William Optics + ZWO
Zenithstar 81 with the ASI2400MC Pro
between the corrector and the sensor, for 60.2 mm of back focus.
21.8 mm of spacers puts the Zenithstar 81 with the ASI2400MC Pro within 0.1 mm of target, inside tolerance.
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 Zenithstar 81, William Optics specify 60.2 mm. That is the distance from the back of the corrector to your sensor, and it is what everything below is working towards.
The ASI2400MC Pro contributes 17.5 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 38.5 mm, leaving 21.7 mm for spacers.
The closest available stack is 21.8 mm, which lands +0.1 mm from target. That is inside the 5.7 mm tolerance, helped by the corrector's 5.2 mm of adjustment.
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 |
Astrodymium M48 fine adjustment ring 1.0mm BuyAgena Astro · US | 1.0 mm | M48 |
Astrodymium M48 fine adjustment ring 0.8mm BuyAgena Astro · US | 0.8 mm | M48 |
| Corrector to camera | 42.8 mm | 60.2 mm − 17.5 mm camera = 42.7 mm needed; this stack is +0.1 mm |
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
- 447 mm at f/5.5 (from 559 mm native)
- Sampling
- 2.74″/px
- Field of view
- 4.61° × 3.07°
- Sensor
- 36 × 24 mm, 5.94 µm pixels
Where to buy this setup
Where to buy the William Optics Zenithstar 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 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
ZWO specify "Back Focus Distance 17.5mm" for the ASI2400MC Pro, measured to the camera's own front face with no adapters fitted.
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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