RE: ATM Diagonal

[BOB PRABOWO <BOB@bahana.co.id>]

Hi Asaf,

>1. One said that 1.3" is the maximum, and a second said that 1.4" is.
I do not know the answer to that, but I guess you should get the
diagonal optimizing progaram. Go to Mel's page and you can download the
program there.

>2. I don't know where I should place the diagonal _in_ the "light Cone"
>
>(my tube diameter is 7" [well, it's not really a tube, because it's an 
alice-like telescope]).

I don't know if anybody has responded to this but here's how determine
where to place the focuser and the diagonal. The diagonal should be
placed at the center of the tube if you place your  mirror on the center
of the tube :-)

I don't really remember what the equation is in Berry's book but it'
something like the following. BTW, I send my respond to the list so if
I'm wrong someone can correct me.

Remember that the distance from the mirror to the eyepiece must equal
the focal length. So if your telescope's f.l is 48" than the distance
from the mirror to e.p must be 48". But you have to take the diagonal
into account. The diagonal is needed to direct the light from the mirror
to the eyepiece at the side of the tube.

So, the focal length  of the scope must equal the mirror to diagonal
distance + diagonal to eyepiece distance, or in equation: 
(1)  FL=m.d Distance+d.e Distance.

To determine the diagonal-eyepiece Distance you must consider the
diameter of the tube (radius of the tube actually), the height of the
focuser and the focuser travel, the equation is:
(2)  d.e Distance= d/2+H+T, 
where d is diameter of the tube, H is height and T is focuser travel.

With those two equation, we can calculate the d.e to determine the m.d. 
Let say your mirror is 6" f/8, the f.l =48", your tube d= 7" and your
focuser height=2", and assume the focuser travel is 1" (could be less).

d.e Distance= 7/2+2+1= 6.5". 
With this, determine the m-d distance:
m.d Distance = FL-d-e= 48-6.5= 41.5".

So, your focuser hole must be placed 41.5" from the surface of the
mirror. That's also where the diagonal should be place because the
diagaonal is right in front of the focuser.

In practice however, when determining the focuser hole's position you
must also consider the distance from the end of the telescope tube to
the surface of the mirror (the back end  of the tube to mirror surface).
You have to include this distance in your equation. 
So, you have to use another equation: 

(3) F = E+FL - (d/2+H+1) where
F = focuser hole position, 
E = end of tube to mirror surface distance
FL = focal length
d = diameter of the tube
H = focuser height, and 1= focuser travel.

Example:
Now, for your telescope, the E is, say, 4" the rest are the same as
above. Then we get:
F= 4+48-(7/2+2+1)
  = 45.5
The focuser (and also the diagonal) should be placed at a distance of
45.5" from the end of the telescope tube.
I hope the explanation is clear enough to be understood,  if I make
mistakes here please correct me guys.
I hope this helps Asaf,
Good luck

Bob P. Sumitro
Jakarta, Indonesia

True Victory is Self Victory!
Harmonize yourself with the
Divine Parent Mind -
salvation lives right within
your own body and soul.

-Morihei Ueshiba-



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