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telescope making manual - uploaded by Corona-o.pdf |
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Size 0.7Mb Date Jan 10, 2006 |
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....
. The diameter of the telescope tube should be about 2" larger in diameter than the diameter of the objective mirror The plastic liner may be carefully peeled out of the inside of the tube.Slow, careful peeling helps keep the liner in one piece and makes it easier to remove....
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Sawing off the corners of the tailgate at the pencil marks. Now the tailgate should fit inside the telescope tube. (Plane or sand to fit if necessary.)...
The mirror mounting blocks are designated PART F in the plywood cutting plans. The mounting blocks are screwed in place inside the telescope tube. The mirror sits on the two bottom blocks and should just clear the top blocks under no circumstances should the mirror be pinched or squeezed between the blocks. If the blocks are cut from 3/4" plywood and are 1" wide (they should be about 4" long), we will probably have to place two blocks with the 3/4" side "up" and two blocks with the 1" side "up" in order for the mirror to fit nicely (see diagram above)....
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the design for a "Binocular Box" which is easy to make from odds and ends, and can make your viewing much easier. In the U.S. and Canada there are magazines : "Sky and Telescope," "Astronomy," and (in Canada) "Sky News" which contain advertisements for supplies and accessories, such as eyepieces, focus-mounts, mirror-kits etc. Willman-Bell, publishers, also supply books, mirror-kits and other materials of interest for amateur astronomers....
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....
As the depth of curve in the average 6 or 8 inch mirror is usually less than 1/10", these tools work fine...
(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...
SMOOTHING the MIRROR:
The grit sizes supplied with most kits are #80, #120, #220, #320, #500, then 12, or 15-micron, 5, or 8micron. The last of these are often Aluminum Oxide rather than the Carborundun or Emery grits. The numbered grits are finer as the numbers increase and the micron sizes are finer as the micron-size decreases. These should be kept in separate containers and , if stored together - should always be kept with the finer grits above the coarser ones if they are ever stacked one above the other...
Now you are ready to start again - but this time with the next finer grit size (220) You will continue this way, until you finally reach the finest grit size. When you finish with the 220 size grit, re-check your focus and use it with either the tool on top (to increase the focal-length) or with the mirror on top (to reduce the focal length) After this step - you continue with the finer grits - changing the position of the tool and mirror every fifteen minutes or so, during all the next finer grit sizes to preserve the curvature of the mirror's surface. The time required to remove the pits left by the previous grit will vary, usually becoming less time needed as the grit sizes decrease. For the 220 grit you may need less than a couple of hours work, and as the grit size reduces the time spent will become even less. The best guide though - is to examine the surface very...
A few additional notes:
The pitch should be heated slowly and not made too hot or it will lose some of the natural solvents it contains. A fairly thick creamy consistency is best - fluid enough to pour easily but not too 'runny' Be sure to have the mold ready before the pitch is melted and ready to pour. The sooner it is poured and cooled the less solvents will be lost. The last time I made a pitch lap - I cast a 'block' of pitch, This was made a little softer than the formula above (I had a deeper mirror to polish) I added about 1 ounce of beeswax and about 8 to 9 milliliters of linseed oil to 1 lb. of pitch. The 'block' I cast was about 10" long by 4-1/2" wide and 5/16" thick. This was heated in a tub of water at 95 degrees F. for about 5 minutes. It was then possible to cut 3 or 4 strips 3/4" wide x 4-1/2" across before re-warming the block again to cut more strips. Afterwards the strips were cut into squares (3/4" x 3/4") in both cases I did this with a cold knife, without the pitch breaking. This seemed to me to save a lot of time. The squares were then fixed to the tool in the normal way: before applying the squares the tool was coated with a very thin layer of turpentine, then the squares were heated, one by one, with a candle flame untill the bottom of the square was 'wet' and then they were applied to the tool and gently pressed to ensure good adhesion. Following this the whole tool complete with the new pitch squares in place was heated in the 95 deg. water for a few minutes. Then with a layer of Aluminum foil placed on top of the mirror, the warmed tool (or lap) was pressed firmly on top until the squares were all evenly pressed to the shape of the mirror this needs to be done carefully - so that the squares don't get compressed too much and run together. It is better to do it two or three times - applying more gentle pressure each time until they appear properly pressed. Wetting the back of the tool also allows one to see the squares underneath, so that the degree of pressing of the squares can be seen. When you are sure that things are o.k. lift off the tool and gently peel off the Aluminum foil. The pattern on the foil may also give a good indication that the tool is correctly pressed. After this 'cold pressing' can be done. (see below) Before the next step - a word about polish.In most mirror-kits the polish supplied is Cerium Oxide. This is a very fine pinkish colored powder which polishes faster than the Optical Rouge which was used in the past. Rouge although the best of all polishes is extremely messy and stains everything it contacts. Very few people use it today. An alternative, which is somewhere between the polishing speed of rouge and Cerium Oxide is a product called Zirconium Oxide. It is available from many suppliers of kits and is a clean white powder - and is the polish we recommend....
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