The optics do look in pretty good condition for their age.
Quote:
Originally Posted by Zed
How do I measure the height of the focuser ?
I guess it has to be measured from the tube to the top of it, but fully extended or fully collapsed ? I'm asking just for future reference, just in case I decide that it is worth improving.
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Eric made a good comment about considering cost, its a one I often make myself to people considering building scopes. But if you would like to upgrade here is what you need to measure and think about.
Rack the focuser out until it is about half way, perhaps to where the 22mm eyepiece comes into focus. Without the eyepiece in it, measure from the side of the tube to the top of the focuser. That will be the height of the current focuser. But if you replace it the new one will most likely be a different height, and you then need to take into account the focal length of the telescope - a change in focuser will probably require a new tube to be made as well if the current tube is not long enough.
I mentioned about measuring focal length before; to be sure about the length try this (this is also what you need to measure up with the new focuser and tube if you go that way). I wrote this some time ago when I was trying to put some notes on telescope making together.
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1. Know the focal length of your mirror (f).
2. Measure the average height of your focuser.
Rack your focuser out about half way and measure from where it sits on the tube to the top of the draw-tube. This is (h).
3. Measure the diameter of the tube and halve it to get (r). If the tube is quite thick, add the tube thickness to r as well.
4. Put the primary mirror in its cell and hold it against the outside of the tube, marking where the top edge of the mirror sits. Make sure all bolts and knobs etc are contained within the tube. At this stage you may also like to mark a ring around the tube where holes for the cell are to be drilled.
Alternatively, sit the cell, with the mirror in it, on a table and accurately measure the height to the top edge of the mirror. Mark this distance on the outside of the tube.
5. You can now calculate the distance (d) for the focuser placement
d = f - (r + h)
Once you have d, you can measure from the mark you made on the outside of the tube for the top mirror edge to where the centre of the focuser should be. Mark this spot, then drill or cut your hole out.
6. Once you have your focuser placement, you can look at placement for the spider. You want to leave enough room between the spider and the secondary mirror cell for adjustment and collimation purposes. In other words, don't put the spider too close to the focuser (but don't put it too far away either).
7. Note- You do not have to be super-precise! The travel in the focuser will compensate for small errors in measurement. This is one reason you must remember to extend it when measuring h.
Also, make sure you place the primary mirror cell and the spider perpendicularly in the tube. This will make final collimation of the telescope much easier to achieve
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Hope that helps.
