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telescope making manual - uploaded by Corona-o.pdf |
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Size 0.7Mb Date Jan 10, 2006 |
How much is this going to cost? Well... an 8" f/7 mirror from Coulter with diagonal costs $219.95, plus
$15.00 for shipping. If you can build an 8" scope for twice the cost of this, consider yourself lucky! You can buy a "Big Three" scope for only a little more; you can also buy used for less... Something else must be motivating you to "build your own." This Webpage is for you... and your daughter and/or son!...
The telescope tube should be about 2 inches wider in diameter than your objective: A ten-inch diameter objective mirror requires a twelve-inch diameter tube. An eight-inch diameter objective mirror requires a ten-inch diameter tube. A six-inch diameter objective mirror requires an eight inch diameter tube....
Refer to the above drawing and plywood "cut pattern" throughout the following step-by-step
instructions. Label all pieces (in pencil, chalk, or crayon) as they are cut! We will build our telescope from the "inside out," beginning with the tube....
Note: Cut the telescope tube the same length as the focal length of your mirror. Then cut the eyepiece hole back from the front end of the telescope tube by the radius of the tube. That is, for a 10" diameter tube, cut the eyepiece hole 5" from the front end; for a 12" diameter tube, cut the eyepiece hole 6" from the front end. These distances are for mirrors about 1" thick. If you have a thick mirror (i. e. 2"+) the hole should be moved up toward the front end of the tube an extra 1" to compensate. (i. e., a 12" tube with a 2" thick mirror would put the hole 5" from the front end; a 10" tube with a 2" thick mirror would put the hole 4" from the front end. A more important Note: Now, the above note ONLY works if you make the homemade focuser AND follow all the details of tailgate construction, use Sonotube, etc in these plans. If you don't, you will have to do some simple arithmetic. Folks: it is extremely easy to drill your focuser hole in the wrong place! Please read the following email and my response:...
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A furring nail (or rubber furniture glide), is hammered into each block before the blocks are screwed into the telescope tube. The furring nails prevent the mirror from rolling out the front end of the telescope tube. To determine the placement of the furring nail, allow room on the block for the width of the mirror, plus an extra 3/4" for clearance. Please note: the drawing at left is for bottom blocks (1" high); for top blocks, they should be turned 90-degrees and only be 3/4" high before nailing furring nails or rubber furniture glides on....
Preparing to screw the mounting blocks into the tube. Pilot holes for each of the screws may be made in the main tube 3" from the rear end....
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Cradle Boards (Part B)
BALANCING THE TUBE 1) To balance the tube we will need to install the primary mirror, the spider with the diagonal mirror in the telescope tube, an eyepiece, as well as any 1X finder or normal finder that you plan on using with your telescope. Remember: HANDLE MIRRORS WITH CAUTION!!! 2) Slide the telescope tube into the Tube Box. The fit of the tube should be snug in the Tube Box. If the fit is much too loose, a piece of masonite or thin plywood, or shim shingles (the kind you used for the spider) may be glued inside the Tube Box to tighten the fit as needed. 3) Slide the Tube Box along the tube to the spot where the weight of the telescope is balanced at the middle of the Tube Box. Use a broom stick handle to aid you in finding this balance point. Knowing this "balance point" will aid you in determining where (how high) to attach your Cradle Boards....
The way to get the alignment perfect is by fiddling with the position of the spider in the tube. When you have it just right, you can glue the spider in place. Apply a line of glue on either side of each shingle where it contacts the tube--100% black silicone glue works well here. (Note: Before installing the spider you may wish to screw a small eye-hook dead-center into the 90 degree cut end of the wood block that supports the diagonal. After the spider is installed, another eye-hook may be screwed into the telescope tube, and a string may be tied between the two eye-hooks. This will protect the objective mirror if the spider is accidentally knocked out)....
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If we moved the point source slightly to one side the image would move slightly to the opposite side and could be seen by placing a screen at that point. Below is shown the same mirror (B) but with parallel light rays coming to the mirror surface from distant objects - stars, planets etc. In this case the light rays meet the mirror surface at differing angles from edge to edge of the mirror. They are bent further inward and form an image at what is termed the FOCUS of the mirror. Once the source of incoming rays is a few miles away - the rays are effectively parallel, and they will meet at the 'focus' of the mirror. This point will be a point halfway between the mirror and the center of curvature of the mirror's surface....
around. The cleats should be positioned close to the disc, at 3 points close to but not quite touching it, so that we can turn it as necessary. For an 8" disc we would apply about a half to one teaspoonful of #80 grit and sprinkle it around the surface. (NOTE: for 4" 5" or 6" discs we would recommend using #120 grit for the rough grinding -this of course works less quickly, but you need a certain minimum number of strokes in the rough-grinding process to ensure that the 'spherical figure' is obtained) Then apply a small amount of water and spread the mix with our finger to make a reasonably good mix of water and grit. If the mix is too dry, then the discs will tend to bind and be hard to move - if so - we add a little more water and try again. If it is too wet the grit will wash out over the edges of the tool . With a little practice you will soon get to know the right amount to use. When grinding - if the mixture is good you will be able to tell by the loud grating noise as you rub the discs together. If you have to work on a bench which does not allow you to walk around - then to work around the mirror evenly you need to turn both the mirror disc and the tool disc about a quarter turn (but not exactly a quarter turn each time) so that you will grind across a different chord of each. You will in effect "walk around the tool" by turning it - but you will make your strokes across the bench in the same direction all the time. When you turn the discs try to do this methodically. Center them first, then turn both discs together, now turn the top disc only. This way you will have turned each of them about a quarter turn. Then you off-set the top one and continue with the next set of strokes. (Remember to vary this 'quarter turn' - as we want this turning to average out so that both discs become ground out evenly across their whole surfaces.) We continue with the off-set stroke constantly working our way around the mirror . The mix dries out after just a few minutes of work - ( 2 to 4 minutes) and the loud grinding sound dies away. The discs get...
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