ATM 12.5 mirror box/upper cage dimensions

[sr.hughes@att.net]

I am in the process of designing a 12.5 F6 Truss using 
the Kriege/Berry Dobsonian book. There are some 
dimensions that are not adding up and I don't have a 
Obession scope to look at to see visually what is going 
on.

For the mirror cell on the 15 inch the inside diameter 
of the mirror box is 16.5". If the center of the mirror 
box and center of mirror location point are the same, 
then that leaves 8.25" from the center of the mirror to 
the top inside dimension of the mirror box. The mirror 
radius will take up 7.5" of the 8.25". We now are left 
with .75". The top protecting pin is said to be 1" 
diameter plus the 1/8" gap between the mirror and pin 
now adds up to 1.125" which is greater then the .75" 
remaining. I am missing something here? 

I scaled the 15 inch to my current 12.5" and concluded 
that my inside dimensions of the mirror box would be 
13.75" but I was running into the same calculation 
problems as the 15" example I just described.

For the upper cage, Kriege recommends the length of the 
struts be 60% of the mirror diameter. For my 12.5" that 
would be 7.5 inches. I will be ordering a Astrosystems 
secondary holder and spider and I know the length of 
this assembly is 7.5 inches. The uppercage rings (wood 
thickness) will add another inch to the strut length of  
7.5 inches but that only leaves 1 inch longer length 
then  the secondary/spider assembly. Should I increase 
the strut length to 9 inches like the 15inch model has?

My last question regards to the ratio of the mirror box 
inside dimensions vs the upper cage dimensions. 
According to Newt my upper cage ID should be around 
14inches (about 1/4 inch larger then what Kriege 
recommends but I have read the archives and saw other 
people had the same problem with front aperture 
vignetting and learned the why, the archives are a great 
resource...kudos to the organizers). I plan on mounting 
my truss blocks on the outside of the box as Kriege 
recommends. Kriege also says that typically the mirror 
box dimensions are 5% larger then the upper cage. Since 
my cage is 14" ID should I just scale the mirror box 
from that dimension? I guess that answer depends on my 
previous question about the cell.

Thanks for your help.

--Shannon Hughes