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Dachs, yes, it affects the position of the primary relative to the secondary, closer, in order to take up the extra distance between the mirror's edge and the inside of the tube. This will in turn mean that you will need a larger secondary mirror to capture the light escaping around it, which in turn means loosing light to a larger secondary obstruction! You need to get the light to come to focus somewhere just above the height of the focuser, with the draw tube in all the way. Unless the primary is closer, you won't achieve focus.
There is a balance between the size of the secondary and the %obstruction it produces and the distance the path of light needs to overcome. Too small a gap between the mirror and the tube, and you lose how wide a field the mirror can see. Too large a gap you increase the size of the secondary as explained above. And you need to add the focuser to the mix.
That is why I suggested you have a look at the scope design sites. These produce diagrams to help you visualize what goes on.
I'm having to be very careful with the 17.5" dob I'm rebuilding. I have an existing secondary I'm using, so I need to know what parameters I need to build around it and the primary to optimise the light that enters my focuser, and minimise the % obstruction of the secondary.
Simply put, it is maths applied to a direct physical project. Nothing more. It really is not complicated. Once you visualise the situation at hand, year 6 maths solves all your problems.
With respects to air circulation, if you leave some sort of gap system within the mirror cell, heat discipation would be quick for a 6" scope, even using PVC tubeing. People attach small 12V fans to their mirror cells to assist with this cooling.
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