Askar + ZWO
FRA500 with the ASI2600MM Duo
between the corrector and the sensor, for 55.0 mm of back focus.
16.5 mm of spacers puts the FRA500 with the ASI2600MM Duo exactly on target.
Assumes the Askar F3.9 0.7x Full Frame Reducer (0.7× reducer) between the two.
The imaging train
Where this number comes from
The Askar F3.9 0.7x Full Frame Reducer wants 55.0 mm between its rear shoulder and the sensor.
The ASI2600MM Duo contributes 17.5 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 38.5 mm, leaving 16.5 mm for spacers.
16.5 mm comes to exactly 16.5 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 |
| 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
- 350 mm at f/3.9 (from 500 mm native)
- Sampling
- 2.22″/px
- Field of view
- 3.85° × 2.57°
- Sensor
- 23.5 × 15.7 mm, 3.76 µm pixels
Where to buy this setup
Where to buy the Askar FRA500
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 ASI2600MM Duo
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 0.7x Full Frame Reducer
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
ZWO publish "Back focus distance 17.5mm" for the ASI2600MM Duo 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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