For many years filar micrometers had been unobtainable and although the occasional classical brass example does appear they tend to get snapped up by collectors and placed on the shelf. Over the last 10 years, however, a number of firms and individuals in the UK and USA have produced commercial instruments and there are at least two sources of supply at the time of writing. (See the references for further details.)
The RETEL micrometer is made in the UK from duralumin alloy and consists of a fixed and movable parallel wires and a PA wire at 90°. The movable wire is driven by an engineering micrometer capable of about 12 mm of travel and readable to 0.001 mm using the vernier. The PA circle is calibrated in 1° intervals and again a vernier allows this to be improved to 0.1°. The wires are made from artificial fibre and are 12 |im thick which means that for short focus telescopes a Barlow lens is needed to reduce the apparent size of the wires in the eyepiece. Spider thread is better in terms of thickness but it is difficult to fit and needs regular replacement. The man-made fibre is extremely durable - I have had no breakages in 13 years of continual use involving many thousands of individual settings.
The van Slyke micrometer is made in the USA from a solid block of aluminium and again features an engineering micrometer to drive the movable wire whilst a range of optional extras such as digital readout are also advertised. Unfortunately, as this was being written the micrometer has been transferred to the manufacturer's discontinued catalogue but was still available as a custom order.
A comparison between the two made by Andreas Alzner can be found on the Webb Society web page.3
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