Problems finding focus with a DSLR

Introduction

One of the most common issues raised by clients is that they’ve put a DSLR onto their reflector telescope and, try as they might, they can’t focus on the sky. Users of Newtonian telescopes (particularly the smaller ones, 150mm aperture and below) often have trouble mounting a digital camera onto the telescope.

In this article, I’ll show you why you’ve got the problem, and more importantly, what you can do about it.

What should happen

Newtonian scopes

A small Newtonian telescope

A Newtonian telescope is the type with the mirror at the back. The focuser is at the side of the tube, up near the front. It’s a common and popular arrangement, particularly on a Dobsonian mount, because large mirrors are much cheaper to make than large lenses. It’s the best way to get a good view of dim objects like nebulas or galaxies.

What’s more, you can use a Newtonian telescope to do astrophotography, just as long as it’s on a mount that tracks the stars. The best type of tracking mount to use would be an equatorial, where you can take exposures of several minutes (if you’re set up well). But you can use an alt-azimuth tracking mount for exposures of around 15 seconds. Longer than this, you’ll start to get “field rotation” where the stars on the outside of the image will start to grow trails, like this:

image rotation
Image credit telescopemount.org

Putting the camera on and focusing

t-ring

Getting a DSLR camera onto one of these scopes is normally a pretty straightforward process. You need a “t-ring” (like this one), which attaches to the camera body using its proprietary connection. The t-ring ends in a female “t-thread”, which is an industry-standard 42mm diameter and 0.75mm thread pitch.

2" adapters with t-threadMany focusers (or these 2″ adapters) have a male version of this thread on them, so you can simply attach the t-ring (and the camera) onto the focuser.

Celestron universal t-adapter

If your focuser doesn’t have a thread on the end, and you can only slip in a 1.25″ eyepiece and tighten the thumbscrew, you need to use the jigger in the photo. It’s a Celestron “universal t-ring adapter”. This is a simple tube that fits into the focuser like an eyepiece and has the required thread on the outer end.

For most Newtonian telescopes, the connection is now complete. You can look through the camera and focus on the Moon, stars, or nebulas.

A small Ne

…or not

But for some Newts, particularly smaller ones, you can find yourself in a situation where you just can’t make it focus on the stars. Specifically, you can run the focuser in to its limit and not get anything in focus. You just can’t quite get the camera close enough to the telescope tube. This is a frustrating problem.

What’s going on?

Images created by telescopes

All telescopes work in the same way. They bend light so that it makes a sharp image somewhere, and you use a eyepiece to look at that image.

Did you know that you can actually see this image? Get a scope with no eyepiece and point it at the Moon. Hold a piece of paper in front of the eyepiece holder, and move it up and down until you see a small, but sharply-focused Moon projected onto the piece of paper. This is the focal point (or focal length) of the telescope. This point is fixed by the mirror and can never move.

By the way, don’t try this exercise with the Sun or you’ll risk starting a fire!

It’s only for thing a long way away (like the Moon) that the image happens at the telescope’s focal point. For objects closer to the telescope (say, a whale off shore), the image is created a bit further away. This scribbled diagram shows these situations.Focusing

How to use the focuser to see the image

The telescope’s focusing mechanism is essentially two barrels that roll up and down, one inside the other. The eyepiece (which has its own, much shorter focal length) sits in its holder at the end of the focuser.

Remember that the place where the image will fall depends on how far the target is away. When the focal point of the eyepiece is exactly on this image, you will see your target in focus. Rolling the focuser will move the eyepiece (and its focal point) so you can get this done. You can see the gap between the eyepiece and its focal point in the diagram above (I’ve marked it “FLe”).

Even when you’re in focus looking at an object far away, say, the Moon, the focuser should be racked out a couple of centimeters as in the photo. This gives you a bit of wiggle room both ways if you want to look at something closer or your eyesight is a bit wonky.

Why are small Newts so much of a problem?

Because Newtonian telescopes have their eyepiece in the side, you can’t make a focuser that racks in and out a long way. When you rack the focuser in, the barrel starts to protrude into the main tube, obscuring the main mirror. Even more, and the barrel crashes into the secondary mirror. This means that there’s not a whole lot of wiggle room for the focuser.

On a tangent, yes, of course you can design a Newt so it can take a DSLR without having this problem. In fact, there are two ways of doing it. You can either move the secondary mirror further down the tube closer to the main mirror. Alternatively, you can move the primary up the tube towards the secondary.

These Newts are known as “astrographs”, and an example is the saxon 200DS. They do have a couple of disadvantages, though. First, you have to rack the focuser out a long way to use an eyepiece, and second, they’re not very good for focusing on nearby objects.

So why is the DSLR camera the problem?

Newtonian telescopes are typically designed so the focus is between 7 and 10cm from the tube. (You can see this in the photo below.) This provides enough room for the body of the focuser plus they eyepiece. It also allows any other fittings they need, such as filters, plus a bit of wiggle room for people with weird eyesight.

Focus distance for an eyepiece

The problem happens when you remove the eyepiece and put the camera on. Getting back to the diagram with the moon and the paper, you need to position the sensor of the camera right on the image cast by the mirror of the telescope.

DSLR cameras have the sensor placed deep inside the camera body. You can see that inside this photo of my Pentax. (This distance is normally called “back-focus”, but its technical name is “flange focal distance”). This is a throw-back from when they used actual film which had to sit in with enough room for the film canister as well. Most DSLRs have around 40-44mm between the front of the camera and the sensor.

Thinking about the moon on the paper again, you have to position that sensor onto the spot where the the Moon was sharpest. For some scopes, even if you rack the focuser all the way in, that point is still short of the sensor.

If it were the opposite problem, you’d run out of focus travel trying to get the camera further away from the tube. In this case, you could put some spacers in, but you can’t add negative space.

So, what do you do?

The solution – a Barlow lens

The answer is that you add a short lens called a Barlow. A Barlow isn’t actually designed for this. It’s a concave lens that effectively doubles the focal length of the primary mirror of the telescope. The main reason for using a Barlow is to increase the magnification of your scope. It does this by straightening out the converging light rays slightly. The useful side effect of this is that it moves the focal plane outwards slightly, giving you more room to insert the camera. You can see this in the scribbled diagram below.

Adding a Barlow lens draws the focal plane away from the telescope

But it doesn’t always work

A week or so back, I had a small (150/750) Newtonian that I had to test for a client. The scope itself was working perfectly, and the go-to mount was tracking fine as well. To prove the tracking was going, I thought I’d take a photo of the Orion Nebula, which was in view at the time.

I grabbed my DSLR and a t-ring and attached it to the scope. Of course, I wasn’t able to find focus. A photo of my DSLR on the scope is near the top of this article.

No problems, I thought, and dashed inside to grab my Barlow. The problem really started when I found that even with the Barlow, I wasn’t able to find focus. I was stumped for the night.

Next morning, I came into the shop rather crestfallen, thinking that the solution I’d suggested to so many customers wasn’t working.

You have to use the right Barlow

After thinking for a bit, I decided to have a look for other Barlow lenses. The setup I’d used the previous night was with a standard Barlow that didn’t have a t-thread cut onto the end. But I found another type that did have the t-thread. This other Barlow is specifically for attaching cameras to scopes. The clever designers have made it snuggle deeper into the focuser, leaving less room at the end. It’s even shorter because I didn’t need to use the t-thread adapter.

The two Barlow lenses are side-by side in this photo.

Two different design Barlow lenses. One is closer to the secondary.

I’ve marked the overall length of the lenses in blue. You can see the one on the right is much shorter, because its lens has been burrowed deeper into the telescope. This allows the camera to get significantly closer to the telescope tube. Using the Barlow on the right gives me that bit more space. I can use this to move the camera closer to the telescope tube. Moving the camera, I can get the sensor onto the focal point of the mirror.

Hooray, I’m in focus!

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Mondays, Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, or how to take photos of the sky. He can even help you to see some rare birds.

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3 Comments


  1. Hi, I’ve just discovered your article searching for a solution as I have this problem with my Nikon D5600 and Celestron Astromaster 130EQ. Your article mentions the alternative barlow lens but not what make/model the shorter lens is. Can you clarify please?

    Reply

      1. Hi, sorry for my delayed reply.
        And thank you for your quick response.

        Reply

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