Re: ATM Some help with "Alice" type design

["Bob Scholtz" <rscholtz01@sprynet.com>]

Johnny,

No I don't have an AutoCAD file viewer

The focuser board doesn't have to line up with the box; better from an
esthetic perspective but not crucial.  You just don't want the focuser board
or truss tubes in the light path.

Now, you don't say how wide the channel is; you don't want the truss tube to
bottom in the channel or the alignment when you draw the tube into the
channel won't be consistent.  For 1.25 in tubing I calculate the channel
width with a 1/4 in. depth to be 1 inch wide max.  This places the bottom of
the channel and the tube wall line to line.  I'd go with a 1/16 in to 3/32
in. clearance which reduces the channel width to 57/64 in.  Once you've done
this calculation and figured out the channel dimensions, then you can move
the lower channel around in the block to get the tube the right distance
from the mirror box wall.  You don't have to have the channel in the center
of the block.  If the side of the block gets a little thin on the mirror box
wall side so what; you have the box wall to reinforce it.

Also, you don't give the size of the mirror, but you don't want the truss
tubes to be in the light path so you may have to move the mirror off center
in the mirror box away from the truss tubes.  My 'Alice-like' scope has the
mirror off center for that very reason.  Given all this; if the focuser
board is not exactly even with the mirror box wall it really doesn't matter,
you just make up any difference in your diagonal holder. Finally, if the
truss tubes are very long, 1 1/4 in OD may not be stiff enough.  On my 10 in
f 5.6 scope the truss tubes are 49.5 inches long and 1 1/2 inches OD.  The
real 'Alice is an 8 in f 4.3; in this case the 1 1/4 inch tubes are fine
because the tubes are short.

Hope this helps,

Bob



----- Original Message -----
From: "Johnny" <takmais@swbell.net>
To: "Bob Scholtz" <rscholtz01@sprynet.com>
Sent: Sunday, July 22, 2001 9:24 PM
Subject: Re: ATM Some help with "Alice" type design


> do you have an autocad file viewer?  itd be easier for me to draw this.
but
> i'll try to refrase the question.
>
> i'll start with a few givens:
> -we want to keep the plane of the focuser board lined up with the plane of
> the mirror box correct?
>
> -the overall thickness of the channels is 1/2" for both top and bottom
> channels.
>
> -the width is 1.75" in my case
>
> -the placement of the bottom channels is limited in its movement in the
> direction of the mirror box wall, but unrestricted in the opposite
> direction.
>
> -tube diameter is 1.25"
>
> -channel is 1/4" deep.
>
> ok, if the channel is cut 1/4" deep then the truss tube centerline will be
> 1/2"(focuser board thickness) + 1/4"(channel thickness) + 5/8"(radius of
> tube) giving 1 3/8" from the outside face of focuser board to the tube
> centerline.
>
> now, the placement of the bottom channels must put the centerline of the
> tube at 1 3/8" from the side of the mirror box.  if the width of the
channel
> blocks was 2" then we know that we've got 1/2" for the mirror box
thickness
> + 1" from the edge of the truss channel block to the centerline of the
truss
> tube.  that gives a distance of 1 1/2" from the outside face of the mirror
> box plane(the one thats even with the focuser board plane) and the
> centerline of the truss tube.  thats 1/8" too far and the only i can see
to
> compensate for this would be to put an 1/8" shim of some sort under the
> truss channel block on the focuser board.  This is what i ment when i said
> there was a relationship between the width of the channel block and its
> thickness.  if one used a different width channel block, say 1.75" wide,
> then we would still have the same 1 3/8" from the outside face of the
> focuser board to the centerline of the truss tube, but we'd then have
> 1/2"(mirror box thickness) + 5/8" (edge of channel block to centerline of
> truss tube giving only 1 1/8" which puts us 1/4" to far in.  this would be
> easier to correct though because one would simply move the channel 1/4"
away
> from the side of the mirror box.  So you see my predicament?  there seems
to
> be the need for some exact ratio of thickness to width that would be
needed
> to make this work perfectly.
>