Takahashi Extender-Q 2" Adapter

Product Overview

A 2-inch adapter that fits the visual section of the Takahashi Extender-Q, opening the train up from Takahashi's M42 x 0.75 fitting to a standard 50.8 mm barrel. With it in place the Extender-Q accepts 2-inch star diagonals and 2-inch eyepieces — the wide-field oculars that will not fit a 1.25-inch train at all.

Takahashi lists it as the 50.8 Adapter (EX-Q), part TKA00596, with a published light path length of 39.5 mm. It is machined from black-anodized aluminium and retains the accessory with two thumbscrews rather than one, which is what keeps a heavy diagonal square rather than letting it tilt on a single contact point.

Who It's For

This is the part you need if you use the Extender-Q visually and own 2-inch eyepieces or a 2-inch diagonal. The Extender-Q's own visual section does not take them directly; this adapter is what bridges it.

It has no application in an imaging train — a camera connects to the Extender-Q through the photographic path, not through a 2-inch visual barrel.

Key Features & Design

• M42 x 0.75 female telescope side: threads directly onto the Extender-Q's visual section.
• 2-inch (50.8 mm) accessory side: takes standard 2-inch diagonals and eyepieces.
• 39.5 mm light path length: Takahashi's published figure for what this adapter contributes to the optical train.
• Dual thumbscrews: two contact points hold a heavy diagonal square instead of letting it pivot on one.
• Black-anodized aluminium: non-reflective inside the light path and light enough not to shift focuser balance.
• No-play fit: machined to Takahashi's mechanical tolerance, so a loaded diagonal does not rock in the seat.

Recommended Uses

• Using 2-inch wide-field eyepieces behind an Extender-Q on a Takahashi refractor.
• Fitting a 2-inch star diagonal where the extra optical path and larger clear aperture are wanted.
• Upgrading an existing Extender-Q setup from a 1.25-inch to a 2-inch visual train.
• Holding heavier oculars securely where a single-screw fitting would allow tilt.

Compatibility and Accessory Notes

• Fits the Takahashi Extender-Q visual section via M42 x 0.75 female thread. It is designed for that fitting specifically and is not a universal 2-inch adapter.
• Accessory side: 2-inch (50.8 mm), holding 2-inch diagonals and eyepieces with two thumbscrews.
• Light path length is 39.5 mm. That is the figure to add when you are checking a train against your focus travel. Our Takahashi Back Focus Guide lists the light path length of every Takahashi adapter alongside the back focus figures for each telescope.
• Mechanical only. There is no optical element in it — it does not correct, magnify or reduce.

If you are not sure whether your eyepieces will still reach focus with this in the train, send us your telescope, Extender-Q configuration and eyepiece list and we will work it out with you.

Good to Know Before You Order

• It requires an Extender-Q. The adapter threads onto that product's visual section and is not a standalone telescope accessory. If you do not own one, tell us what you are trying to connect and we will find the right part.
• It is a mechanical adapter. No glass, no correction, no change to focal length or focal ratio.
• Visual use only. Imaging trains take the photographic path through the Extender-Q instead.
• It adds 39.5 mm to the optical path. On tubes with limited inward travel that figure is worth checking against the back focus guide before ordering — and we will check a specific combination for you.
• 2-inch accessories are heavier. The dual thumbscrews are there for that reason, and the fit is machined without play, so a loaded diagonal sits square.

Frequently Asked Questions

Is it difficult to set up?

No — it threads onto the Extender-Q visual section, and the diagonal or eyepiece slides in and is held by the two thumbscrews. There is nothing to align.

Do I need an Extender-Q to use it?

Yes. It is made for that product's M42 x 0.75 visual section. Tell us what you are connecting and we will confirm the right adapter if you do not have one.

Does it change magnification or focal length?

No. It is a mechanical adapter with no optical element. The Extender-Q itself supplies the 1.6x factor.

How much does it add to my optical train?

39.5 mm, per Takahashi's published figure. The back focus guide on this site lists it alongside every other Takahashi adapter so you can add up a train.

Can I use it with 1.25-inch eyepieces?

With a 2-inch to 1.25-inch reducing adapter, yes. We stock those.

What is it made of?

Black-anodized aluminium, machined to Takahashi's standard tolerance so the seat holds a loaded diagonal without play.

Bottom Line

In short: Takahashi's 50.8 Adapter for the Extender-Q — M42 x 0.75 in, 2-inch out, 39.5 mm of light path, held on two thumbscrews so a heavy ocular sits square. Purely mechanical, and it is what lets a 2-inch diagonal onto an Extender-Q train.