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Folks  I  have a  question on BackFocus,

given that i have a Reducer/Flattener asking for 55mm, and Camera Sensor to Flange distance of 17.5mm leaving me to fill a gap with 37.5 mm of spacers what sort of 'Tolerances' am i working with, will simply purchasing an Astro 7.5mm and 30mm tube spacer do the job and that the Astro industry has the tolerances all buttoned up with acceptable parts or will there inevitably be some fine tuning required when the images start to tell a story. OR, is the Reducer/Flattener more tolerant of a little stackup error in the parts.  for me it comes down to whether I go with an adjustable Tube spacer or Fixed . 

some insight appreciated, 

Gary ( am kinda just setting up with the hobby )

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I finally went with one 20mm fixed tube adaptor and T2-VAREX adjustable (18.00mm / 13.85mm), gave me a final range of 38.00mm/33.85mm towards a target length 37.5mm. for some reason the fixed tube did not seat down accurate against the 0.8x Reducer/Flattener Face so this arrangement gave me the adjustment i needed. I should be within the ±0.5mm tolerance with my verniers. cheers again.

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Dont forget that any filters that are in the light train may also increas or decrease the spacing The best thing to do is get it as close to the manufacturer's specifications and then then test it. It can happen that you need to move it a bit to get the best results (or maybe accept a certain amount of cropping).


Wait until you try fitting an OAG and a filter wheel in there, and then wrestle with getting the guide camera in focus at the same time as the imaging camera. Stiff drinks may my required at this point! 😆

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