From: "Kevin H Williams" 
To: 
Reply-To: "Kevin H Williams" 


I am building my first telescope

6" f/5
diagonal with a minor axis of 1.6"

I was hoping the list participants could review my calculations before I
make any cuts

from barry's "build your own telescope" I get this formula, all
length are in inchs

(distance from diagonal to front of mirror) = (focal length) - (diagonal to
focal plane distance)

diagonal to focal plane distance = (collapased focuser height +1) + (1/2
diameter of tube)
with my numbers = 3 + 1 + 4.5 = 8.5

focal length = (f/x * mirror diameter) with my numbers focal length = 5*6 = 30

(distance from diagonal to front of mirror) = (focal length) - (diagonal to
focal plane distance)
with my numbers = 30 - 8.5 = 21.5

so I get distance from the front of the mirror to the center of the diagonal of 21.5

now I need to make a decision about tube length, if I want to put the
mirror cell 1 inch from the back end of the tube, and the mirror cell is
2.5 inchs long

the distance from the back of the tube to the front of mirror is 3.5 I want
the distance from the focuser to the front of the tube to be 6.00

so tube length is (distance from front of the mirror to the center of the
diagonal) + (distance from the back of the tube to the front of the mirror)
+ (distance from the center of the diagonal to the front of the tube)

so I get tube length to be 21.5 + 3.5 + 6 = 29 inchs

any reason to back the tube shorter or longer? would it hurt to make it longer?

when I plug this into Newt 2.5, I get a YES on "Diagonal to small to
admit 100% ray" - how bad is this, and what are the ways to fix this?

hope this is not too confusing, I have searched the internet for software
to do these calculations, but could not find anything, it probably sounds
simple to someone more experienced, but not to me, and I want to do it
right the first time

thanks for any help
kevin

--- BBBS/NT v4.00 MP