The 55 mm figure comes from the T2 imaging standard, where a camera, a T-ring and a flattener were designed around one agreed distance. It became a default because it usually worked, and defaults that usually work are the most dangerous kind.
One flattener, four telescopes, four figures
The William Optics FLAT6AIII is a single 0.8× reducer that fits several William Optics refractors. It does not have a required back focus. It has one per telescope, and William Optics publish them separately:
| Telescope | Required back focus | Difference from 55 mm |
|---|---|---|
| Fluorostar 91 | 60.4 mm | 5.4 mm |
| Gran Turismo 71 | 64.1 mm | 9.1 mm |
| Gran Turismo 81 | 62.1 mm | 7.1 mm |
| Zenithstar 81 | 60.2 mm | 5.2 mm |
Nothing about the corrector changes between those rows. The optical prescription of the telescope in front of it does, and the corrector has to sit at whatever distance that prescription demands.
What being wrong looks like
A spacing error does not defocus your image. You will refocus, the centre will look fine, and the stars in the corners will be elongated. William Optics' guidance is that a sensor too close to the optical plane stretches stars radially and one too far stretches them tangentially, and that distortion appearing equally in all four corners points to back focus rather than tilt. (One independent guide cautions that the radial and tangential correspondence can reverse for some correctors, so confirm by changing the spacing a little.) That is why this problem survives so many clear nights: the failure mode looks like tilt, or like a bad night of seeing, or like a lens that is not as good as the reviews said.
It is also why the tolerance matters more than people expect. A corrector's spacing tolerance is typically a fraction of a millimetre, which is smaller than most single spacers you own.
How to know which case you are in
- Find your corrector's documentation, not your telescope's, and not a forum post.
- Check whether the figure is given per telescope. If the corrector fits more than one scope, assume it might be until you have read otherwise.
- Check what the figure is measured FROM — the corrector's rear shoulder and its rear thread are not always the same plane.
- If nobody publishes a figure for your combination, do not substitute 55. A number with no source behind it is worse than an admitted gap.
That last rule is the one this site is built on. Where no manufacturer publishes a required back focus for a pairing, the page says so and refuses to calculate, rather than assuming the convention and producing a confident, plausible, wrong answer.