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DIY PROJECTS

1. MIRROR MAKING
I am not going to go into detail here about making telescope mirrors. There are various websites which do, and there are a number of good books.  My favourite is The Handbook of Telescope Making by N.E. Howard. Then there is the How to Make a Telescope by Jean Texerau. Both of these describe the construction of 8 inch mirrors. Beginners are often recommended to start with a 6 inch mirror but an 8 inch is not really any more difficult to make and will only cost a little more to complete.  The aditional light grasp though is significant. A good treatment on a 6 inch F10 is Muirdens' Beginners Guide to Making a Telescope.
 
The purpose of this section is to summarise the procedure and perhaps help you decide whether or not to try it.
 
STAGE 1: ROUGH GRINDING. Two glass [or pyrex/borosilicate] disks are required, one called the blank [which will be the finished mirror] and the other is the tool. Your blank must be ground and polished to a precise paraboloidal curve. Depending on the size and focal length your mirrors' curve will have a specific depth of curve. The tool should be fixed to a firm surface. A coarse aluminium oxide  abrasive and water is applied, then the mirror blank is lowered onto the tool. Then a simple back and forth motion of the mirror is applied; both disks should be rotated every 6 strokes or so.Also you take a step around the work. Due to differential wear the mirror blank will become concave whilst the tool becomes convex. The depth of curve on the mirror is checked periodically by holding a straight edge across the mirror and an object equal in size to the depth is placed under this until the correct depth is attained.
 
STAGE 2:. FINE GRINDING. Ultimately the mirror must be polished to an optical finish. This cannot be done from rough grinding. Therefore a seuence of progressively finer abrasives are used to smooth the mirror ready for polishing. The mirror should be checked with a magnifier to ensure the smooth is progressing. It is also very important to clean up thoroughly between abrasives to avoid contamination. The same simple back and forth stroke can be used.
 
STAGE 3: POLISHING. The mirror is polished against a surface of pitch. This needs to be applied to the tool blank. The pitch is melted and then poured on the tool. While the pitch is still soft, the mirror should be smeared with polishing compound [rouge] and then pressed against the pitch. It is very important that the pitch surface corresponds to the mirror curve. A sharp blade is then used to form square facets n the pitch. These are typically about 1 inch square. When the tool is ready polishing can begin. When polishing a different stroke is required. This is the so called W stroke. The mirror should be rubbed against the tool, having applied rouge, in a W pattern with about 1/3 overhang. The mirror should be regularly checked with a magnifier to monitor progress of polishing. Disks should be rotated every 6 strokes or so.
 
STAGE 4: FIGURING. Now this is where things get tricky. The curve on the mirror must match certain calculations. The most common method for doing this is the Foucault test. A so called knife edge tester is used to measure the figure on the mirror compared to the calculated numbers.
 

2. MIRROR TESTING
There are a variety of ways to test a telescpe mirror. The best are termed star tests which involve testing the mirror under real observing conditions. One of these is called the Ronchi test. This requires a Ronchi grating; this is a slide which has a grating [of around 100lines/inch] printed or formed on it. They can homemade [see web] or commercially purchased. Whilst observing the grating is placed between the eye and focuser and the image defocused A pattern of lines then becomes visible. See below.
The images at left show what you might see at your scope with a Ronchi grating. What you want to see is A. These straight lines show a well corrected mirror. B and C show over and undercorrection of the whole mirror. D is much worse. The overall figure seems ok but the hooks on the lines idicate turned down edge - a serious problem. E and F show raised and depressed regions respectively
2. FINDER BRACKET
The image below shows the construction of a simple and cheap finder bracket. It uses easy to obtain DIY store parts.
3. Unit Power Finder
The pic above shows the construction of a simple unit power finder. These work very well. Two nesting tubes needed. The inner tube carries a red LED mounted on a card disk. The small rectangle shows a resistor which is required for the LED, to prevent blowing. The pinhole is formed in a piece of tin foil using a new needle. The long lead on an LED goes to the positive side of a battery, and the shorter lead goes to negative. The outer tube carries a small biconvex lens which collimates the light beam. 
4. GRP MOULDING.
Glass Reinforced Plastic [GRP or fibreglass] is a very useful material for telescope making. To make mouldings a mould is required. This can be done in a few ways. I have discovered that moulds can be made by carving the mould into a 12mm sheet of MDF  using a router.
The GRP is readily available from Halfords.
The pic at left shows moulds created by measuring out the part on 12mm MDF and then routing out to a depth of 6mm. These are parts for the tube base, mirror cell and tube top ring for an 8in Newtonian. Vaseline can be applied as a release agent.
To the right is a pic showing the mould with GRP inserted. This is the base of the mirror cell.
The image below shows the first stage of clean up. a rotary sander was used to smooth the exposed back of the part.
Next the mould is removed by cutting and breaking off. See left.
The pic below shows the part removed from the  mould
To the left is the completed cell using the GRP part. It's been painted matt black. The long screws are for mirror adjustment, and there are tabs at the edge to hold the mirror in place.
Here is the base ring of the scope. Also shown is the tube ring for the top of the tube.
5. MAKING A FOCUSER
Focusers can be made in a variety of ways. The rack and pinion which uses gears and the Crayford which runs on a pair of bearings and a rod. The latter is easier to make. It is possible to to make a Crayford without any special tools. Mecanno or siimilar can be used to good effect.
If you do have a lathe and some basic machining skills it is possible to make custom made good ones. I built a Crayford made on a lathe. It is a bit unusual in that spur gears are used from the motor. It worked well. The drawer tube was made by boring and turning a piece of 2 inch round bar. Another piece was was formed with a bigger radius and then cut out of the cylinder, used as the pillar to carry the bearings. These very small bearings come out of junked computer drives.  The handset is single speed and runs as a fine focusing aid. The motor was a very high quality DC running at 5V and having a built on gearbox. This motor was obtained surplus at a very good price. It held all the eyepieces I had. 
The Crayford has enjoyed great success and is widely available commercially. However it can suffer from tensioning problems. The tension needs to be adequate for large eyepieces and even cameras; but the tension needs to be loose enough to move.  The geared rack and pinion is much better in terms of load carrying capacity due to friction of the gears.