here are a series of photos of the prominences of yesterdy and today wirth the spectroscope and different diffraction gratings : there was a technical detail that i wanted to experiment about : the anamorphosis .
In 1868 when Janssen and Secchi started to observe the solar prominences daily with a spectroscope they didnt use diffraction gratings : it hadnt been invented yet! they were using prisms . And the way they were using it didnt produce anamorphosis (or shrinking) of the images.
I've always noticed that my images were a bit flatter that they really are compared to photos taken with real H alpha filter. So i browsed through books about spectrography etc... and I found that there is a config that allows no deformation of the final image : aka LITTROW design.
Here are some results... (with and without LITTROW mode)