I have a 6", f/6 mirror from the 1970s that I rebuilt into a DOB. Using a sonotube and Poplar wood, rather than a metal tube and press-board, it is very lightweight for taking out.
I bought a 3 vane spider for it and it is well made. But the vanes are made of 1/8" screw stock, painted black. So I did some rough calculations.
6" mirror (full size full thickness, real Pyrex).
1.52" secondary. 25.3% central obstruction.
Each vane (3" length over mirror - 0.76" secondary radius = 2.24" of each exposed (in the light path) spider vane, X 3 vanes = 6.72" of exposed (in the light path) vanes.
So using the 1/8" thickness of the vanes (machine screw stock) the linear amount of mirror obstructed by the vanes is 0.84" (6.72" x 0.125"), or 14% more obstruction.
If I'm figuring this correctly, that works out to an additional 14% obstruction, in addition to the 25% for the secondary or a total of 39% total linear obstruction?
If I'm mis-calculating this, please let me know.
What I ended up doing, was to replace the screw stock with 1/2" flat brass, which is 0.016" thick. The additional obstruction from this is, with painting added, about 2.25% additional obstruction, for a total of just less than 28% total obstruction.
So that is a big difference.
I ask if this line of thinking is accurate since I do see some commercially available scopes with very thick (plastic) spider vanes. Some are made for imaging, but some are for visual also. I would guess, some come close to 45% obstruction.
With spider vanes, does the diffracted light go into the Airy rings, or does it add to the overall background of the FOV.
I have the feeling, I may be missing something.