The "Heath Monochromator" -
Philosophically and Practically a True Academic/Industrial Team Project
 The "Heath Monochromator" project started in 1964 as part of my post-doctoral work with Howard Malmstadt at the University of Illinois, Urbana and ended as a consultant for the Heath Co. around 1966. At Illinois, I was designing a spectrometer for research to begin in the fall at the UW, Madison, when it became evident from looking at a balsa wood model I had made that parts of the same design could be used in many undergraduate teaching labs as the core of a complete spectrophotometric system, both solution molecular and flame atomic emission and absorption. The core of that imagined system was the short-focus, bench top-sized Czerny-Turner instrument that came to be known later as "The Heath Monochromator".

The initial focal length of 0.35 meter was set based on the whole instrument ending up half the size of a lab bench at the U. of I. Noyes lab! Given that, the grating was specified at 1200 grooves/mm and slit widths to be adjustable down to 10 microns so that a slit-width limited resolution of 0.1 nanometer could be achieved while still allowing first order scanning through the visible spectral range of a 1P28 photomultiplier.

The sine bar design for grating rotation was mine, based on literature review and a personal consultation with Mr. Ernie Davidson of ARL instruments at a Pittcon meeting. The slit designs were from Jack Haynes of Hansvedt Engineering, Urbana, with whom I worked for two years on many other mechanical aspects of the instrument. Thus use of a stepper motor to drive the grating, synchronized to a stepper driven chart recorder was the idea of Dr. Christie Enke, then of MSU, Lansing, MI., and implemented by Wayne Kooy of the Heath Co.

All of the mirrors were made from scratch by Bill Herdman of 3B Optical in Gibsonia, PA. For many years, Bill cut and coated optics for me, ranging in quantities from one 16" sphere to a couple thousand 2" parabolas. The gratings were supplied in quantity by the Jarrell-Ash Company, Waltham, MA.

Howard coordinated the academic side, Neil Shimp of the Heath Co. coordinated the industrial side, and Jack and I handled the manufacture and design issues of what became an outstanding example of a true team project.

 The optical schematic (above right) and inside picture (right) show the great care with which these early designs were developed, largely by Jack Haynes, but with my input. The use of cast aluminum supports was done so the finished and optically-aligned product could be shipped direct from the Heath Co. in Benton Harbor, MI., to a customer without the need for field installation. Massive entrance and exit slit side support plates allowed the monochromator to be sandwiched into other instrument parts without special optical alignment procedures. The long sine bar allowed a 32 tpi drive screw to be ground at sensible cost. The optical path was folded with relocatable folding mirrors so that either input or output could occur sideways or straight on.

The grating could be flipped in its mount (difficult) or negative diffraction orders used (easy), so that the entrance and exit slit sides could be reversed.

The whole design worked wonderfully well, as long as it was initially carefully aligned at the Heath factory. I worked out that procedure using an early model HeNe laser, several plumb-bob, fluorescent, white strings, and a Pen-Ray® Hg lamp.

For a monochromator to work, the grating equation must be superimposed onto the physical alignment of the grating itself. This in turn requires that the grating be essentially perfectly perpendicular to the "zero-order normal" as shown above here. Also, the rulings must be perfectly parallel to the axis of rotation of the grating and to the vertical entrance and exit slits.

This alignment can be achieved by pitch and tilt adjustment screws as shown at the far left. Exact grating rotation can be achieved through the use of precision ball bearings. All of this was done with a minimum of cost by superb engineering design of the Heath and Hansvedt engineers working together with Jack Haynes and I.

Lots of design, alignment, and fabrication work went into the sine bar grating drive system. The arguments against a sine bar drive are that it "wastes lead screw length" and "traverses too slowly". We had the lead screw ground at 32 tpi in quantity and cheaply, so wasted length was not an issue. Jack Haynes, Neal Shimp, Howard Malmstadt, and I collectively designed the two-motor, poker-chip clutch mechanism in a late night session so that wavelength alignment could be maintained while switching between fast slew and slow, stepper scan motors. We made the sine bar length adjustable for factory calibration purposes, and then firmly locked in place for use.

Undoubtedly, the two-sliding-pins idea of Jack Haynes was a major breakthrough for inexpensive fabrication. Prior to this, sine drives used sapphire ball ends sliding across the surface of ultra-precise optical flats. They were very expensive. Jack saw that two precision ground rods, cheaply made as hydraulic piston parts, would provide sufficient sliding smoothness if the sine bar were long enough. By mounting the grating on two, angular-contact, back-to-back ball bearings, such a long lever arm could be supported. The result was a nut carrying a drive pin, with a driven pin making point-to-line contact with it (see the magnified inset in the picture at the right).

The nut was driven by the lead screw. The whole nut assembly was prevented from rotating by a guide rod. The nut itself was about an inch long so that a large number of threads were in contact with the lead screw all of the time, averaging out any local variations in internal surface roughness or pitch.

It all worked wonderfully well.

In a short focal length monochromator with a grating of relatively low number of grooves per mm., the resolution of the instrument is mostly determined by how well the optics transfer a geometrical image of the entrance slit to the center of the exit slit at any particular grating angle. In this monochromator, the burden of assuring geometrically exact image transfer was put on the mechanical alignment of the slits and mirrors.

The slits are built on a precisely machine mounting plate, which starts the optical-mechanical reference. The entrance and exit slits are ganged together with a precision chain drive, as shown at the right. The alignment starts by setting the slits vertical to earth normal with a micro plumb bob and mercury lamp. Then the entrance slit is made narrower than the exit slit. Mirrors are adjusted until the entrance slit image is in between the jaws of the exit slit. Then the exit slit is closed until its width matches that of the entrance slit, The drive sprocket connecting the two slits is then locked in place.

In an alignment procedure like this, the collimator mirror and camera mirror are already aligned, and the grating is at its zero-order position, all determined relative to the position of the entrance slit. This instrument plane is used to define all of the other angular positions, as well as grating and mirror tilt.

Clearly, the slit mechanisms are critical to the performance of the monochromator. Commercial bilateral mechanisms cost in the vicinity of $900 (in 1964 dollars!). Since the entire monochromator was to retail for $1000, their use was out of the question. Jack Haynes revealed his mechanical genius by designing an excellent true bilateral slit assembly that could be made for $15, yet performed as well or better than the higher priced commercial units. He did this by taking advantage of the fact that anything placed on top of a Parallelogram whose opposite sides pivot about its top and bottom apexes will traverse a parallel trajectory to its initial position (top illustration). Slit blades placed on top of two parallelograms pivoted in opposite directions by equal amounts will displace from a common bisector remaining parallel. (bottom illustration). Slight vertical motion is inconsequential.

The slit blade parallelograms are made by cutting a "keyed rectangle" out of their center, with the key holes coming very close to the edges. These are the flexible hinge points. It is remarkable how strong the blanks are after they have been heat treated, even though they are very thin at the hinge points.

The slit jaws are made from the machined blades by first grinding the edge to a sharp taper, and then "razor stropping" each one until any nicks or gouges in the edge have been removed.
A cam is placed against the side of the slit blades. A gear rotates a screw in a long nut to push against a roller on the bottom of a bar to which the cam is fixed. This moves the parallelograms and near their top flex joints and bilaterally opens the slit jaws. It is a brilliant idea!

Possibly one of the most challenging aspects of the monochromator project was its geometrical alignment. All of the components had to be made to follow the "zero order pathways" as diagrammed above. Also, the two slits had to be set to an identical width, both absolutely and with respect to each other. It took most of my first year at the UW to get these procedures worked out, using a combination of a (then new) HeNe laser, Hg germicidal lamp, fluorescent strings, and plumb bobs.

The most interesting step was using the HeNe laser diffraction pattern to set the slit widths. This was done by measuring the valley-to-valley width of the first diffraction maximum of the 6328 line of the laser as projected on an index card placed over the collimator and camera mirrors. The method is shown in the adjacent diagram.

The laser was placed to illuminate either the entrance or exit slit, and an index card with a mm. scale printed on it placed in front of either the collimator or camera mirror respectively. The slit was then narrowed to 0.01 mm. as indicated by the positions of the valleys of the first diffraction maximum. Another set of marks was made for a width of 0.020 mm., and a third for 0.0040 mm. These three settings were used to calibrate each slit by adjusting the position of the pusher end on the parallelogram sine bar mechanism (see above).

The two slits were ganged together by locking the gear on the shaft that turns the slit pusher rod. A very precise, plastic chain then tied the two slits to a mechanical counter on top of the monochromator casting. The procedure for the slit calibration was mine, based on single slit diffraction experiments I did at Illinois while taking a physics minor. The slit drive wheels and pusher, and the ingenious slit coupling chain, all came from Jack Haynes.

The slits still had to be tilted to be parallel each other and to the grating ruling lines. This was done with a jewelers eyepiece and Hg lamp. When they were all parallel, the Hg spectrum would lie parallel to the monochromator base over two diffraction orders (+ and -) and the image of the entrance slit would fill the exit slit uniformly as the Hg green line at 5461was slowly scanned. A bit of fussing around could assure this.

Finding the center of the round, reflective mirrors and the plane, reflective grating, in the dark, was a challenge. The horizontal center of all three of these elements had to be located accurately since this determined the "zero-order" configuration of the instrument, and, thus, the zero wavelength setting of the sine bar. The vertical center was not as critical, since all this did was assure that the full apertures of the grating and camera mirror were illuminated.

To locate this center, a small nick was made in the horizontal center of the aluminum collimator and camera mirror holders with a sharp knife. Then a white thread that had been washed with soap and had a small lead blob attached to its bottom was allowed to hang over the edge of the mirror holder as a plumb bob as shown at the right here. The same was done for the grating.

When the HeNe laser spot hit the string, as mirror angles were adjusted, it glowed brilliant red. At any other time, the laser spot was faint. By adjusting the plane side mirrors from each slit, and then adjusting the angles of the camera and collimator to hit the center of the string hanging in front of the grating, an almost perfect zero-order configuration could be achieved. Jigs to hold the laser perpendicular to the entrance and exit slit side plates made it possible for technicians at the Heath Co. to align every monochromator quickly and easily this way.

There were accessories made for the monochromator that were not successful because they were quickly made obsolete by research that led to improvements in the competition's hardware. An example is shown at the left. This is the flame/AA unit, which should have sold well because of its combination of flame emission, atomic absorption, and atomic fluorescence features. But, it was based on the use of a Beckman® total consumption burner. This was made obsolete by the slot burner, which use air/aceylene rather than oxy/hydrogen as fuels. Another problem was that atomic fluorescence never became a popular technique, and flame emission was relegated to a niche set of alkali determinations. AA became the standard "inexpensive" method for trace analysis of metals, and that soon used only a low resolution, high aperture monochromator.

This was all unfortunate, since a great deal of engineering and design went into this accessory.

Inside and outside views of the assembled and aligned monochromator are shown in the photographs below, left and right. There are two features of the monochromator that have been obsolesced by time. One is the use of mechanical counters for reading out the wavelength and slit widths. These could now be encoded and presented digitally, although there is nothing fundamentally wrong with the mechanical approach. A deeper problem is the use of discrete transistors in the stepper motor drive. IC's would make this much simpler today.

Some 1200 of these monochromators were successfully made, aligned, sold, and shipped both as single units and as part of a larger spectrophotometric system before the Heath Co. sold the design and rights and went out of the spectrometer business. It is my understanding that today, about 40 years after the first balsa wood model was struck, these monochromators and their associated spectrometric accessories, are still being used in some of the educational environments for which they were first envisioned and designed! I learned an immense amount about what it takes to market an instrument by being part of this excellent team.