Askar + QHYCCD
FRA600 with the QHY268C (SBFL - Jan 2023 onward)
between the corrector and the sensor, for 55.0 mm of back focus.
19.5 mm of spacers puts the FRA600 with the QHY268C (SBFL - Jan 2023 onward) 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 QHY268C (SBFL - Jan 2023 onward) contributes 14.5 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 35.5 mm, leaving 19.5 mm for spacers.
16.5 mm + 1.0 mm + 1.0 mm + 1.0 mm comes to exactly 19.5 mm, so this combination lands on target with nothing left over.
What to buy
| Part | Length | Threads |
|---|---|---|
ZWO M54 to M48 Adapter 21mm BuyAgena Astro · US | 21.0 mm | M54 → M48 |
ZWO M48-M48 Extender 16.5mm BuyAgena Astro · US · also Teleskop-Express (EU) | 16.5 mm | M48 |
Astrodymium M48 fine adjustment ring 1.0mm BuyAgena Astro · US | 3 × 1.0 mm = 3.0 mm | M48 |
| Corrector to camera | 40.5 mm | 55.0 mm − 14.5 mm camera = 40.5 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
- 1.85″/px
- Field of view
- 3.20° × 2.14°
- Sensor
- 23.5 × 15.7 mm, 3.76 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 QHYCCD QHY268C (SBFL - Jan 2023 onward)
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
QHYCCD publish "14.5mm（±0.2）" for the QHY268C (SBFL - Jan 2023 onward) 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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