A DIY 177mm f/8 "Sort of" Cooke triplet for 4x5 format from off-the ...

This is posted on LFF as well. Not sure if I should post here or in apug's large format subforum, but I flipped a coin and it came up tails, so...

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.

There's a limit to what you can do with off-the-shelf optics so don't expect the same quality as a true Cooke. But that aside...


This covers a 4x5 format. Reference the layout for lens orientation (showing f/16 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 # LA-A Diameter: 2"
http://www.thorlabs.com/thorproduct....umber=LA-A

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

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

Airgap between lens 2 and lens 3: 4.0

Lens 3: Part # LA-A Diameter: 2"
http://www.thorlabs.com/thorproduct....umber=LA-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 Barrel is Delrin. That's pretty robust stuff. The opto-mechanical engineers I work with wrap my optics in it quite often for more demanding applications than this. It's a better choice than brass, to be honest.

Catalog optics are not conducive for making imaging objectives: they are just not the right shape or glasstypes. Typical use is for simple laser and telecommunications devices. It's no joke that I tried earlier in my career to do this and failed, and was only successful after a decade's worth of additional design experience. This little self-imposed challenge is kinda the optical design equivalent to making some complex mechanical device out of bar stock and sheet metal without access to a machine shop, except more complex.

Part of designing the lens is knowing what design type has a chance of succeeding and what doesn't. Petzval's are all about glasstypes, lens shape, and lens speed.. . It requires a greater selection of glasstypes than available from the catalog vendors, and a different selection than what's available. A Petzval designed from catalog lenses isn't possible.