Taking a photo using a DSLR and small Newtonian reflector

Focusing with a small Newt and a DSLR

A slightly longer version of this is on the Optics Central blog page.

One of the most common (and irritating) problems that owners of Newtonian telescopes have is getting a DSLR onto them – and actually get the image in focus.

Well of course you want to take a photo!

One of the most common (and irritating) problems that owners of Newtonian telescopes have is getting a DSLR onto them – and actually get the image in focus.

Of course, people want to take photos of the Moon or other things when they’ve got a telescope and a DSLR – who wouldn’t? But actually getting the two things together is a little complicated.

To make the physical connection, you need at least two things.

• First the telescope’s focuser has to have a specific thread (called a t-thread) cut into it. If your focuser doesn’t have one of these you can get a little tube with an eyepiece-shaped tube and the required thread.
• Second, you need an adapter called a t-ring, which connects the t-thread on the focuser to the DSLR. Obviously, the t-ring has to be for the right brand of DSLR.

This is something like what the first two things get you:

This looks all good so far, but there's a problem that you may or may not have (depending on your scope's geometry). This brings me to a third requirement:

• Third, you need to have enough space to get the camera’s sensor onto the focal plane.

 What (I hear you ask) and where is the focal plane?

The focal plane

It’s the actual image created by the mirror (or lens). When you look through the eyepiece, you’re actually looking right at it. You probably never knew it’s there, because it’s normally invisible!

There is a way to reveal your focal plane though. Next time you’re out at night looking at the Moon, take the eyepiece out of the telescope. Take a piece of paper and hold it in front of the focuser. You’ll see a blob of light on the paper. Next, move the paper closer to or further away from the focuser. You’ll find a point where the blob of light turns into a small, nicely focused image of the Moon. This is the focal plane.

Here's one of my scribbled diagrams to help you see this.

But I can't get it to focus

With smaller Newtonian scopes (under about 150mm), there's a good chance you won’t be able to get the camera into the space between the focuser and the focal plane. You get a camera onto the scope, but try as you might, you can’t get it to focus on anything – it’s all a blur.

The reason for this is there’s not enough room between the focuser and the focal plane to fit the camera. You’d have to roll the focus in beyond the point where it stops, effectively burying the camera into the side of the telescope. Not a good look.

The way to get a focused image in the camera is to position the sensor exactly on the focal plane, just like the piece of paper with the image of the Moon from last time. What we need is some way of moving the focal plane further out, giving us enough room for the camera.

Pull the focal plane out with a Barlow

There is a way: you use a Barlow. This is a concave lens that is normally used to increase magnification, but its side effect is to push the focal plane out. My next scribbled ray diagram shows what’s going on here.

 

Without the Barlow, the light rays come to a point close to the focuser. Here, there’s not a lot of room to fit in a camera. But with the lens, the light rays nearly straighten out, the focal plane being further away from the focuser. There's more room between the focuser and the where the image is formed, so there's plenty of room to get your camera's sensor onto the focal plane.

Job done…?

But it still didn't work. Why not?

The other day I had to take some test photos through a small Newt. I got it home and set it up, only to find I couldn’t make focus.

So I fetched my Barlow, but still couldn’t make focus. The night was a bust.

It turns out there are different sizes and designs of Barlows. The one I was using was too long. There's another  one, which is able to burrow much further into the focuser. You can see the two in the photo. The one on the right is the one I used, and the one on the left is the one I should have used. You can see the t-ring (the camera attachment) is much further down, closer to the focuser.

This one is the one you use for photography. Using a short Barlow, your camera gets as close as possible to the telescope, and at the same time the focal plane is drawn out towards your camera. This will allow your sensor to get focus.

The rest is up to you!