reaspn for using a focal reducer

Forums Spectroscopy Focal reducers for Alpy! reaspn for using a focal reducer

Robin Leadbeater

Hi Tony,

See for a comparison of two different setups

Dont worry too much about matching and focal reducers though. There is no need in general to match the focal ratio of the scope to the ALPY.  You can use any focal ratio scope with the ALPY down to f4. It will not change the spectrum shape or resolution. The only reason for using a focal reducer is to better match the size of your star image to the size of the slit so you can collect more of the light and go fainter. This depends only on the telescope focal length (not focal ratio) and your seeing. If the star is smaller or comparable to the slit, then you are better off without one. Even if the star is bigger than the slit, if the star is a bright one, you may still decide not to use a reducer to avoid the risk of chromatic aberrations.