Adjustable Focus (continued)
Another approach is to use two tubes with slightly different diameters to build a sliding (telescoping) focuser. Cardboard mailing tubes or PVC pipes are possible sources for the sliding tubes. Sticky-back felt, from your local arts and crafts store, can be used to create a nice friction fit between the two tubes. Be sure to leave a lengthwise gap in the felt to allow air to escape as you are focusing. The felt can also be used to line the inside of the tubes to reduce reflected light inside your lens.
Here's an example that uses this approach for a rather improbable 400 mm lens.