Where Does the Barlow Lens Go on a Telescope? A Step-by-Step Guide

Updated November 2026

A Barlow lens goes between the diagonal (or the rear of the focuser on a refractor without one) and your eyepiece, so the light passes through it before reaching the eyepiece. Placed correctly, it multiplies the magnification of whatever eyepiece you attach to it. This guide shows you where it belongs, how to seat it, and what to check when the view looks off.

Understand the Light Path First

Light from the telescope’s main optics converges at the focal plane inside the focuser. The eyepiece then magnifies that image. A Barlow lens is a negative lens that sits in front of the eyepiece and effectively lengthens the light path, pushing the focal point farther out. That is why it boosts magnification. Because it changes the focal point, it must sit in the light path before the eyepiece, never after it and never at the objective end.

Shopping for one? See our guide: 5 Best Barlow Lens for Telescope Picks Compared (2026).

Where to Place It Step by Step

Use this sequence on most setups:

1. Start with the telescope focuser fully racked in, with nothing inserted yet.
2. If you use a diagonal (typical for reflectors and many refractors viewed at a comfortable angle), insert the diagonal into the focuser first and tighten its thumbscrews.
3. Thread or slide the Barlow into the diagonal’s eyepiece holder, or into the focuser directly if you are not using a diagonal.
4. Insert your eyepiece into the Barlow’s receiving end and tighten its clamp.
5. Focus by turning the focus knob slowly until the image sharpens.

The order matters. The Barlow sits between the diagonal and the eyepiece, not between the focuser and the diagonal, unless the manufacturer’s instructions for that specific Barlow say otherwise. Some Barlows are designed to sit inside the focuser for compatibility with certain eyepiece sizes, so check the packaging.

Matching Barlow and Eyepiece Sizes

Barlows are sold with barrel sizes such as 1.25 inch and 2 inch. The Barlow’s inlet (the end facing the diagonal) should match your diagonal or focuser size, and the outlet (facing the eyepiece) should match your eyepiece. Adapters exist for mismatched sizes, but each adapter adds length and can shift focus. Keep the number of adapters to a minimum.

Setting Focus After Adding the Barlow

Adding a Barlow moves the focal point outward, so you will usually need to rack the focuser out farther than you did with the eyepiece alone. If you hit the end of your focus travel without reaching sharp focus, the Barlow may be seated too far in, or your eyepiece may need to sit deeper in the Barlow’s holder.

Common Mistakes

• Placing the Barlow in front of the diagonal instead of behind it, which changes the optical path and often prevents focus.
• Leaving the eyepiece loose in the Barlow, which lets it shift during observing.
• Stacking several adapters, which adds unnecessary length and can introduce wobble.
• Expecting a sharp image at very high magnification when atmospheric seeing is poor. Steady air matters more than the Barlow.
• Forgetting to recenter the target after changing magnification, since the field of view shrinks.

Recommended Gear

A simple set of matched adapters helps when your eyepieces and focuser sizes do not line up cleanly, reducing the number of loose connections in the light path. View on Amazon

A quality diagonal keeps the Barlow in a stable position and gives a comfortable viewing angle for most targets. View on Amazon

For more on how these pieces fit together, see Barlow Lens Before or After Diagonal: Where to Place It (2026) and How to Use a Barlow Lens on a Telescope: A Step-by-Step Guide for Beginners. If you are still deciding whether one is worth adding, Is a Barlow Lens Worth It? 6 Picks Compared for Every Telescope covers the trade-offs.

FAQ

Can I put a Barlow lens directly on the telescope without a diagonal?

Yes, on many refractors and some reflectors you can insert the Barlow straight into the focuser. Check that the focuser can accept it and that you can still reach focus. Using a diagonal is common for comfortable viewing angles.

Does the Barlow go before or after the eyepiece?

The Barlow always goes before the eyepiece, meaning between the diagonal or focuser and the eyepiece. Putting it after the eyepiece will not produce the intended magnification.

Why can’t I reach focus after adding a Barlow?

The Barlow moves the focal point outward, so you may need to extend the focuser farther. If focus is still out of reach, check that the Barlow is seated fully and that the eyepiece is inserted at the correct depth.

Conclusion

Place the Barlow lens between your diagonal or focuser and your eyepiece, then refocus after each change. Keep the connections secure and limit adapters to preserve a steady light path. With the order right, you can increase magnification without losing sharpness.

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