From: "Jim Miller" 
To: ,
        
Reply-To: "Jim Miller" 


and if you are near sighted make sure to test the infocus without your
glasses on to make sure you can reach focus with all the eyepieces in that
condition as well.

almost made that mistake myself!

jtm

----- Original Message -----
From: 
To: 
Sent: Thursday, March 06, 2003 10:15 AM Subject: FW: ATM focuser placement
calculations and tube lenght



Kevin,
 I typed your numbers into Newt and didn't get the obstruction. I assumed
your tube ID was 8.75, and tube thickness 0.125, and added 0.25 extra
travel on the focusser.  Tube length came out as 31.75.

Otherwise, your calculations look about right.

1. Don't worry too much about getting it perfect; mount the focusser and
the diagonal first, and just prop the mirror cell in the back in about the
right position.  Check out focus using all of your eyepieces.  Move the
cell toward or away from the diagonal until the focussing range works.  If
you care about terrestrial viewing, test it on the closest range you want
to view (closer objects need the eyepiece farther out).  And don't forget
any cameras you intend to use.  Anyway, once you have the objective in a
good position, then drill and mount the cell.

If you are really unsure, leave the tube length overly long in the back
(say 3"), mount the objective as above, and cut off the excess.  Keep
enough tube back there so your objective cell hardware doesn't get hit if
the tube is placed that end down, say 1" beyond the hardware.

Oh, I just remembered that Berry likes mirror cells that mount to the back
of the tube, not the inside.  The disadvantage to this is that you can't
move the cell very far toward/away from the diagonal to get the position
right.


2. Add more tube length to the front to help minimize dewing.  Your limits
are vignetting and what becomes too awkward in length or weight.

3. Don't fret too much about getting 100% edge illumination across the
entire field if you intend to use this primarily as a visual instrument.
Most people can't even detect 70% illumination.

4. Pay attention to prepping the inside of the tube to eliminate stray
light.  It's easier to do before you've the optics mounted, and you won't
get dust on them.  There are lots of ways to do it: baffles, textured flat
black paint, velvet, flocking paper. It'll help your contrast.  After you
are done cutting and painting, clean out the inside of the tube really well
-- optics collect dust.

=Matt


Original Message:
-----------------
From: Kevin H Williams Kevinw99{at}ezwv.com
Date: Thu, 6 Mar 2003 09:14:38 -0500
To: atm{at}shore.net
Subject: ATM focuser placement calculations and tube lenght



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




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