From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Matthew L. Brown" 
>
>
>I seem to recall his focal length was fairly short, around 40 inches, for
>which 100 yards would be adequate on eyepiece placement.  The problem with
>using the sky is you are doing things in the dark.  Plus I found it far
>easier to set this up with the tube horizontal, to make it easier to move
>the main mirror around.  In the end, however, I think I checked my setup
>against a star on the horizon before I drilled.
>
>=Matt
>


Hi
 First, I think my ROC method is the simplest but with just
a little math ( very little ) one can focus the telescope on any close
object and then calculate the right location for the infinite location.
 He already knows the approximate focal length and that simplifies
things a lot. You just use the thin lens formula ( every ATM'r should know
this formula ).

 1/f = 1/a + 1/b
  f = focal length
    a = image distance
    b = object distance


 Lets say he pointed the telescope across the street at the neighbors
window( no one was home or taking a shower ). He knows that the window is
275 feet. He also know that his focal length it about 40 inches ( I don't
know what it is suppose to be ). We just plug these numbers in:

  1/40 = 1/a + 1/275*12

solve for a = 40.49 inches.

This means that the location that the eyepiece is at for the window, across
the street should be 0.49 inches farther out than it would be for a star.
Since we didn't really know what the real focal length was, one might ask
how much error is there if it were of by say 15%.

 1/40*8.85 = 1/a + 1/275*12

solving for a = 34.35  where f was 40*0.85 = 34

 So, even if the real focal length was off by 15% shorter than
the estimated focal length, we were only 0.15 inches wrong on our placement
of the stars focal posistion. This is usually within the range of the
focuser.

 Still, the ROC method is quite simple and only requires
a tape measure, flash light and a piece of paper. You can even use the
piece of paper to divide by 2. Dwight

--- BBBS/NT v4.00 MP