spectrographs and focal ratio

Forums Spectroscopy Focal reducers for Alpy! spectrographs and focal ratio

Robin Leadbeater

The key thing is that if your star image is already comparable/smaller than the fixed slit width, adding a focal reducer to reduce the focal length will not give you any more throughput or resolution even though the spectrograph might be able to accept a faster beam. You just end up adding more unwanted glass and risk chromatic aberrations.

As an example, taking Steve’s 120mm f7 scope, the 23um ALPY slit covers 5.6 arcsec of sky so in typical seeing the star is already smaller than the slit and reducing the focal length further will give no benefit. (If it was possible to reduce the slit width, you could take advantage of the extra potential resolution but the ALPY slit only has a single fixed slit width)

If we consider my setup though (An f10 C11 with 280mm aperture) The slit is 1.7 arc sec and my seeing is typically 3 arc sec so unless I use a reducer I lose a lot of light at the slit. At f5 the slit is 3.4 arcsec and a much better match to my seeing