The Star Analyser  200 used as an Objective Grating

mounted in front of a camera lens



Although the sensitivity is much lower of course due to the small aperture, this arrangement does have two significant advantages over the more usual configuration of  the Star Analyser mounted in the converging beam between the telescope and camera sensor. 


  1. Since the light beam is parallel though the grating, the aberrations in the  objective grating configuration are reduced compared with the usual arrangement of mounting the grating between the telescope and camera and therefore the resolution is potentially higher.  See here for an example of the SA100 used with a telephoto lens and Digital SLR camera.      

  1. Because the camera lens has a shorter focal length compared with a telescope, extended objects like comets, planets, planetary nebulae etc appear more point like. This  allows spectra to be obtained of objects which could not otherwise be recorded successfully using the  Star Analyser with a  telescope. See here for example the spectrum of a comet recorded using a 100 l/mm grating  (bottom of the page) using an early  prototype of the SA100 mounted in front of a camera lens on a modified webcam

Using the SA200 instead of the SA100 in this configuration gives the same length of spectrum with a lens of half the focal length.  This has some advantages:



  1.   When measuring extended objects where the resolution may be limited by the size of the object in the image.


  2.   The shorter focal length  is  more forgiving in terms of focus errors


  3.   There is less vignetting at the same aperture with the shorter focal length lens

        (eg there is no vignetting with the Canon 350D and EF-S 18-55mm zoom at 55mm shown above)  


  4 .   Depending on the quality of the lens, the resolution is potentially higher if the star image size is smaller with the shorter focal length.


  5 .   When using a fixed camera mount to produce a trailed spectrum, the trailed distance is shorter with the shorter focal length giving a brighter spectrum in the image.


  6 .   The SA200 can be used at 50mm focal length, within the range of the standard short focal length zooms typically supplied with DSLR

        still giving the advantage of the  higher resolution potential of the configuration but the SA100 needs a longer focal length telephoto lens



There are some disadvantages to consider however:


  1.   The wider field means there is greater risk of interfering background stars and spectra.


  2.   The sky background is brighter.

  3.   Star images are potentially undersampled and the resulting narrow spectrum increases the risk of artifacts due to insufficient coverage of rows of pixels,

       particularly with colour cameras and where the spectrum is not exactly horizontal.  Stacking frames or defocusing slightly can be used to reduce any

       potential problems though and this is not a problem if you are using a fixed camera to produce a trailed spectrum.



Below are examples using the SA200 mounted in front of a 50mm f1.8 camera lens and an ATIK 314L+ cooled astro camera (shown top right of this page.)


Gamma Draconis (K5iii)  and Jupiter

(Note the detailed features in the spectra which would be unresolved with the Star Analyser used conventionally in the converging beam between telescope and camera)




Comet R1 2012 Lovejoy

(Note how the comet appears almost stellar in the wide  field image, producing a sharp spectrum)