Checking out the Sigma 500 mm f/4 XQ mirror lens

Those of you familiar with this blog will no doubt recognize this troubling fondness I have for vintage camera telephoto mirror lenses.   I’m always on the prowl for that inexpensive yet high image quality compact mirror lens that will give me that telephoto focal length reach that I can throw into my knapsack for that upcoming African Safari vacation.  Recently, I discovered  in the late 1960s Sigma produced a high end (denoted by the XQ designation) 500mm mirror lens with a native f/4 speed.   That is a full two stops faster than the standard f/8 mirror lens and dictated a lens structure that was 6 lbs in weight, 8 inches in length and 6 inches in diameter.  Sigma thoughtfully designed in a carrying handle to allow you to manage that heft!

The lens has a minimum focusing distance of about 50 feet so in order to conduct MTF tests you have to use a T-thread extension tube to bring that down to a more manageable 10 feet, but this tube, along with a 2x convertor, was supplied with the lens is typically missing from vintage finds.  If you’re amateur astronomer like myself, you’ll have plenty of these pieces lying around.    And as an astronomer, I was particularly intrigued that Sigma decided to utilize a helical focuser housing a weak biconcave lens that internally focuses instead of moving either the primary or secondary mirror like some other less expensive lenses.  Optimal optical performance of a cassegrain reflector occurs at  only one fixed distance of separation between the primary and secondary mirrors whereas most mirror lenses use existing lens housing manufacturing tooling to move the secondary mirror in and out.  On disassembly of the lens, it became evident that Sigma chose not to follow the popular Maksutov design, but rather used the Albert Bouwer’s design that is also found in the Zeiss Mirotar.  The 500mm f/4.5 Mirotar lens goes for $10k nowadays unless you can find the East German version (Zeiss Jena) for about half as much.

History is a funny thing.  A Dutch spectacles manufacturer named Hans Lippershey is credited with inventing the telescope but Galileo is the name popularly associated with its invention.  Dimitri Maksutov patented his all spherical meniscus corrected cassegrain reflector design in 1941 but Dutch engineer Albert Bouwers actually predated Maksutov by a few months even while working in Nazi occupied Netherlands and the two did not publish until after the war.  The Bouwers design is particularly elegant (and simplifies manufacturing) because all surfaces have concentric spherical radii but suffers from a great deal of spherical and chromatic aberration.  Maksutov addressed this by departing from purely concentric symmetry such that one of the faces of the meniscus becomes more convex and the whole system is slow, at f/15 and greater.   Bouwers’ solution involved adding an additional weakly positive lens in front of the meniscus separated by an air space.  Correction of these optical aberrations will be challenging for this lens at f/4 and my first impressions were that the images were too soft for my liking.  Then I discovered the inner surface of the meniscus was coated by this terrible layer of haze (not fungus but I still don’t know what causes it).

The haze resisted removal with organic solvents, weak acid, mild abrasive pastes and even superfine steel wool.  It was evidently embedded in the surface glass layer.  The optics do not appear to be coated with antireflective coatings which would be the common finding of this vintage.  Although I have some experience in telescope manufacturing, I have none with glass grinding which is a dark art practiced by those who also fabricate their own telescope optics.  Still I thought I could try some very light glass polishing with fine 3.5 micron cerium oxide to remove this layer of haze without changing the figure or thickness of the glass … too much.

I needed make a variant of what glass grinders call a pitch lap to keep the polishing even across the entire meniscus surface and also preserve the surface curve.  And a way to measure the surface curve to detect if any appreciable changes had been made post polishing.  My pitch lap was made in my former dental office where I still see patients once a week.  I took an impression of the meniscus lens, made a negative of that and then coated the surface with some steel reinforced JB Weld epoxy and poured it up with dental gypsum stone.  This resulted in a hard replica of the inner surface of the meniscus that the actual lens could sit on and be ground against.  I protected the spotted secondary mirror surface by painting it with a peel away cleaning product called First Contact and then with a layer of cellophane tape.

A spherometer is used to measure the curve of an optical surface and can be a relatively expensive piece of analytic instrumentation.  But you can make your own with a dial gauge costing about $20 and some spare aluminum pieces to create a rigid tripod mounting system for it.

I managed to remove about 80% of the haze and could detect  no real change in the figure of the optics with the spherometer.  The image does appear sharper but when compared to my conventional Sigma 500mm f/4 telephoto prime, or even my coveted Zuiko 500mm f/8 mirror lens, the MTF analysis shows that the lens is not a particularly good performer.  I doubt that removing all the haze will make any real IQ improvement.