heh ... 10 months later and the cardboard tube is still standing in the corner, next to the (unfinished) model railroad ... :/
Couple of problems, that just kept me from progressing ...
1) The tube isn't quite big enough ... I REALLY would like 1" airspace between the incoming light and the mirror edge to minimize thermals ... and the tube is only 7" inside ... not enough.
2) Thick cardboard is heavy.
3) I am concerned with the "stability" of the material ... this tube is designed to be low cost as its only purpose was to serve as a disposable support to hold a paper product.
In the meantime, I've reconsidered my original plan. I will stick with the hexagonal tube, but will use plywood to create support "rings" and use a thin masonite "skin" to make the tube. The advantage of the masonite is it will 1) provide a finished surface on the outside; 2) a rough surface on the inside (to help mitigate grazing reflections); and 3) it is dimensionally stable from temperature and humidity.
I have worked up an inverted primary mirror cell, that will be more compact (important for keeping the overall tube length down); while allowing for easy removal of the mirror (by removing the three collimation nuts from the bolts).
Let's see how long it takes me to make progress or change my mind ....
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