The Jones Medial Refractor

 

Features:

• Color Free across the full visible spectrum
• Coma Free Diffraction limited flat field
• No field tilt unlike most TCTs
• Compact shorter tube compared to Schupmanns
• No optics near focus sensitive to dust
• Convenient focus position
• Protected silvered mangin lens
• Unobstructed fast design
• All spherical, no aspheres

The Jones Medial is a refractor using a single crown front element which is corrected by a mangin corrector as in the Schupmann family.   It is closer to the Honders Medial, however, as it has no intermediate focus making a lot shorter tube.  In fact when developing this design I started with the Honders Medial, tilting the lenses to avoid the obstruction using two Chief type correctors.  Using Zemax I found that only one positive correcting lens was needed as in the original Honders.  Tilting the meniscus lens, however, introduces field tilt.  To correct for the field tilt I changed the folding flat to a very weak convex which reduces the F ratio only slightly. The result is a telescope highly corrected like the Honders but unobstructed and an un-tilted field.

  Zemax Prescription

Chromatic focus shift

Construction:

The lenses were diamond generated, ground and polished using techniques I have in my videos: http://www.youtube.com/user/opticsed       The mangin lens is easy to test as both curves can be tested with the KE test although I used a Zygo interfereometer.  After getting the concave curve good you can test the convex side through the back to a good null as the SA is only .011 waves!  The concave curve on the front lens can be KE tested but the convex side isn’t a null when tested through the back with roughly ½ wave of SA and so I matched the Igram generated on Zemax but it could also be zone tested like a mirror with the KE test. I generated another piece of Pyrex with the same curve as the mangin convex to use the same tooling and cored the middle without testing, assuming the middle should be good.  The small field lens is an off-the-shelf OptoSigma coated lens.  The mangin is mounted in a 6 point edge flotation holder. It has three flexible aluminum clips that can rotate with black nylon screw heads to hold the lens.  The convex side of the meniscus was silvered and then protected with lacquer and paint.

If the tilt of the mangin is not right it will introduce prismatic color where a star is spread out into a spectrum which might correct for atmospheric dispersion if one wanted to do so.  Push-pull set screws are used on the corner to adjust this tilt.  The secondary is mounted on a thin aluminum bracket that can be bent by tightening the mounting screw to adjust the secondary in the Y direction and can be rotated by an eccentric tip screw that fits in a slot in the bracket.  Turning the screw rotates the bracket enough for alignment.

A light baffle made from aluminum flashing covers the secondary. (not shown).  The small field lens is mounted on a hinged plate which can be adjusted by turning a threaded rod near the eyepiece focuser.  It corrects for astigmatism.   I checked the mangin by using a 10 inch newt with a fiber optic source.

In the process of working on the baffle the scope fell off the mount falling to a concrete floor.  This crazed the edge of the mangin.  I made another lens but it fell off the test stand!  So I went back and used the first lens since the craze wasn’t deep enough to have an image effect.

Field lens holder and Focuser                     Secondary mirror

This is a very interesting design.  Roger Ceragioli mentioned this design has some similarities to a Hamiltonian-schiefspiegler on page369-371 in his book. That design used a mangin third lens that was either wedged or de-centered.  Performance wise it’s not as good as my design (I filed a preliminary patent disclosure in mid 2011 before his book was out).  In my design if you slow the f ratio down to F/15 and increase the BFL the spots get amazingly small for light across the entire visible spectrum.  So much so that the design can be scaled up to a 40 inch scope and still be diffraction limited across the spectrum!  All in all this design should obsolete the very good Schupmann design as it has a wider better corrected field of view, no field tilt, no optics near focus and a much shorter tube (about 56% as long).  It’s also interesting how aberrations are controlled in this design.

Longitudinal color is controlled by the power of lens A and B.

Lateral color is controlled by the power and spacing of lens C.

Spherical aberration and field coma are controlled by bending lens A and B

Coma and astigmatism from tilting lens B is controlled by the secondary and field lens tilt.

Field tilt is corrected by the curved secondary tilt.

Prismatic color is controlled by lens B tilt.

My scope is very portable and light weight and with its mid-tube eyepiece position I can sit down and view any sky declination.  It works very well as Zemax said it would.  In fact since it is a much shorter tube and has a better corrected wider field it should obsolete the very good Schupmann design.

Ed Jones