William Optics + QHYCCD
Gran Turismo 81 with the QHY268C (SBFL - Jan 2023 onward)
between the corrector and the sensor, for 62.1 mm of back focus.
26.6 mm of spacers puts the Gran Turismo 81 with the QHY268C (SBFL - Jan 2023 onward) exactly on target.
Assumes the William Optics FLAT6AIII (0.8× reducer) between the two.
The imaging train
Where this number comes from
The William Optics FLAT6AIII does not have a single back focus figure — it depends on the telescope it is fitted to. On the Gran Turismo 81, William Optics specify 62.1 mm. That is the distance from the back of the corrector to your sensor, and it is what everything below is working towards.
The QHY268C (SBFL - Jan 2023 onward) contributes 14.5 mm of that on its own — sensor to front face. Its M54 thread does not meet the corrector's M48 directly, so the ZWO M54 to M48 Adapter 21mm bridges them and adds 21.0 mm. Together that is 35.5 mm, leaving 26.6 mm for spacers.
20.0 mm + 5.0 mm + 1.0 mm + 0.6 mm comes to exactly 26.6 mm, so this combination lands on target with nothing left over.
What to buy
| Part | Length | Threads |
|---|---|---|
ZWO M54 to M48 Adapter 21mm BuyAgena Astro · US | 21.0 mm | M54 → M48 |
Astro Essentials M48 extension tube 20mm BuyFirst Light Optics · UK | 20.0 mm | M48 |
Astro Essentials M48 extension tube 5mm BuyFirst Light Optics · UK | 5.0 mm | M48 |
Astrodymium M48 fine adjustment ring 1.0mm BuyAgena Astro · US | 1.0 mm | M48 |
Astrodymium M48 fine adjustment ring 0.6mm BuyAgena Astro · US | 0.6 mm | M48 |
| Corrector to camera | 47.6 mm | 62.1 mm − 14.5 mm camera = 47.6 mm needed |
Links to retailers are affiliate links. They cost you nothing and they pay for this site. They do not affect which parts are recommended — the stack above is whatever lands closest to target.
What you get
- Focal length
- 382 mm at f/4.7 (from 478 mm native)
- Sampling
- 2.03″/px
- Field of view
- 3.52° × 2.35°
- Sensor
- 23.5 × 15.7 mm, 3.76 µm pixels
Where to buy this setup
Where to buy the William Optics Gran Turismo 81
Affiliate link — they may earn this site a commission and change nothing above. No prices shown: this site is static, and a stale price is worse than none.
Where to buy the QHYCCD QHY268C (SBFL - Jan 2023 onward)
Affiliate link — they may earn this site a commission and change nothing above. No prices shown: this site is static, and a stale price is worse than none.
Where to buy the William Optics FLAT6AIII
Affiliate link — they may earn this site a commission and change nothing above. No prices shown: this site is static, and a stale price is worse than none.
Read next
Sources and caveats
QHYCCD publish "14.5mm（±0.2）" for the QHY268C (SBFL - Jan 2023 onward) without stating whether that figure includes the adapter supplied in the box. We use it exactly as published, which is correct for the camera in the configuration it is sold in. If you have removed or added an adapter at the camera end, adjust accordingly.
We have not recorded the thread gender of every part listed above. The lengths are right; check that the faces actually mate before ordering.
Every figure on this page was read from the manufacturer’s own documentation and last checked in September 2026. Manufacturers revise them — ZWO and QHY both have, mid-model — so that date is part of the answer.
Check these figures against your own manuals before buying. If something here is wrong, tell us and we will fix it.
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