Building Lenses

[Nodda Duma]

The periskop is a symmetrical version of the landscape lens, meaning the prescription layout is roughly a mirror image around the stop. The landscape lens is a single meniscus in front of or behind the stop. Symmetry reduces the odd aberrations (coma, distortion, lateral color).

Spherical aberration is minimized by the lens shape.

Distance from the stop to the lens is chosen to compensate for field curvature. Typically the distance is set to flatten sagittal field curvature since it is less pleasing to the eye than longitudinal.

The ideal lens shape and distance from the stop can be calculated from the corresponding aberration formulae.

Alternatively, it takes only an hour or so to bang out an optical design in Zemax: 15 minutes to get the Periskop solution and 45 minutes of tinkering with better corrected variations before undoing and saving the Periskop solution.

Note as a landscape lens derivative, the design family is more appropriate for wider fields of view i.e. landscapes rather than portraits.

[Jim Jones]

Tim, the main advantages of the Periskop design are probably wider coverage, not better performance of a small distant subject like the Moon. A two element 2000mm achromatic telescope objective should give better performance than two 4000mm meniscus lenses used in the Periskop design. Surplus Shed doesn't list such an objective now.

[Tim Meisburger]

Thanks Jim. That makes sense, but I already ordered the lenses. If it doesn't work, I'm only out fifteen bucks (including shipping). Using the periskop design will at least increase speed, bringing me to something less than f/64 wide open. Nodda explained above how to calculate the distance between the lenses, but I didn't really understand it, so I'll just scale a diagram of a periskop (I'm guessing an inch and a half). Hopefully that will get me close enough to get an image.

[Nodda Duma]

Tim, you don't have to calculate the distance, just adjust the distance until the image on the ground glass looks "good" and pleasing to the eye. This will very likely be the distance for corrected sagittal field curvature. Pay attention to the off-axis image for this. Make sure to keep the distances from the iris to front lens and to rear lens the same.

If you knew the specifications of the lenses -- radii of curvature and thickness -- then I could model in Zemax and give you a starting point. But scaling from a layout is a good idea and will get you close. Hopefully you have a way to accurately adjust their positions without decentering.

[Nodda Duma]

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There used to be a lens designer hanging around the APUG forum, talking about lens building and offering up some designs, but I haven't been there in a while, so he may be gone.

That was me. I'm still around.

[mdarnton]

Yup, that was you. Thanks for showing up!