Re: ATM Great focusers...

["Dave Pitou" <dpitou@ix.netcom.com>]

Greg,
        I haven't seen any plans for sled focusers, but they aren't hard to
design. The basic concept is that a plate with the eyepiece moves along the
tube, not at right angles to it. Given this concept you have to figure out:
1)  A way to guide the plate so that it moves straight and smooth. I use 2
steel rods and 4 linear bearings, so the plate slides along the rods. These
are normal items for a machine supply house along with W.M.Berg or Stock
Drive both in NY, or surplus.
2. A way to move the plate. I've used a friction drive on one of the rods,
but a threaded rod parallel to the traverse rods works better, and can be
motorized. A friend also used a rack and pinion which worked great.
3. A way to mount the diagonal to the focus plate. I made my focus plate
wide so that the two spider vanes would come in at a significant angle. The
plate is cut at this angle and the vanes are screwed to it. Where the vanes
converge I made a block with a 3/8" hole in it to hold a Novak diagonal
holder. The vanes need to be thicker than with a standard tensioned spider,
but 1/16" steel should be fine for any diagonal under 3".
        For all this work you get unlimited focus travel, (mine has 8"), no
rocking or backlash (depending on how you move the plate), and the eyepiece
can be set so it's just barely out of the primary light path, so it's as
low as it can be, hence the smallest possible diagonal.
        They are ideal for fast Newtonians, but one of the nicest I've used was on
a 12.5" F8 which was optimized for planetary work. The diagonal size was
1.25", and he still had a good size fully illuminated field!

dp2


----------
> From: Greg Furtman <gfurtman@spacestar.net>
> To: Dave Pitou <dpitou@ix.netcom.com>; Anderson, Jay
<andeja@mail.va.gov>; 'SMTP:atm@shore.net'
> Subject: Re: ATM Great focusers...
> Date: Thursday, July 17, 1997 5:23 PM
> 
> At 08:02 PM 7/16/97 -0700, Dave Pitou wrote:
> >For fast newts sled focusers work great. They are kind of a pain to
make,
> >but you get any amount of travel you want and they always stay low to
the
> >tube. A downside is you need to design a thicker spider to minimize
> >diagonal flexure, so diffraction can be higher depending...
> 
> Dave,
>       Do you have any plans for building a sled focuser or know of any web
pages
> or magazines that would have some designs?
> Greg Furtman
> Furtman Woodwork & Design
> 27426 County Rd H
> Webster, WI  54893
> (715) 635-7928
> gfurtman@spacestar.net
> 
> Custom woodworking, including cabinets, furniture,millwork, & telescope
parts.