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DeepSkyMan posted a topic in Discussions - Scopes / Whole setupsOkay folks, so, for a variety of reasons, including but not least an appalling sequence of poor weather since my last post on this particular topic, I have been unable to obtain an accurate measurement of the focal length off my 8” EdgeHD incorporating Baader SteelTrack Focuser and Click-lock Diagonal. I managed to remedy this situation over the weekend just past. Initial measurements based on adding the optical path of each of the components together, had the complete resulting optical path at about 2400mm, however this was not bourne out in practice. Using my 22mm T4 Nagler, at 2400mm focal length this should have given approx 45’ TFOV. Using M81 and M82 as a yardstick (approximate separation 36’), they should easily have been visible in the same field of view, however they were not. Extrapolating back from these values seems to be indicating a focal length somewhere between 3000 and 3225, which makes the revised speed of the system a whopping F15 to F16. This must be having a significant detrimental impact on the sub-aperture correction inherent in the Edge HD design, so I’m thinking of removing the Steeltrack from the system and replacing with a FeatherTouch or equivalent. This measurement is actually consistent with a drift measurement I made last year, (instructions courtesy of Don Pensack), but discarded due to it being so far removed from the expected result nearer 2400mm. But the proof as they say is in the pudding. So for those considering adding a Crayford to their EdgeHD, based on this experience I would say don’t. From feedback on this topic in CN, it is very possible that it is something to do with either the slightly faster primary/secondary combination in th EdgeHD and/or the sub-aperture correction in the baffle tube, possibly resulting in significant vignetting. (Thanks to Don Pensack and Edggie from CN, for their input). If you are interested in reading the CN thread discussing this you can find it here: https://www.cloudynights.com/topic/574002-calculating-effective-focal-length-of-an-edgehd/ Kind Regards Paul.