Cutting a scope and maintaining alignment?

[colinsk]

While cutting a tube sounds easy, maintaining alignment and re-collimating a telescope seems a little challenging to a non-metal worker. Can anyone share some tips and tricks to make it easier for those of us who have not done it?

Personally I would remove the objective and cut the tube in a lathe and re-collimate. That is not what successful modders are doing so I would like to hear of what solutions people have come with?

[Merlin66]

Cutting a donor OTA is straight forward...
1. Check and check again your measurements are correct...
2. Remove the focuser
3. Have a look inside the OTA and make sure there are no baffles which will be affected
4.Stuff a clean towel inside the OTA to prevent filings etc getting inside the tube.
5. Use a long strip of wide card, wrap it around the tube at your mark and then apply a strip of masking tape around the outer surface of the tube following the edge of the strip. This will ensure the cut is reasonably square.
6. Remove the card strip and double check the cut length
7. Use a fine bladed hacksaw to slowly cut through the tube ( they are usually only a mm or so thick)along the edge of the masking tape.
8. Check the final length.
9. Clean up the cut edge gently with a fine file.
10. If you are replacing the focuser: Add a strip of masking tape around the outer suurface of the OTA following the new cut edge, slip the focuser back in until the should sits hard on the new cut edge, gently withdraw it until the thre mounting holes are just visible, mark the position of the three holes on the masking tape on the outer surface of the tube.
11. Measure the position of the hole centres relative to the focuser shoulder - mark then onto the masking tape
12 Use a pilot drill (2mm or so) to drill the new mounting holes in the OTA - check by sliding the focuser into final position - if OK then open the holes up to match the screw size (they will be 4mm or so)
13. Remove the focuser, remove the cloth and brush, shake out any filings etc - remove the masking tape and re-fit the focuser etc - job done!

To check the collimation of the focuser (or adaptor plate etc) there are two things to do:
1. Cover the objective with a piece of light card/ white paper where the centre of the objective is marked.
2. Insert a laser collimator into the focuser and check the spot hits the centre mark. Tilt or adjust the focuser (adaptor plate) until this is achieved and tighten into place.
3. Replace the laser with a Cheshire collimator, and illuminate the side hole with a bright lamp (LED torch works well)look through the Cheshire and you'll see a bright circle and doughnuts from the reflections of the lens surfaces in the objective. These should all be concentric.
4. Using the collimating screws at the objective (if fitted)bring the circle/ doughnuts into alignment. If there's no collimating screws at the objective, look at how it could be adjusted - some have small screws at the side similar to the focuser - slightly loosen and tilt/ adjust as required.

This will ensure that the focuser is square to the mechanical axis and that the objective is square to the focuser.
Hope this helps.

[marktownley]

The only thing I would add to what Ken has wrote in order to make sure the cut on the tube does not 'drift' offline when you do the sawing - place the tube rings either side of the line you want to cut, with a 'blade width' separating them, then, when you do saw the tube rings will act as a guide and keep the cut line going where you want it to. May take a bit of paint off the tube ring but better than the cut going offline...