A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can build

Yesterday was kind of slow at work, so I played in Zemax while I worked on some other stuff. Basically I was wondering if I could design a Cooke Triplet in a reasonable large format focal length using lenses you can buy out of a catalog. I figured some intrepid DIY'er might be interested in the results and maybe put one together. The lenses are all available from Thorlabs.

Of course, keep in mind that "you get what you pay for" :) There's a limit to what you can do with off-the-shelf optics. But that aside...


This covers a 4x5 format. Reference the layout for lens orientation (showing f/8 aperture setting).

Attachment 129094

Focal length: 177 mm.
Sharp on-axis, soft off-axis at f/8
Should be pretty sharp at f/16.

Lens 1: Part # LA1384-A Diameter: 2"
http://www.thorlabs.com/thorproduct....umber=LA1384-A

Airgap between Lens 1 and Aperture stop: 9.0 mm
Airgap between aperture and Lens 2: 2.0 mm

Lens 2: Part # LD4735 Diameter: 1"
http://www.thorlabs.com/thorproduct....tnumber=LD4735

Airgap between lens 2 and lens 3: 4.0

Lens 3: Part # LA1050-A Diameter: 2"
http://www.thorlabs.com/thorproduct....umber=LA1050-A

Back focus is ~156 mm.



Blacken all lens edges with Speedball Black India ink (I forget the part #, you can get it at a crafts store like Michael's).

Notes for lens 1: Mounting should allow for a clear aperture diameter of at least 35mm.

Notes for aperture: The aperture should provide a maximum 22.2 mm for f/8. For the aperture, you can use Waterhouse stops or an off-the-shelf aperture such as Thorlabs D25S: http://www.thorlabs.com/thorproduct.cfm?partnumber=D25S Make sure you maintain the 2mm airgap.


Notes for lens 2: The fused silica material acts as the "flint" I couldn't find a larger diameter lens like this, which limits the f/# to 8. When mounting you need to provide for a clear aperture diameter of 23mm or you'll get vignetting. This gives you about 1mm edge mounting thickness which is sufficient to mount the lens. Alternatively, you can edge bond in a tube with RTV *not* hard epoxy.


Notes for lens 3: Mounting should allow for a clear aperture diameter of at least 33 mm.


The design consists of two outer plano-convex outer lenses (the positive crowns) and an inner equi-concave lens (the negative "flint"). Finding a suitable flint was tough because catalog lenses are made out of either N-BK7 or N-SF11 and the combination of those two is not conducive to color correction. The fused silica of the chosen lens seems to work well, but performance does suffer. A custom lens would help performance quite a bit, but I'm limiting this to what you can buy off-the-shelf.


Blacken the inside of your barrel.


Good luck, have fun, and share the results!


Regards,
Jason

P.S. Feel free to ask questions

Kewl!! :)

Interesting, thanks.

I have a number of LF triplet lenses, including a couple of modern Multi Coated Geronars, however the 3 TTH Cooke triplets I own are quite different - the element are surprisingly quite thick and they are large lenses (length & weight) for their FL and aperture.

Ian

edited the thread title to clarify this is a build-your-own-lens project.

Well, this is very exciting!
Any tips on barrel material?

Quote:

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Originally Posted by Will Frostmill

Well, this is very exciting!
Any tips on barrel material?

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Anything will work that will hold them in place without blocking the optical path and which allows you to mount to a lens board. I once mounted a laser beam focuser in a pair of rolled-up notebook paper tubes. Had a sliding focus and everything. Only cost me about 3 swear words.

There are lens mounts available at thorlabs, Edmund optics, etc, but they are a little pricey. Since they're 1" and 2" diameter lenses, maybe you could make the barrel out of standard PVC pipe. Or look on McMaster-Carr's website. If it were me I'd go by Home Depot and dig around or root around in the basement.

If you have access to a lathe then you can of course machine up a barrel with lens mounting steps and retainer rings. Allow about 0.005" clearance for the lens diameters (so the lenses don't get smooshed when the metal contracts at lower temperatures). Paint the inside with flat black!

Thanks, that's a really, really, comprehensive answer. Particularly nice that you added the bit about the 0.005" clearance.

Sounds like may be a project for 3D printed mounting plates and PVC tubes...

If you can make a 3D printed mounting plate then you can make a 3D printed lens barrel.

Hi, thanks for putting the effort into this design! Your description made me curious what it's like to design a lens when constrained to only off-the-shelf parts. Does your software allow for automatically varying the choices of elements from a preselected catalog, or do you have to pick the elements and then just vary the distances? I've spent the last few evenings working in OSLO-EDU trying to get some intuition for how lens designers work. Reading through some of your other posts it seems like you're a great resource for us here, thanks for joining!