ZWO M42-M42 Extender

You bought a good telescope. You got a quality camera. You used a field flattener. You set up everything carefully. You took a 3-hour exposure on a clear night.

You opened the image. Stars in the center are sharp. But the edges? Stretched. Oval. Ugly.

You didn't do anything wrong. You're just missing one thing — the correct back focus distance. And the fix is a small aluminum tube that costs Rs. 5,130.

The ZWO M42-M42 Extender is that fix. Simple, precise, and exactly what your imaging train needs.

The Back Focus Problem — Why It Happens to Everyone

Every field flattener, focal reducer, and corrector lens has a required back focus distance. This is the exact gap between the corrector's last lens and your camera sensor.

Miss that distance by even 2mm — your corners look bad. Miss it by 5mm — your entire frame looks stretched.

The problem? Your camera nose piece, your filter, your filter wheel, your OAG — they all add thickness. When you add everything up, the total is almost never perfect. You're always a few millimeters short or long.

The ZWO M42-M42 Extender solves this. You pick the size you need — 5mm, 10mm, 15mm, or 20mm — and add it to your imaging train. Your back focus distance becomes exact. Your stars become sharp. Edge to edge. Every single frame.

What Makes This Extender Different From a Random Spacer?

You can find cheap aluminum spacers everywhere. Daraz has them. Local camera shops have them. But here's the problem with those — they are not machined to exact tolerances.

A spacer that says "10mm" but is actually 10.4mm ruins your entire back focus calculation. You'll never know why your images look slightly off. You'll keep adjusting, keep imaging, and keep getting frustrated.

The ZWO M42-M42 Extender is CNC-machined aluminum. CNC means a computer-controlled machine cuts every unit to the exact same length. No variation. No rounding. If you order 15mm, you get exactly 15mm.

That precision is the difference between sharp corners and stretched stars.

Full Technical Specifications — Everything You Need to Know

One side is male. The other side is female. This means you can stack multiple extenders together. Need 25mm? Stack a 10mm and a 15mm. Need 30mm? Stack two 15mm units. Total flexibility.

How to Pick the Right Size for Your Setup

This is where most people get confused. Here's a simple way to figure it out.

Step 1: Find your field flattener's required back focus distance. This is in the manual or on the manufacturer's website. Most common value is 55mm.

Step 2: Measure all the components in your current imaging train. Add up the thickness of your camera nose piece, filter, filter wheel (if you have one), and any other spacers you already use.

Step 3: Subtract your current total from the required back focus distance. That number is your shortfall — and that's the extender size you need.

Example: Required back focus is 55mm. Your current train adds up to 45mm. You need exactly 10mm more. Order the 10mm ZWO M42-M42 Extender.

Not sure about your calculation? The Sky Deep team offers free consultations — they'll do this math with you for your exact setup.

Where Does This Extender Sit in Your Imaging Train?

Think of your imaging train as a chain. It starts at your telescope focuser and ends at your camera sensor. This extender fits anywhere along that chain — between any two M42-threaded components.

A typical setup looks like this:

Telescope Focuser → Field Flattener → ZWO M42-M42 Extender → Filter → Camera

Or with a filter wheel:

Telescope Focuser → Field Flattener → Filter Wheel → ZWO M42-M42 Extender → Camera

Because one end is male and the other is female, it threads in cleanly without any adapter or extra hardware. It locks tight and stays firm during the entire imaging session — no flex, no tilt, no movement.

Who Actually Needs This Extender?

You just added a field flattener to your setup. Flatteners change your back focus requirement completely. Almost every setup needs some spacers after adding a flattener.

You switched from one camera to another. Different cameras have different distances from the thread to the sensor. When you change cameras, your back focus shifts. You need to recalculate and re-space.

Your images look sharp in the center but bad at the edges. This is the classic sign. Sharp center, oval corner stars — almost always a back focus issue. A correctly sized extender fixes this.

You added a filter wheel or an OAG. Both of these add physical thickness to your imaging train. That extra thickness changes your back focus distance. You need to compensate with a spacer.

If you're building a full deep sky setup, browse the deep space imaging collection at Sky Deep — everything from telescopes to cameras to spacers is in one place.

Stack Multiple Extenders for Perfect Spacing

Here's a feature most people miss. Because one end is male and the other is female, these extenders stack together perfectly.

Need 25mm total? Stack a 5mm and a 20mm. Need 35mm? Stack a 15mm and a 20mm. You can build almost any spacing you need with different combinations.

Many astrophotographers buy two or three different sizes. They mix and match until they hit the exact back focus their setup requires. Once they find the right combination — they never touch it again.

Check all available accessories including different spacer options in the camera accessories collection at Sky Deep. And if your setup also needs an M42-to-M48 conversion, the ZWO M42-M48 Extender (16.5mm) handles that. For T2-to-T2 spacing needs, the ZWO T2-T2 11mm Extender is the right choice.

Why Buy From Sky Deep — The Only Authorized Dealer in Pakistan

Sky Deep is Pakistan's ONLY authorized ZWO dealer. This is an official designation from ZWO — not a self-claimed title.

When you buy from random marketplaces, you risk getting a spacer that is not machined correctly. A 0.5mm error in a "10mm" spacer means your back focus is wrong — and you'll never know why your images look off.

From Sky Deep, you get:

1. Genuine ZWO products — precision-machined to exact specifications
2. Free nationwide delivery — Lahore, Karachi, Islamabad, Peshawar, everywhere
3. Free gear consultation — they'll help you calculate your exact spacing before you buy
4. Proper after-sales support — if something is wrong, they fix it

Sky Deep is also the official learning partner of Lahore Astronomical Society. They have helped hundreds of Pakistani astrophotographers build correct, well-calibrated imaging trains.

Visit the Sky Deep homepage or reach out on the contact page before buying. Not sure which size you need? Book a free consultation — their team will calculate your exact back focus shortfall.

Build Your Complete Imaging Train

The extender is one piece of the puzzle. Here's everything else you need:

A sharp apochromatic refractor gives you flat, color-correct images across the full frame. Browse apo refractors here — these are designed specifically for deep sky imaging.

A cooled camera reduces noise in long exposures dramatically. Color cameras are here and monochrome cameras are here.

A stable equatorial mount tracks stars accurately. Without precise tracking, no amount of back focus correction will save your images. Browse equatorial mounts here.

And to fight light pollution in Pakistani cities, add the ZWO Duo-Band Filter to your imaging train. It blocks street light and captures only nebula light.

To learn how to calculate back focus and build your imaging train correctly from scratch, the Sky Deep Astro Knowledge Hub has free guides waiting for you.

Order Now — 10 Units in Stock

Pick your size — 5mm, 10mm, 15mm, or 20mm — and order directly.

Order the ZWO M42-M42 Extender here

Not sure which size to pick? Book a free consultation — Sky Deep's team will do the calculation for your exact setup.

Questions Everyone Asks Before Buying

Q1. How do I know which size — 5mm, 10mm, 15mm, or 20mm — is right for me?

You need to calculate your current imaging train length and compare it to your field flattener's required back focus. The difference tells you which size to order. If you're not sure, contact Sky Deep with your telescope, camera, and flattener model. They'll do the calculation for you for free.

Q2. Can I stack two of these extenders together?

Yes — that's exactly what they're designed for. One end is male and the other is female, so two extenders thread together perfectly. Stack a 10mm and a 15mm to get 25mm. Stack two 20mm units to get 40mm. You can build almost any spacing combination you need.

Q3. Will adding this extender affect my image quality?

No. There is no glass or optical element inside this extender. It is a solid aluminum spacer. It only changes the physical distance between components. Your image quality depends on your telescope and camera — not this spacer. The only thing it changes is whether your back focus distance is correct or not.

Q4. What's the difference between this and the ZWO M42-M48 Extender?

Both solve back focus problems — but for different thread configurations. The ZWO M42-M48 Extender connects an M42 component to an M48 component and is a fixed 16.5mm. This M42-M42 Extender connects two M42 components and comes in four size options. Which one you need depends on your thread sizes and how much spacing you need.

Q5. Is this compatible with non-ZWO cameras and accessories?

Yes. M42x0.75 is a universal thread standard used across many brands — not just ZWO. This extender works with any M42-threaded camera, filter wheel, field flattener, or accessory regardless of brand.

Sky Deep — Pakistan's largest astronomy store. The only authorized ZWO and Celestron dealer in the country. Trusted by astronomers, built for dreamers.