STEP BY STEP
A card template is useful to test that you’ve measured correctly. Draw around the tube to get a circle, then carefully measure the bayonet connector of the right-angled finder and draw its shape in the centre of the circle you’ve drawn.
Carefully cut out the aperture in the end of the tube, ensuring you follow any safety precautions recommended by the tool’s manufacturer. Remember – because you drew inside the template aperture, you will need to cut outside of the marked lines.
If you used a film can, cut it to 25mm long and reinforce the cut edge with insulating tape. Mate the tube to the finder and rotate it through 90°. Tighten the locking ring on the finder until the tube will no longer rotate.
Cut out the template and make sure it fits the finder. If it does then – ensuring the template is properly positioned on the tube end – draw around the aperture with a permanent fine-point pen (non-permanent marks may not survive the next step).
Use a fine file to smooth any rough edges and ‘tweak’ the shape of the aperture. Experiment to determine the appropriate amount of loop tape to give a snug fit to the eyepiece of the polarscope. Film canisters usually need less tape than end-caps do.
Fit the finder to the polarscope and ensure that the reticule is visible in the centre of the field of view. If it isn’t, this is usually because it is tilting slightly, so you will need to adjust the amount of loop tape inside the tube.