wide spectrum

Robin Leadbeater

Yes mine was one of the original kit versions. Having to assemble it from a box of bits taught me a lot about how it works. You can still see the assembly photos and drawings on Olivier Thizy’s website


I have asked for confirmation about the main mirror adjustment on the ARAS forum. 

>it does increase the height of the image. However I think this might not matter 

The large change in width of the spectrum with the mirror position is strange. I am pretty sure I do not see this but I need  a clear sky to check and the run of good weather has broken at this end of the country. 

A wide spectrum, even if it does not affect the total flux is not ideal as you have to sum more rows, introducing more noise from the camera and sky background.

If the spectrum is wide when the star is focused on the slit (ie the flux is maximum), this can be due to astigmatism. ie the focus of the collimator lens in the dispersion direction (as set up by making the lamp lines sharp) is different to that in the vertical direction.  (The spectrograph design has some inherent distortion of the image but this is small) .  I had this issue once when I overtightened the grating in the holder, distorting it slightly into effectively a cylindrical lens. I am note sure how the gratings are supported in the latest design.  A possible test for this could be to check the zero order image of a star without the slit in place.  In this configuration the grating is effectively a mirror so the image should be round. I have not tried this though.

When the grating was not pinched I did not see any significant difference in focus between the guider and  the spectrum image. (ie when the star was in focus on the slit the spectrum was narrow.) This is true for both my  ALPY and LHIRES.