What Linwood said. I'll add that it's a good idea to not fixate on a single exact number when it comes to back focus distance. There are manufacturing variations in the objective or mirror of the scope as well as the corrector that will combine to slightly alter what the back focus distance turns out to be. In other words, the distance on paper will land you pretty close and on slow scopes it might be just fine, but faster focal ratios will require some more tuning. In it end, it might be that the BFD wasn't 80.8mm, it might be 80.2 or 81.1 or something close, but not exactly what the documentation says.
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With that in mind, I always slightly undersize my ensemble of adapters and extensions by 1 or 2mm. I then use shims, available in an assortment of diameters and sub-1mm thicknesses, to bump the sensor out some more until it its inside the corrected field. When fine-tuning this distance, the on-paper value isn't what you follow, it's what you see in the corners of your frames.
On Jun 1, 2021, at 16:00, ap@... <ap@...> wrote:
The best example I have seen is the spacing guide in this web site about half-way down. It shows what happens to an in-focus image when spacing is too short or too long.
The center will always look good, since you can reach focus in the center. But without proper backfocus the corners are then distorted (well, they are always somewhat distorted but the right backfocus gives them the minimum distortion, which is sort of its definition).
My understanding of a filter is it adds, meaning if the backfocus from OTA to camera was specified as 146.5mm (e.g. my C11) then when I add a 3mm Chroma filter, I add 1/3rd that thickness, and now use 147.5mm in total.
I find that the most confusing things about backfocus are all the (legacy I think) descriptions of 55mm and DSLR spacing, where the ASSUMPTION is you are starting with a specific set of camera and T-Adapter lengths, and so they only talk about the rest of the equation.
The second most confusing is the point of measurement. My Refractor is measured from the shoulder of the male threads of the last component, i.e. the point closer to the OTA at the base of the threads. My SCT is measured from exactly the opposite, on its male threads you measure from the end of the threads, the further point from the OTA. This is a detail often hard to find, but can vary by 10mm or more. Indeed in that opt article it shows the backfocus measurement point matching my refractor, but WRONG for my SCT.
And yes, backfocus helps keeps Precise Parts gainfully employed. 😊