An oddly-designed Barlow included with some StarSense Explorer telescopes.

An unusual Barlow that's shipped with (at least some) Celestron StarSense Explorer telescopes has to be assembled in an unusual way in order to be able to focus on stars.

We’ve been selling the new Celestron StarSense Explorer telescopes for a few months now.

The StarSense Explorer is a nifty new development in the world of beginner to intermediate telescope mounts. It uses the camera on your mobile phone to determine which way the scope is pointed by looking at the stars and consulting a database. It then guides you to whatever you want to look at. Very clever.

Of course, our stock sold out before we were able to grab one to use as a display model, so I still have yet to play with one. But that’s not the point of this post.

The point is the strange Barlow lens that comes with the StarSense Explorers.

A Barlow is inserted into the converging rays of the telescope, making them converge less. Put simply, this pushes the focal point (where the light actually converges) further back from the objective lens, increasing the focal length (magnification) of the telescope. You also have to move the eyepiece back.

I've done a quick and dirty ray diagram here. Without the Barlow, the image is formed at F. With the Barlow in place, the focal plane moves back to F'. This can be an advantage, as I've discussed here.

We had a series of queries from customers who had had trouble using the Barlow lens that was supplied with the telescopes. Try as they might, users simply couldn’t get anything in focus with this Barlow.

Eventually we got one back, and tested it on a scope. Nothing. We could focus on a tree just outside the window a few metres away, but nothing further. Clearly there was something wrong.

We contacted Celestron and described the problem. After some thought, they identified what the issue was and told us what to do.

This particular Barlow has to be inserted directly into the focuser prior to attaching the right angle and the eyepiece. I’ve shown this here.

We had set it up like other Celestron Barlows (and pretty much every Barlow I've used before), going focuser – right angle – Barlow – eyepiece. The next photo shows how you do it with standard Celestron (and other) Barlows.

Once we set the train up in that way, we were able to find focus in distant objects, although the magnification was greater than expected.

Celestron is in the process of updating their telescope instructions.

Further reading here.