Re: ATM Making baffles out'a masonite??

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Ed,

>I didn't mean to start _ that _ thread again... 

Why not? ;-)

>Part of my rationale for installing baffles is to stiffen the tube.

>What information do you need? The tube is 18.25 ID, the mirror to focuser hole is 56.75
>inch, the focuser hole to front of tube is 8.75 inch. The diagonal is current 4 inches, 
>and
>I am baffling for a 1.75 degree FOV ...

the focal length = 71.5" so here we go:

1.75 deg FOV is 1.75*71.5/57.3=2.18" dia (won't quite fit a 2" focuser, but no doubt fine 
for  photo).

A line from one edge of the FOV (-1.09") through the other edge of the diagonal (2.0") at 
(71.5-56.75=14.75")will hit the tube wall at (9.125+1.09)*14.75/(2.0+1.09)=48.76" from 
focus or about 22.75 from the mirror center - say 22.5" in front of its edge. 

Light passing by one edge of the tube will hit the opposite edge of the mirror at an 
angle to the optical axis of arcsin(8+9.125)/(56.75+8.75)=15.2 deg. The normal to the 
mirror here is about 57.3*8/(2*71.5)=3.2 deg, so it will be reflected at an angle of 
15.2-6.4=8.8 deg, and hit the tube wall at 1.125/sin(8.8)=7.35" from the mirror edge.

The part of the tube that needs to be baffled is thus from 7.35" to 22.5" in front of the 
mirror edge - this area should be either hidden from sight or from being lit by reflected 
light from the main mirror.

A line from 7.35" in front of the mirror to the opposite edge of the field will have an 
angle arcsin((9.125+1.09)/(71.5-7.35))=9.2 deg. Here comes a tricky part: light passing 
the tube edge reflected to hit the opposite tube wall will hit it at something less than 
8.8 deg, but if we use 9.2 deg here we'll be on the safe side: the baffle should be 
halfway between 7.35 and 22.5 up=appr 15" from the mirror edge, and 
0.5*(22.5-7.35)*sin(9.2)=1.21" high. This is a bit too much for the 1.125"+ clearance, so 
you would prefer 2 baffles, a little more than 0.6" high and (7.35+15/4)=11.1" and 
(22.5-15/4)=18.75" from the mirror edge respectively - not an exact solution but should 
be close enough to make sense. 

(with a tube of 27.3" inner dia, you would not need baffles at all - if my calculations 
are correct. With a tube that reaches out to the center of curvature, you need no 
baffles, regardless of diameter!)
Nils Olof