Backfocus or Correcting the Corrector

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Since a couple years I own a focal reducer for my Schmidt Cassegrain. It didn’t come with a whole lot of documentation or manuals, but I knew that it was supposed to reduce the focal ratio from 1/10 down to 1/6.3 while at the same time correcting the image. I have used it mostly to visually observe larger objects that wouldn’t quite fit in the field of view otherwise. 


The reducer/corrector for changing the focal ratio to 1/6.3


When starting with photography I wanted to give it a try as I was hoping to benefit from the image correction as well as from the fact that a shorter focal length would be more forgiving in the presence of small errors in tracking and focusing. In general that worked as expected, however I was really surprised when the plate solving of my first images showed a focal ratio of 1/4.8 rather than the expected 1/6.3. That is how I started to realize the importance of the backfocus distance of a device. The corrector, like many optical devices, is calculated with a specific assumption on the distance to the camera or eye. In the case of my reducer that distance was 82mm. I learned that if you differ too much from that distance it will result in a different focal length and issues like increased vignetting or aberrations. 


The reducer is directly attached to the telescope and in the early days I needed to follow that with a diagonal mirror to avoid the camera to hit the mount.  I turned out that the camera distance created by that is much larger than the desired 82mm. There are correctors that have a longer backfocus, that may have been consistent with adding a diagonal. However, my model was not one of them.


With the NexStar mount a diagonal was required to prevent
the camera from hitting the base of the mount


Initially I addressed the issue by placing the reducer behind the diagonal (which was not that easy as the connections are all wrong). Eventually, with a new mount I was able to remove the diagonal from the configuration entirely and come up with a configuration that meets all backfocus requirements

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