Turn a Telephoto Lens Into a Spotting Scope or Telescope




In an old Galen Rowell book of photographs, he mentioned an adapter that he could put on his long telephoto camera lens to turn it into a spotting scope.
After some searching, I realized that they were very hard to come by. So I purchased a 45 degree erect-image diagonal and used some old telescope eyepieces and never got it to work: it wouldn't focus.
Then, just before an upcoming trip to Yellowstone, I realized the problem. I needed a Barlow lens in front to shift the focal point back far enough to accommodate the extra length.
Thus, my original project was reborn.
Here is a picture of the completed spotting scope.

Step 1: Gather the Stuff (spend the Cash)

You'll need the following stuff for the project. You can get the stuff cheap at a local astronomy club swap. Prices are estimates for 2009, purchased new.
1) Silicone glue (or any sturdy glue or epoxy). I used silicone because it can be undone to some degree.
2) A telescope eyepiece with a 1.25" barrel (the most common standard). I use a 25mm Celestron Plossl for wide views, and a 10mm Celestron Plossl for high power. (~$50 each)
3) A 45 deg erect image diagonal with a 1.25" barrel (for a spotting scope) OR a 90 degree diagonal with a 1.25" barrel (for a telescope). The barrel needs to be threaded on the inside. Diagonal barrels are often threaded inside to accept filters. I bought my 45deg one from Orion (~$45).
4) A 1.25" 2x Barlow. The lens element holder needs to be unscrewable from the barrel, and the threads need to match the diagonal. Mine did. It is a bottom-of-the-line Celestron ($40)
5) A telephoto lens. There needs to be room to shove the 1.25" barrel of the diagonal about 1" into the backside of the lens. (Some zoom lenses have lens elements right near the opening, which is true of my Canon 85mm to 200mm lens and you'll have to figure out how to get that to work). I'm using a Canon 100-400mm IS zoom lens. (Lots of $$)
6) A lens cap for the back end of the zoom lens (the side that connects to the camera).
7) A 1.25" hole saw. (~$8)

Step 2:

Unscrew the lens element from the Barlow.
Attach the lens element to the end of the diagonal (the end that will stick into the lens).

Step 3: Drill the Cap

Mark the exact center of the lens cap. Drill a 1.25" hole in the cap. Use a drill press if you can. If the hole is not exactly centered, the image will suffer.

Step 4: Make Sure It Can Focus

You can skip this step if you are using a Canon 100-400mm IS lens.
Point the camera lens toward a distant object.
Put the eyepiece in the diagonal/Barlow assembly. Put the drilled cap on the lens.
Put the Barlow end into the camera lens. Hold it roughly where it will be when it's glued to the cap. Look through the eyepiece and adjust the camera len's focus. Make sure that the system can focus. If not, try moving the Barlow out a bit. For my Canon 100-400 lens, the system could focus for the longest (400mm) case when the diagonal was right against the cap. If this is not the case for you, you may need to fashion a spacer between the diagonal housing and the cap. The spacer must be the same thickness all the way around to guarantee that the barrel of the diagonal is aimed straight down the barrel of the lens.

Step 5: Glue the Diagonal Onto the Cap

Take the Barlow lens off the diagonal again, so you don't get glue on it.
Put the diagonal into the hole in the cap. You'll want the part that holds the eyepiece to be outside, and the shiny chrome part of the diagonal on the inside.
It's important that the diagonal barrel is exactly perpendicular to the camera opening (otherwise, the image will suffer). Check the fit. On mine, there was no issue and the housing of the prism in the diagonal sat flush against the lens cap. But if it tilts, you'll need to shim it or file it till it doesn't.
Connect the cap to the lens and figure out which way you want the diagonal to be facing. For a 45 degree one, you want the angle to point away from the tripod mounting hole on the lens.
Mark that orientation. Check it. You won't be able to change it once the glue sets.
Glue the diagonal in place.
Let it dry.

Step 6:

Attach the lens element to the end of the diagonal (the end that will stick into the lens).
Insert an eyepiece into the diagonal and you're ready to use your new scope.

Step 7: How Did It Perform?

The performance of the scope was way better than I expected. I don't see any pincushion effects, stars are very sharp, and colors are fairly pure. I did notice a very minor yellow halo on one side of the moon. A higher quality Barlow may improve this.

It turned out to be an ideal instrument to show the kids pronghorn antelope in Lamar Valley, Yellowstone.

Pointing the thing with a generic tripod (no slow-motion controls) is pretty tricky. I tend to prefer 25mm eyepiece so the magnification is not too high, and the field of view is bigger.

I estimate the magnification to be about 32x with the 25mm and 80x with the 10mm eyepiece. I think the Barlow is doing a little more than 2x in this configuration, so it may be a bit higher. Since the clear aperture of the lens is about 65mm, I could probably go to a 6mm eyepiece and still get more out of the image, but my tripod would be extremely hard to aim at 130x.

If you make a telescope (as opposed to a spotting scope), one drawback of this scheme is that you can't rotate the diagonal. On an altitude/azimuth mount, this may not be a big deal, although when you look at things low on the horizon, you may have to lower the mount since you can't spin the diagonal sideways.
I doubt it will work well on an equatorial mount, unless you can spin the whole telephoto lens in its mount. Having said that, the Canon lens cap can actually go on in 3 different ways (120 degrees apart), so it may be an ok compromise.

I should also mention that although the lens is a zoom lens that can be as short as 100mm, it actually will not come into focus for distant objects unless the zoom is set between 200mm and 400mm.



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    13 Discussions


    3 years ago on Introduction

    Kenko makes them, they are $140. They support Canon, Nikon, Pentax, and Sony, and have a number of different eye pieces.


    4 years ago on Introduction

    Hi there great instructable!
    Would a cheapskate option be to track down a damaged film slr camera of the same mount and just chop off and then reseal everything but the mirror and viewfinder? The pentax K mount monocular converter costs a small fortune on the rare occasion that they come up on ebay but a K1000 where the battery corroded is dirt cheap.

    1 reply

    Reply 4 years ago on Introduction

    Cool idea. It will be a little weak on magnification though.
    I just looked through my canon 20D through the lens. The image is about 1x magnification at about 55mm. This means the focal length of the view finder lens is about 55mm, since magnification is just the ratio of the two lenses.
    That means for a 400mm lens, the magnification is ~7x through the lens. That's a little less than you'd get with most low end binoculars.
    But if you use a 2x barlow and a 10mm as I've described, then the magnification is 400mm*2/10=80x.


    4 years ago on Introduction

    Don't you need filters for that? I mean, for sure you can't watch an eclipse with that right? Or maybe... ?

    1 reply

    Reply 4 years ago on Introduction

    For a total solar eclipse, where the moon fully blocks the sun, you don't need any filters. If there's any bit of the sun showing, you'd need a filter to not go blind!


    7 years ago on Introduction

    did you try having the barlow on the eyepiece instead of on the diagonal? That could help with lenses that have back element further back and also adjusting for various lenses gets a little easier.

    1 reply

    Reply 7 years ago on Introduction

    I didn't try it but I'd be surprised if it works. The camera lens focus is inside the lens. I believe the barlow has to be inside the focal point. After the diagonal would be way outside the focal point.
    Let me know if you get it working. I'd be interested to see your solution.


    A much better (and easier & a lot cheaper) way to convert a camera lens into a scope is by using an eyepiece with an erecting element already built in. Then you can skip the diagnol and the barlow. A few years ago, before we had instructables, I wrote an article on how to make one like this. It was geared towards a Nikon audience, but obviously, the concept works with any brand of lens. It's still online here:

       - DC

    1 reply

    Reply 7 years ago on Introduction

    Wow, that's pretty cool. Nice work.
    Your link is broken and I couldn't find any erecting eyepieces available in the US anymore, but some of your UK links are still good.
    There are two advantages to the way I did it. First, on a spotting scope, it's nice to have a 45 degree prism, so it's a little more comfortable, or you don't have to raise your tripod so high. I'm 6' tall but my tripod isn't.
    Second, it is nice to be able to use a few different eyepieces depending on how far I want to look or how wide I need my field of view to be. I can even use my zoom eyepiece (when I buy one, its on my list!).
    I had all the other components already, so I only needed the diagonal, so the total cost was not so important for me.
    Thanks for the comments.


    9 years ago on Introduction

    But why would I want to turn an expensive telelens into a scope?

    1 reply

    Reply 8 years ago on Introduction

    If you only want to carry one heavy hunk of glass on an airplane, you don't have to pick between spotting scope and telephoto. It has way more magnification than a pair of binoculars (see the last page on performance).
    If you're headed to Australia, you can take pictures of the wallabies and then check out the tarantula nebula that night!