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Amateur Telescope Making
Here is some advice on making your own telescope
For a good explanation of telescope basics go here or to Celestron International
Tips
for Building a Newtonian
Telescope
Newtonian Telescope Kit
Building A Pier
The easiest telescope design for the amateur to make is the Newtonian. This basically consists of a large primary mirror for collecting the image at one end and a small secondary mirror at the other end that directs the image to the eyepiece for viewing. The primary mirror is held in place by an adjustable frame attached to the telescope tube called the primary cell. The secondary is held in place by another adjustable frame called the spider. A hole is cut into the telescope tube above the secondary for a focuser. The focuser holds the eyepiece, which magnifies the image. Most amateurs use high quality Plossl eyepieces. There are three types of focuser, rack and pinion, crayford and helical focusers, the choice depends on what the telescope will be used for. The rack and pinion is suitable for general use. The rack and pinion allow the greatest amount of adjustment and work well with all types of telescopes, but does not allow fine adjustment or support heavy weights. There is usually some image shift involved with rack and pinion focusers, due to the coarse nature of their adjustment. This can be overcome by using an additional helical focuser for fine adjustment. The crayford is the strongest, most stable focuser for mounting cameras and other equipment. The crayford is very stable for attaching cameras and other equipment on larger telescopes. The helical focuser allows the finest focus adjustment with zero image shift and is also very stable for attaching cameras. It is widely used to replace rack and pinion focusers on SCT telescopes to illiminate image shift. The best choice is probably a crayford type focuser, providing very good adjustment capability with the ability to support heavy loads for additional equipment.
There are two types of mount for the telescope, the Equatorial mount has right ascension and declination scales for keeping track of the telescopes position and can also be adjusted to track the stars as they move. This mount is usually purchased; however there is another type of mount called the Dobsonian which can easily be made from plywood. Most amateurs will start with a Dobsonian mount.
Materials
The telescope tube is usually made from a spiral wound cardboard tube called a sonotube. You can also make your own tube from metal but generally, the best option is to purchase a sonotube from one of the telescope and equipment suppliers. If you are building a telescope for the first time you will want to buy all the pieces ready made and assemble them into the final product. There are two types of mirrors, soda lime glass or Pyrex. The only difference between soda lime glass and Pyrex during use is that glass takes longer to adjust to temperature changes, the image quality is the same for both. The mirrors need to be ground to a precision of 1/8 wave or less to produce a high quality image, avoid mirrors that have an accuracy of 1/6 wave. When buying mirrors buy the primary and secondary at the same time and specify a mirror set. Mirror holders, focusers and eyepieces are pretty much standard, just make sure you buy the correct ones. The mirror holders should be adjustable and must be the correct size to fit your mirrors and tube. Eyepieces for telescopes have either 1 1/4" or 2" diameters. The larger telescopes, above 8" should have 2" focusers and eyepieces. Again, most amateurs use high quality Plossl eyepieces. To find suppliers where you can purchase all the parts look in our Space and Planetary Science links section.
List of Astronomy Software you might find useful
Tips When Assembling Your Scope
If any of the mirrors attract dust or grease leave them until the scope is assembled. After the scope is assembled use a soft cloth and lightly clean the grease and dust. Use acetone or isopropyl alcohol as the cleaning agent. If there is only dust, remove the dust with a clean soft camera brush. Minor dust can be left as it will not affect the image much and is unavoidable once you are using the scope unless you have an airless vacuum chamber!!.
1. Drill the
focuser hole in the center of the tube and slightly larger than
the focuser
2. Attach the secondary mirror and spider first
3. Position the secondary mirror in the center of the focuser and
adjust the mirror so the hole at the end of the tube is centered
in the secondary mirror and focuser. You will need a
collimating eyepiece with crosshair to position the secondary in
the center of the focuser. You will also need to offset the
secondary mirror slightly to optimize the optics.
4. Measure the height of your focuser in its midway
position above the tube and add half the tube diameter to
determine the distance above the secondary mirror.
5. Subtract this distance from your primary mirrors focal length
to determine the distance from the primary mirror to the
secondary.
6. Attach the primary mirror to the mirror cell.
7. Measure the distance from the face of the primary to the screw
holes on the primary cell. Add this distance to the
distance calculated in 5. This is the distance your primary
mirror cell will be from the center of the focuser hole.
8. Measure the distance to the primary cell from the
focuser and mark the screw holes
9. Attach the primary cell and mirror to the tube
10. Test the scope by pointing at the Moon and focusing to
determine whether the primary is correctly positioned. The
image will not be centered as the scope is not yet
collimated. The image should come into focus when the
focuser is at its midway position. You will have to move
the position of the primary closer or further to the focuser if
you cannot focus the image correctly.
11. Collimate the scope.
Building Your Own Telescope Pier
A permanent mounting for a telescope is desirable in order to have a permanently polar aligned and assembled instrument ready for anything more complicated than simple observing. In most cases, a pier made from cast concrete is the best solution but where this not possible or desirable, here is alternative solution. Here is a pier system I have designed using readily available building materials. The result is a super strong, durable pier for your telescope. The pier is suitable for EQ5, EQ4, CG5 and SCT Fork Mounts (have not tested this).
The pier is based around 100x100mm posts which use standard post supports for attaching to the floor and telescope. Steel posts in 2-3mm thickness are ideal but timber is laso suitable if the pier is not to be very high. Use a 100x100mm square post support for the base. The support bolts onto the floor via 4 mounting holes on each corner. The posts simply bolt into the square post support. If the fit is less than perfect, adjust with wedges until a completely tight fitting is obtained. Use a 100mm concrete type post support for the top of the pier. This support has a center hole and a couple of off axis holes for 10mm bolts. Attach a 150mmx150mmx3mm steel plate to the top of the post support. Drill matching holes in the steel plate. The bottom bolt of the telescope equatorial mount goes through the center hole of the steel plate and post support. Preferably weld the steel plate to the post support or use a second countersunk bolt to fix the plate in position on top of the post support.
A series of 4mm holes should be drilled in a semi circular pattern to attach guide bolts. The guide bolts prevent the equatorial mount from swivelling around on its azimuth axis when the telescope is moved manually.
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A home made motorized scotch mount is here.
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Text and Images Copyright ©2004-2009 by Rob Kanen