William Optics + ZWO
Zenithstar 61 with the ASI533MM Pro
between the corrector and the sensor, for 56.0 mm of back focus.
33.0 mm of spacers puts the Zenithstar 61 with the ASI533MM Pro exactly on target.
Assumes the William Optics FLAT 61R (0.8× reducer) between the two.
The imaging train
Where this number comes from
The William Optics FLAT 61R does not have a single back focus figure — it depends on the telescope it is fitted to. On the Zenithstar 61, William Optics specify 56.0 mm. That is the distance from the back of the corrector to your sensor, and it is what everything below is working towards.
The ASI533MM 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 33.0 mm for spacers.
16.5 mm + 16.5 mm comes to exactly 33.0 mm, so this combination lands on target with nothing left over.
What to buy
| Part | Length | Threads |
|---|---|---|
ZWO M42 (T2) to M48 Extender 16.5mm BuyAgena Astro · US · also Teleskop-Express (EU) | 16.5 mm | M42 → M48 |
ZWO M48-M48 Extender 16.5mm BuyAgena Astro · US · also Teleskop-Express (EU) | 2 × 16.5 mm = 33.0 mm | M48 |
| Corrector to camera | 49.5 mm | 56.0 mm − 6.5 mm camera = 49.5 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
- 288 mm at f/4.7 (from 360 mm native)
- Sampling
- 2.69″/px
- Field of view
- 2.25° × 2.25°
- Sensor
- 11.31 × 11.31 mm, 3.76 µm pixels
Where to buy this setup
Where to buy the ZWO ASI533MM Pro
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Sources and caveats
ZWO specify "Back focus distance 17.5mm/6.5mm" for the ASI533MM 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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