Related to the Riccardi Reducer I’ve had different information. All distances are taken from the M63x1 thread. Teleskop-Austria the dealer of my telescope and APM-Telescopes said 80 mm. The previous owner of the scope said 78-80mm. In order to get an overview on the used distances I collected available data, put them into a diagram and fitted a curve.
The result is shown in the figure. It reveals that the range 78-80 mm is a good value to start with the 550 mm focus length of the Esprit. The exact value has to be figured out by experimentations. To make a long story short: During 17 nights from June to August 2016 I finally came to the best distance of 78.5 mm, if simultaneously a filter with 2.1 mm thickness is in the train. Assuming the filter accounts with 1/3 of its thickness to the optical way, the effective back focus in my case is 77.8 mm.
My imaging system needs 39.5 mm back focus. After figuring out the optimum reducer distance it was easy to design an adapter. This connects he M63x1 Riccardi holder to the M54x0.75 thread of the filter wheel. The Riccardi holder fits into the M74x1 thread of the focuser and fixes the Riccardi inside the focuser tube and consumes 13 mm back focus. Again the difficulty is the material-saving design of the filter wheel and its very small wall thickness. Well, ought to look for some better quality in future…
Find a drawing of the ordered adapter here. I summarized an overview of the whole chain in a simple sketch. Maybe this is helpful for somebody who need this information. The sketch shows the camera at the right hand side and the connection to the focuser at the left hand side. Part „A“ carries the Riccardi at its left and is screwed into part „B“ which itself is screwed onto the focuser. Performing this way the Riccardi disappears in the focuser.
This design realizes 78.5 mm back focus for the Riccardi, which is at the „shorter end“ of possible the back focus lengths. This gives the opportunity to extend it by inserting fine tuning rings. My tests revealed that this is notnecessary, hence I’m using it exactly in that configuration. Beneath is a picture showing the assembled Riccardi ready for insertion. The upper end goes into the focuser and the filter wheel is connected to the lower end. In the middle the part „B“ of the above given sketch can be identified.