Quote:
Originally Posted by AstralTraveller
Sorry if this is obvious but - why?
I imagine the secondary size is determined by the aperture, f-ratio, tube diameter, focusser height and travel and whether you are optimising for visual or imaging.
cheers,
Dave
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I can't see why either .... if the secondary is too small taking into account the above , then not all the fov will be properly illuminated as well (may not be important in a dob used visually only, but full fov illumination is desireable if you want to do prime focus imaging and use the full fov).
Think I'll stick with the PVC tube based rebuild - I have already bought my new tube clamps and and a new spider and can't be bothered redesigning the OTA at this stage , maybe V3 will become a truss tube in 3 or 4 years.
My design has a 2.5" diagonal , means 6.5% area based obstuction or 25% diametrical obstruction. Field illumination for my current design is better than 90% out to 25mm from the centre of the fov. A bigger diagonal (a 3" would have given me better field illumination but at the cost of high obstruction - plus I would have had to buy a new diagonal and a new diagonal holder too.)