Sky-Watcher + Player One
Evostar 120ED with the Poseidon-C Pro
between the corrector and the sensor, for 55.0 mm of back focus.
37.5 mm of spacers puts the Evostar 120ED with the Poseidon-C Pro exactly on target.
Assumes the Sky-Watcher 0.85x Reducer/Corrector for Evostar 120ED (0.85× reducer) between the two.
The imaging train
Where this number comes from
The Sky-Watcher 0.85x Reducer/Corrector for Evostar 120ED wants 55.0 mm between its rear shoulder and the sensor.
The Poseidon-C Pro contributes 17.5 mm of that on its own — sensor to front face — which leaves 37.5 mm for spacers.
30.0 mm + 7.5 mm comes to exactly 37.5 mm, so this combination lands on target with nothing left over.
What to buy
| Part | Length | Threads |
|---|---|---|
Apertura M48 Extension Spacer Ring 30mm BuyHigh Point Scientific · US | 30.0 mm | M48 |
Baader M48 extension tube 7.5mm BuyAlpine Astronomical · US | 7.5 mm | M48 |
| Corrector to camera | 37.5 mm | 55.0 mm − 17.5 mm camera = 37.5 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
- 765 mm at f/6.4 (from 900 mm native)
- Sampling
- 1.01″/px
- Field of view
- 1.76° × 1.18°
- Sensor
- 23.5 × 15.7 mm, 3.76 µm pixels
Where to buy this setup
Where to buy the Player One Poseidon-C 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 Sky-Watcher 0.85x Reducer/Corrector for Evostar 120ED
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
Player One specify "Back Focal Length 17.5mm, 12.5mm( without sensor tilt plate)" for the Poseidon-C 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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