[ATM] RE: Questions from a new Dob Builder Was: Hello list,some basic questions for a new Dobsonian builder

[ntent <ntent@pctcnet.net>]

Hello, Mike:
    I'm not sure what you mean by "electronic form," but I can only 
guess you mean pictures of some sort; if not, then the answer is no.... 
I only have pics I made with Xara. 
    The idea is simple, and uses a movable primary cell for the 
collimation adjustment which, to many, is a good idea, and allows for 
some error in drilling of the holes used to mount the spider.)
    I used a 2 inch long piece of 1 1/4" Dia. hardwood dowel for the 
secondary holder, which has been cut at an accurate 45 degree angle on 
one end, to which the secondary will be affixed. I made a circle 3/16" 
inch from the outside edge, and drilled 3/16" Dia. holes 120 degrees 
apart, all the way through the dowel the edge of each hole touching the 
circle I drew, which gives you a good wall thickness to work with. Then, 
along the long axis of the dowel, slots are cut all the way through into 
the holes.
    I used three pieces of 3/4" metal banding, one end of which was 
soldered (I brazed mine, actually) into what is called a "split pin," 
which is a semi-hardened fastening pin, split along the long axis; I 
used  1" X 3/16" Dia. pins. The three pieces of metal banding are cut a 
bit long, but the same length, to allow for later accurate cutting to 
length.  With the dowel standing flat side down on a piece of wax paper, 
the pin end of each band is centered lengthwise in the holes in the 
dowel, each band has a spacer under it's length to hold the assembly 
"flat," and the outer ends of the bands are splayed/spread to close to 
120 degrees apart.  I then mixed some epoxy and dripped it into the 
holes to keep the pins/bands from moving, and also gives strength to the 
holes/slots. In practice, once the epoxy has cured, I turned the 
assembly over and put epoxy into the other end.
    The following was done using a 3 inch wide piece of the same tube 
used for the OTA....
with the 5/16" holes drilled 1/2 inch from one end..... I used 5/16" 
fine thread bolts, the heads of which were cut off, and the end of each 
"bolt" is slotted 1/4" deep with a hacksaw. I then drilled holes 120 
degrees apart as close as possible to the correct location for the 
spider.  I inserted the bolts using two nuts each...... one 1/8" below 
the slot, with a flat washer on the tube side (the inside of the tube 
piece) and placed into the holes, and one nut on the outside of the tube 
piece, and snugged the nuts down to keep them from moving. For what it's 
worth, I used "buddy nuts" (which are half the thickness of regular 
nuts.) Then the spider is rested on the edge of the scrap tube piece, 
directly above the slot in the bolt heads, with the secondary mount 
centered as accurately as possible. The ends of the bands are then 
marked to the slot depth of each bolt, marked 1,2,and 3 (as are the 
bolts,) the bolts are then removed, and the ends are soldered/brazed 
into the bolt slots. If you use the soldering method, there is a 
low-temp silver solder available, which is much stronger than regular 
solder.
    I hope this is clear to you, and anyone else thinking of using this. 
If you have any questions, please post, or off-list... either one.

You wrote:

Hi John,

That would be great - do you have the drawings in electronic form?
Thanks,

Mike

At 11:25 AM 6/6/2005, you wrote:
Hi, Mike..... hope you don't mind me writing off-list.  I have used a 
simple idea for making the spider that I can send you the drawings of if 
you'd like.... just get back to me and I'll find them and send 
them.....     John

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