astro-optics
page
This page is devoted to some, as yet unfulfilled, experimental spectrograph designs based around common amateur Newtonians and SCTs.
The top figure shows a Newtonian converted into a spectrograph where a tiny convex mirror [reflective slit] at prime focus projects a single star onto the perimeter of the main mirror whence it exits the telescope as a parallel [collimated] beam in a modified Mersene-Cassegrain arrangement as explained below. The collimated beam is intercepted by a reflective diffraction grating which disperses the starlight into a rainbow of colours out through the telescope wall onto a CCD where the spectrogram is imaged. The idea is based on my BAAJ paper [final para] and was submitted in a short paper to Applied Optics in the early 1980's and rejected but still have hopes for its practicality. It uses the minimum of additional optics i.e. reflective slit, grating and camera and part of the primary mirror [<5%] as a collimator
The lower figure shows a SCT version that actually images the spectrum as well but masking from extraneous light would be difficult to say the least !
The figure below shows a small Newtonian converted into a powerful and practical Ebert spectrograph either piggybacked on the main telescope or isolated from the telescope for mechanical and thermal stability with starlight from the telescope focal plane via a flexible fibre-optic feed.
Note: fibre-optic feeds have relatively inefficient throughputs ie absorb precious starlight - see below
Below uses a more efficient Coude arrangement with the minimum number of reflections to degrade starlight.
This shows an Ebert spectrograph in static Coude mode where the [reflective] slit is on the polar axis of the telescope. A plane mirror behind the telescope directs starlight onto the static slit. The preferred floor location will only work with a German Mount because the telescope is offset to the side and the polar axis is not hidden behind the telescope pier. With a Fork Mount an additional plane mirrors must be used to view around the pier. Both Mounts work above the floor with a single mirror as shown as the polar axis line is always on view but spectrograph more prone to vibration..
The Mersene-Cassegrain telescope provides a magnified view but does not produce an image. It is analogous to the Galilean telescope or opera glass but uses reflective surfaces instead of lenses. The above spectrograph is a variant of this design.
text & images copyright Maurice Gavin 2000