Before construction, the measurement tasks of the spectrograph are to be defined. It must be remembered
- the resolution R required to resolve the measurement tasks
- the focal ratio (to match the telescope used)
- the type of spectrograph: slitless or with a long slit
- many more customizable details.
Fortunately, the French amateur astronomer Christian Buil developed an Excel spreadsheet that performs the complex calculations of spectrographs for us: SimSpec. Is a somewhat modified version made by Klaus Vollmann available.
As shown in the screenshot you can see the required information: parameters for the telescope, the CCD camera and the spectrograph. You can study the calculated results for different observing conditions (seeing) and objects and test whether yours own ideas will be fullfilled. The turning of an adjusting variable immediately shows the anticipated effect.
Really important design parameters of the spectrograph are:
- The focal lengths of the optics (collimator and camera, in case of a Littrow spectrograph are both the same) and the telescope design
- The grating constant (lines / mm)
- The used diffraction order (usually 1)
- The total refraction angle (total angle)
- The slit width
The calculation results are shown in the lower part of the sheets. It lists the minimum requirements specified in the optics (minimum diameter of the optical elements) and the dispersion (Angstrom / pixel) and resolution R of the arrangement and the indicated wavelength range.These results are quite accurate in my experience, consistent with the measurable reality. I can only recommend that each prospective spectrograph designer exercises such calculations over again for practice and study the parameters and the resulting characteristics of the virtual spectrograph. It will growing over time a sense of what is important and feasible.