Takahashi Flattener 1.01x for FSQ-85EDX

Product Overview

This flattener extends the FSQ-85EDX's corrected field to a 44 mm image circle — enough to cover a full-frame sensor's 43.3 mm diagonal completely. At 1.01x it changes focal length only fractionally, from 450 mm (f/5.3) to 455 mm (f/5.4), so this is a corrector rather than a focal-length changer.

Who It's For

This is what you need if you image with an FSQ-85EDX on a full-frame sensor and want flat, round stars into the corners of the frame, not just the APS-C centre.

Optical / Mechanical Design

The FSQ-85EDX's petzval design is already well corrected across a smaller field; this flattener extends that correction out to a 44 mm circle by adding the last small amount of field curvature correction the native optical path doesn't reach on its own. The 1.01x factor is nearly unity — the flattener is doing geometry, not speed — which is why the focal length shift is so small: 450 mm becomes 455 mm, f/5.3 becomes f/5.4. Backfocus behind the flattener is 36 mm, with a 56.2 mm metal back distance from the telescope's own reference point, and the front face takes M58 filters directly.

Recommended Uses

• Full-frame deep-sky imaging on the FSQ-85EDX where the native field falls short of covering the sensor's corners.
• Wide-field mosaics where consistent corner star shape across panels matters.

Compatibility and Accessory Notes

The flattener threads M72 x 1.0 male into the telescope side and presents M54 x 0.75 female to your camera adapter, with 36 mm of backfocus to the sensor. The front face's female thread accepts M58 filters directly, ahead of the flattener glass. If you're building out a full imaging train and want the spacer stack confirmed to the 36 mm figure, send us your camera and filter setup and we will work it out with you. See the Takahashi Back Focus Guide for the FSQ-85EDX's full published figures.

Good to Know Before You Order

• This is a photographic corrector. It has no application in a visual train with an eyepiece.
• Backfocus is 36 mm from the rear thread. Corner star shape follows how closely your adapter stack hits that figure — send us your setup and we will confirm the spacer stack before you order.

Frequently Asked Questions

Is it difficult to set up?

It threads directly onto the FSQ-85EDX at M72 x 1.0. The work is in hitting the 36 mm backfocus on the camera side, which is arithmetic — add up your adapter, filter drawer and spacers, and send us the numbers if you want them checked.

Does it change my focal length?

Only slightly — 450 mm at f/5.3 becomes 455 mm at f/5.4. It's a flattener, not a reducer.

Will it cover a full-frame sensor?

Yes — the 44 mm corrected circle covers a full-frame diagonal of 43.3 mm.

Can I fit filters directly to it?

Yes — the front face takes M58 filters ahead of the flattener glass.

Bottom Line

In short: a 200 g, 1.01x flattener that gives the FSQ-85EDX a 44 mm corrected circle for full-frame sensors, at 36 mm of backfocus — send us your camera and filter setup and we'll confirm the spacer stack with you.