliostar 76) is 0.9 A. This explains why the double-limb effect is not visible with these two configurations (when properly tunned to the center of Ha).
- In a telecentric beam, the double-limb effect is barely visible at f/35 with a double-stack Lunt 40 (bandpass at 10% peak transmission = 1.3 A) and completely invisible at f/42 ( bandpass at 10% peak transmission = 1.1 A).

Solar limb spectrum extracted from the Second Solar Spectrum (SS2) atlas (for comparison with the NSO/SPO spectrum).
Source: https://www.irsol.usi.ch/it/data_archive/ss2

Double-limb effect visible at f/35 when the center wavelength is at Ha - 0.3 A.

Double-limb effect is barely visible at f/35 when the center wavelength is rght on Ha.

Reference disk images - 1 m Swedish Solar Telescope

Influence of the position of the filter center wavelength within the Hα line (respectively -0.8 Å, -0.6 Å, -0.4 Å and line center).
1-m Swedish Solar Telescope, CRISP spectropolarimeter, bandwidth of 0.06 Å FWHM (obtained by two etalons in telecentric configuration), adaptive optics and image reconstruction. The field covers about 70".
Source: Goran Scharmer, Luc Rouppe van der Voort, New Capabilities for High Resolution Collaborative Studies with the Swedish 1-m Solar Telescope, The Institute for Solar Physics, Second Hinode Science Meeting, 2008
https://www.hao.ucar.edu/partnerships/hinode/conference-2008/presentations/Session5/T5-1_scharmer.pdf