William Optics + ZWO
Zenithstar 81 with the ASI1600MM Pro
between the corrector and the sensor, for 60.2 mm of back focus.
37.2 mm of spacers puts the Zenithstar 81 with the ASI1600MM Pro 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 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 ASI1600MM Pro contributes 6.5 mm of that on its own — sensor to front face. Its M42 thread does not meet the corrector's M48 directly, so the ZWO M42 (T2) to M48 Extender 16.5mm bridges them and adds 16.5 mm. Together that is 23.0 mm, leaving 37.2 mm for spacers.
30.0 mm + 6.0 mm + 0.8 mm + 0.4 mm comes to exactly 37.2 mm, so this combination lands on target with nothing left over.
What to buy
| Part | Length | Threads |
|---|---|---|
Astro Essentials M42 (T2) extension tube 6mm BuyFirst Light Optics · UK | 6.0 mm | M42 |
Astrodymium M42 fine adjustment ring 0.8mm BuyAgena Astro · US | 0.8 mm | M42 |
Astrodymium M42 fine adjustment ring 0.4mm BuyAgena Astro · US | 0.4 mm | M42 |
ZWO M42 (T2) to M48 Extender 16.5mm BuyAgena Astro · US · also Teleskop-Express (EU) | 16.5 mm | M42 → M48 |
Apertura M48 Extension Spacer Ring 30mm BuyHigh Point Scientific · US | 30.0 mm | M48 |
| Corrector to camera | 53.7 mm | 60.2 mm − 6.5 mm camera = 53.7 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
- 447 mm at f/5.5 (from 559 mm native)
- Sampling
- 1.75″/px
- Field of view
- 2.25° × 1.70°
- Sensor
- 17.6 × 13.3 mm, 3.8 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 ZWO ASI1600MM Pro
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 publish "Back Focus Distance 6.5mm" for the ASI1600MM Pro 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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