Violet Haze, A 90mm f/13 Apochromat

I had wanted a high quality APO refractor for some time. Mostly for photographic use. Opportunity presented itself when Roger Ceragioli offered me a 90mm telescope he had finished the year before and was willing to sell. Working for the Steward Mirror Lab, Roger normally grinds very large optics, things like secondaries for six to eight meter telescopes. But as a hobby he makes somewhat smaller telescopes. This particular lens set had won him a merit award at RTMC in 2002. I had previously seen this telescope and after some negotiation we settled on a price.

The lens triplet is exquisite, providing absolutely perfect airy disks at high power. The photo below shows an example of the out of focus image of Antares taken with the telescope. Pulling out my copy of Suiter’s Star Testing Astronomical Telescopes shows nearly identical images for the ideal diffraction pattern. No wonder the ‘scope won a RTMC merit award.

Photographically it has proven to be almost perfectly free of color, corrected across the spectrum. There do not seem to be any detectable UV or IR halos around bright stars. This is partly a result of good design, and aided by the long focal length of f/13. No field flattener is required, with pinpoint stars across the focal plane.

The focal length is just over a meter, 1178mm to be precise. This has an added benefit, a field of view of 66 x 44 arcminutes. This is just about perfect to fit the Sun or Moon on an APS-C sized sensor, with some room to accommodate poor tracking. With a Canon 20Da this gives a plate scale of 1.12 arcseconds per pixel.

The tube Roger had mounted the lens set in was good for visual use, but not strong enough to support a heavy CCD or DSLR camera. Thus I set about machining a new tube for the scope. Cut from solid aluminum the frame is heavy and VERY solid, typical Cooper over engineering. Tube rings support a Losmandy compatible dovetail plate and an upper plate for mounting a guide scope or another camera. The rear sports a Feather Touch crayford focuser. Contrast is good thanks to a full set of baffles down the interior of the tube.

With the Losmandy compatible dovetail I can mount the tube on either the Losmandy G11, providing a tracking mount, or the smaller tripod build to accommodate Primero. The long tube requires a high mount to use comfortably. Fortunately the adjustable extensions built into the legs on the small tripod allow the mount to be raised nearly a foot.

As with all telescopes I built I have named it, as is tradition across the amateur telescope making community. The name ‘Violet Haze’ is a play on the bright violet anodizing that has become a trademark. Standing high on the Losmandy mount the scope is visible across the observing field. It is also a backhanded reference to the complete lack of a violet haze around a bright object when looking through this truly apochromatic telescope

A few design details…

Shown to the right is a close photo of the collimation screws at the rear of the lens cell. Note the capscrew that serves as the ‘pull’ and the setscrew that serves as the ‘push’ in this traditional push-pull arrangement.

Also seen is the closed cell neoprene foam that seals the gap, but allows adjustment of the lens cell.

To provide the rock solid mounting that is required of a photographic scope a heavy split ring holds the tube securely. The rings are approximately 0.5 x 0.5″ in cross section and connect to 0.25″ thick plates top and bottom. The lower plate being exactly three inches wide with 30 degree beveled sides allowing it to fit into a Losmandy dovetail mount.

The tube started life as a 3.5″ ID x 4.0″OD 6061-T6 aluminum tube. This was machined down to 0.1″ thick along most of its length, but the ends were left near full thickness to allow mounting of the flanges for each end. The design would have been a little simpler if I could have machined a slightly longer tube. The bed of the lathe I had available would only accommodate a 36-inch long piece, and perhaps a bit more at a stretch. Thus I modified the cell and rear focuser mount designs to absorb a little of the tube length.

Note the violet extension tube to the rear of the focuser shown in the photos. The tube has a 2″ slip mount at the front and a Nikon F-mount ring at the rear converting the scope into a 1178mm telephoto lens.

This tube was machined with the rest of the scope. When the focal point was calculated for use with a 2″ mirror diagonal the extension tube length was calculated to match. The tube length allows for the 46.5mm (1.83in) that a standard Nikon F-mount lens requires from the image plane to the lens mounting ring. Thus any film or CCD camera with the same back focus requirement will work. The back focus of a Canon EOS camera body is slightly different, 44mm (1.73in), a difference small enough to be compensated for by the focuser.