Guide Image Quality

Andy Wilson

Hi Steve,

I hear what you are saying, as you should not need to introduce a washer to get a good image. It will be interesting to hear how Kevin gets on.

I am a bit biaised as I started off with the cheaper L200 (by JTW) before upgrading to the Lhires III at about twice the price. These are both Littrow spectrographs with some similar design features. The quality of the guide image I was seeing through my Lhires, while not perfect, was far superior to my L200.

Example L200 guide image with obvious flaring and triple image of the star.

Example Lhires III guide image. Not perfect with a secondary image of the star, but overall better image quality.

To put this in context for anyone following this thread, in spectroscopy the purpose of the guide image is to allow you to locate your target star, position it on the slit and to keep it there. So while a good quality makes this job easier it is not strictly speaking essential. The Lhires III and L200 have a series of mirrors and lenses to relay and focus the guide image to a camera, and these can introduce various image aberrations. For example I was able to get some excellent spectra out of my L200 even though the guide image was not very good.