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).
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
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