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GRO Instrumentation -
The Filar Micrometers
The Unitron Filar Micrometer like most others both new and used are nearly all meant to be attached to microscopes. They have no independent light source because a visual microscope is typically equipped with a sub-stage illuminator allowing the filars (reticules) to be seen quite plainly against a bright field.
In order to use a filar micrometer at night to measure double stars in a dark field there must be an independent light source (preferably red) that can be adjusted, it should be diffuse, relatively uniform and allow the filars to be seen clearly. In order to provide a gentle red glow to back light the filars and dim enough to not interfere with the visibility of the stars to be measured a beamsplitter must be inserted in the light path with an AOI of 45°. I have chosen a 25mm x 36mm x 1mm thick 10:90 beamsplitter from Thorlabs.
The 10:90 is the ratio of the transmission to the reflectance of the beamsplitter, in this case 10% of the light from the illuminator is reflected up to the micrometer reticle and 90% of the light from the telescope is transmitted to the eyepiece.
The illuminator is assembled from Thorlabs components utilizing a 600 grit polish diffuser, a 1.0” diameter lens tube and an Agena adjustable cordless red LED illuminator made specifically for crosshair eyepieces. The separation of the illuminator LED and the diffuser has been optimized to present a 1.0” diameter circle of light directly on the 1.0” diameter diffuser creating a very uniform “glow” that is in turn directed at the 45 degree face of the beamsplitter. Looking through the eyepiece of the micrometer with the star diagonal capped off all that can be seen is a very nice red glow that can be adjusted to be as bright as necessary or invisible as required. Both the moveable filar and the stationary incremental filars can be seen quite clearly even at the dimmest setting right down to the numbers indicating the position of the moveable reticle. It is a very obvious variation on the original Unitron Illuminated Diagonal/Filar Micrometer system they sold as accessories circa 1970. I found a copy of the Unitron 1970 catalog on Neil English's "World of Refractors" website in the "Classic Telescope Archive" and included a copy of the page with a picture of the original Unitron items next to my completed tribute. I did however choose to use a far more robust 2.0" star diagonal as a foundation for the illuminator rather than the 0.965" Unitron used but I did attempt to match the overall Unitron design as well as the painting scheme with most of the parts painted gloss black and the illuminator housing a metallic silver matching the Agena exactly. I think it gets pretty close to looking like the original if the overall size is not considered. The position angle circle was rather small and reads from 0 - 360 degrees divided into two-degree increments. It is nicely equipped with a vernier allowing it to be read easily to one-degree and with the help of the loupe I installed, measurements of less than one degree can be achieved.
It was “fairly” simple to build, most of the parts being made from 1/4" and 1/8" model-makers plywood. The diffuser, beamsplitter and lens tube are from Thorlabs, the 2.0” star diagonal is from OPT chosen because it could be disassembled and modified to allow the installation of the illuminator box. The Unitron Filar Micrometer was on the verge of becoming "surplus" at the University and was rescued just in time but I have seen them advertised on ebay for ~ $100.00 or so. The rest of the components can be purchased for < $200.00 including a sheet of 1/4” thick model maker’s plywood, I found a very nice selection at our local ACE hardware.
Anyone interested in how our astronomical predecessors began the gentle but necessary measurement of double stars recording their relationship to each other as the years passed as well as the height of solar prominences in H-alpha, the size of sunspots, the length of lunar shadows and a few other interesting projects might find this to be a more than worthwhile and satisfying project, I know I did.
I am a consummate “gadgeteer” and just enjoy tinkering with optical components and mechanical widgets.
I scour the antique shops and flea markets, check the Cloudy Nights classifieds and Astromart as well as EBay and Amazon for “goodies”.
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