of optics (spectrograph), telescope and observation camera
The optic elements of the spectrograph must be adapted to the telescope optics. The key parameter here is the equality of focal ratios of telescope and spectrograph. When the F# of the spectrograph is smaller than the F# of the telescope, only a partial area of the collimator (and the grating) is used. In opposite situation, a part of starlight is lost.
On the other hand is to adjust the pixel size of CCD on the resolution of the
spectrograph. There is no advantage if the spectrograph resolve 1 angstrom,
but records 3 angstroms on 1 pixel and integrates 3 resolution units into a
1 pixel value (undersampling). Or, conversely, the resolution element of 1 angstrom
is spreaded over 5 pixel, that is all about the same pixel value measured (oversampling
)? And captures additional noise of the to many participating pixels?
According to the Nyquist Theorem, the spectral resolution element (the most fine structure that is mapped in the spectrum) has to expose 2 pixels. Then the system shows the best statistical behavior.
Now we can distinguish different situations. .
If somebody was LHIRES III want to buy, then I recommend to the telescope, for which the LHIRES was designed and optimized: the C11. I had decided for the C14. But that was an (but not worse) error. Because of the nearly 4 m focal length of the C14 the diameter of the star image in focus counts approximately 60 to 80 µm (average seeing in Germany of about 2 to 4 "). So I have to adjust the slit to 40 µm to cut not too much of the star disk/image . Effectively I lose while about 1 / 2 to 2 / 3 of the starlight on the slit, so that brings the bigger aperture of the C14 compared to C11, ultimately nothing. With 40 µm slit width, but I lose resolution compared to the situation at C11, where would be because of the smaller focal length and thus smaller star picture in focus a slit width of 20 to 30 µm to be sufficient. For this reason, I use a Shapley lens in front of slit to reduce the focal length of C14.
For such decisions necessarily ask SimSpec and run through all the possible alternatives. Identify the main consequences for the selected output parameters (degrees of freedom) in SimSpec: minimum size for the optical elements and characteristic parameters such as resolution, dispersion, etc.
back to content