backfocus.tools

William Optics + QHYCCD

Zenithstar 61 with the QHY268M (SBFL)

Within tolerance+0.1 mm of 1.5 mm
20.6 mmof spacers

between the corrector and the sensor, for 56.0 mm of back focus.

20.6 mm of spacers puts the Zenithstar 61 with the QHY268M (SBFL) within 0.1 mm of target, inside tolerance.

Assumes the William Optics FLAT 61R (0.8× reducer) between the two.

The imaging train

Imaging train from sensor to corrector, 56.1 mm total against a required 56.0 mm: QHY268M (SBFL) 14.5 mm, M54 to M48 Adapter 21mm 21.0 mm, M48 extension tube 20mm 20.0 mm, M48 fine adjustment ring 0.6mm 0.6 mm.sensorFLAT 61RQHY268M (SBFL) — 14.5 mmQHY268M (SBFL)14.5 mmM54 to M48 Adapter 21mm — 21.0 mm · M54→M48M54 to M48 Adapter 21…21.0 mm · M54→M48M48 extension tube 20mm — 20.0 mmM48 extension tube 20…20.0 mmM48 fine adjustment ring 0.6mm — 0.6 mm0.6 mm56.1 mmHover or tap a part for its full spec
Drawn to scale, sensor plane to corrector. Total 56.1 mm as drawn. This pairing requires 56.0 mm; the closest stack the parts table can build lands +0.1 mm from it, inside the 1.5 mm tolerance.

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 QHY268M (SBFL) 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 20.5 mm for spacers.

The closest available stack is 20.6 mm, which lands +0.1 mm from target. That is inside the 1.5 mm tolerance, helped by the corrector's 1.0 mm of adjustment.

What to buy

PartLengthThreads
ZWO M54 to M48 Adapter 21mm
BuyAgena Astro · US		21.0 mmM54 → M48
Astro Essentials M48 extension tube 20mm
BuyFirst Light Optics · UK		20.0 mmM48
Astrodymium M48 fine adjustment ring 0.6mm
BuyAgena Astro · US		0.6 mmM48
Corrector to camera41.6 mm56.0 mm − 14.5 mm camera = 41.5 mm needed; this stack is +0.1 mm

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
4.67° × 3.12°
Sensor
23.5 × 15.7 mm, 3.76 µm pixels

Where to buy this setup

Where to buy the QHYCCD QHY268M (SBFL)

BuyAgena Astro · US

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

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Sources and caveats

QHYCCD publish "14.5mm（±0.2）" for the QHY268M (SBFL) 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.

Measuring your own? The four tools worth owning for this →