Centring Collar for TS-Optics RC8 Focuser Decoupling Adapter (TILTRC8)

The TS-Optics focuser decoupling adapter with tilt flange (TILTRC8) is a worthwhile upgrade for GSO-built 8" RCs, including the StellaLyra, TS, Omegon and Astro-Tech versions. Fitting it means drilling three holes in the telescope's backplate, using the adapter ring itself as a template, with a plastic positioning ring supplied to locate everything.

On my kit, the positioning ring is a very loose fit inside the adapter ring. Measured with feeler gauges, there is 1.6mm of diametral clearance, which is 0.8mm all round once the two are concentric. That means the centre of the adapter ring can sit anywhere within a circle of 0.8mm radius about the true axis, and wherever it happens to sit when you drill is where it stays permanently. For a German-engineered kit that asks you to drill your telescope, I expected a closer fit.

There is a second, more immediate reason for taking up the clearance. The adapter ring is used as the drilling template, so wherever it sits is where the holes go. With that much play, the ring can shift as the drill bites, and the second and third holes then have to agree with the position the first one fixed. Without the collar this is fiddly to control; with it, the ring cannot move and the holes came out accurately placed first time.

In practice 0.8mm is the worst case for decentring, only reached if the ring is pushed hard against one side, and careful positioning by eye would probably get you within a couple of tenths, which an f/8 RC would tolerate. But since the holes are not something you can undo, the collar removes the guesswork from both concerns at once.

I left mine in place after assembly, as the positioning ring also serves to block stray light and the collar closes the remaining gap. If you do the same, print it in matt black rather than a light colour, to avoid reflections near the light path.

Printing

Print upright as a collar, in vase mode, with the line width forced to 0.8mm on a 0.4mm nozzle. Vase mode matters here: with a normal seam you get a bump down one side of the collar, which is exactly the error you are trying to eliminate. An aligned seam is the worst case and random is better, but vase mode removes the problem entirely. A slicer with the Arachne wall generator (PrusaSlicer 2.5+, Orca, Cura 5) will also handle a solid 0.8mm wall well if you prefer not to use vase mode.

Aim for a firm interference fit that you can push home by hand without tools. If it is too tight, sand the outside evenly by rotating it against abrasive paper laid flat, a few seconds at a time. Check roundness by fitting it, removing it, rotating 120 degrees and refitting: the feel should be identical each time.

There will be a bump in the print at the start and end, these can easily be removed with a bit of abrasive paper.

Important

This is sized to the clearance measured on my own parts. Tolerances may vary between batches, and printers vary too, so measure your own gap before printing and adjust the wall thickness to suit. The source file is included so you can change it. It is a drilling jig first and a light seal second, and it carries no load once the adapter ring is bolted down.

One other note on fitting

The instructions say the retaining ring holding the primary mirror and baffle tube "can only be loosened with a certain amount of force", but do not mention the three small hex grub screws that lock it. Loosen those first. Once they were slackened, mine unscrewed with almost no effort. Forcing the ring against them risks damaging the thread, the ring or the mirror.