I am getting ready to order a custom adapter to attach a new telescope to my STXL16200 camera + filter wheel. Diffraction Ltd has always stated the back focus distances on their cameras in both physical and optical terms, and up until now, I have always used the optical back focus specification when calculating how long my adapters need to be. Normally, I calculate how long the adapter has to be using the optical back focus, then subtract 1 mm when I specify the length for the custom adapter. This way I have to use a 1 mm spacer to reach the required back focus. Today, someone on another forum who has some expertise in the topic told me that I should use the mechanical back focus distance when designing my adapter. They also stated that as a rule of thumb, filters will reduce the back focus by 1/3 of the filter thickness. For the STXL16200 with self-guiding filter wheel, the mechanical back focus is 61.85 mm, and the optical back focus (with 3mm filters) is 59.87 mm. These values suggest that there is something besides the filters that is reducing the optical back focus. My assumption is that the window in front of the CCD sensor is 3 mm thick and further reduces the back focus by an additional 1 mm. I think that in the process of writing this thread, I have talked myself out of following the advice I was given to use the mechanical back focus. I would be interested in hearing the opinions of others on the forum on this. Mike
Always use the optical back focus. Period. Please note that machining tolerances are on the order of +/- 0.25 mm, and that these can stack up.
