Askar + Player One
FRA600 with the Neptune-C II
between the corrector and the sensor, for 55.0 mm of back focus.
26.0 mm of spacers puts the FRA600 with the Neptune-C II exactly on target.
Assumes the Askar F3.9 Full Frame Reducer for FRA600 (0.7× reducer) between the two.
The imaging train
Where this number comes from
The Askar F3.9 Full Frame Reducer for FRA600 wants 55.0 mm between its rear shoulder and the sensor.
The Neptune-C II contributes 12.5 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 29.0 mm, leaving 26.0 mm for spacers.
21.0 mm + 5.0 mm comes to exactly 26.0 mm, so this combination lands on target with nothing left over.
What to buy
| Part | Length | Threads |
|---|---|---|
ZWO M42-M42 (T2) Extender 21mm BuyAgena Astro · US · also Teleskop-Express (EU) | 21.0 mm | M42 |
Astro Essentials M42 (T2) extension tube 5mm BuyFirst Light Optics · UK | 5.0 mm | M42 |
ZWO M42 (T2) to M48 Extender 16.5mm BuyAgena Astro · US · also Teleskop-Express (EU) | 16.5 mm | M42 → M48 |
| Corrector to camera | 42.5 mm | 55.0 mm − 12.5 mm camera = 42.5 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
- 420 mm at f/3.9 (from 600 mm native)
- Sampling
- 1.42″/px
- Field of view
- 1.08° × 37′
- Sensor
- 7.9 × 4.5 mm, 2.9 µm pixels
Where to buy this setup
Where to buy the Askar FRA600
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 Player One Neptune-C II
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 Askar F3.9 Full Frame Reducer for FRA600
Affiliate links — 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 publish "Back Focal Length 12.5mm" for the Neptune-C II 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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