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The £5 Micrometer


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The £5 Mk1 Micrometer.

Introduction

As you know my most active part of this hobby is observing, drawing and measuring double stars. The measuring I have completed so far is based on using the eyepiece as a ring micrometer and a stop watch to time the stars transit across the field of view.

The main drawback with this method is the extensive calculations you have to undertake to turn the timings into a measure of a stars position angle (PA or theta) and separation (Rho).

I will continue to use this method so I can refine my accuracy.

There is an alternative method that is similar but is significantly easier on the calculations; it is called the chronometric method.

This method requires 2 elements.

An external position circle or dial and a thread/wire mounted at the focus of the optical system.

This design mimics the filar micometer design in using a thread/wire at the focus.

A good quality micrometer is very expensive and exceedingly rare although I have been offered one on loan. I had to turn this offer down as I could not risk my children damaging such a rare instrument.

As an alternative I decided to build my own external PA dial and internal wire.

The Design

The design is very simple. I purchased a 360 degree protractor from WHSmith (£1.99) and photocopied it onto card. Although the card is orange this was more to do with what I had available rather than a deliberate decision.

I took these photocopies and got them laminated at Prontaprint (£3.53). This was to protect it from the damp at night.

Image1

I measured the central point and with a compass drew 2 concentric circles. The challenge I had was the fact that the top of the 1.25” eyepiece holder is wider than the main body. To overcome this I cut out the inner circle and cut slits radialy to the outer circle. This allows the dial to go over the larger section and then grip the narrower section.

I cut out the pointer from the second dial I had and repeated the exercise. I then placed the 2 dials over the eyepiece holder and held them in place with a plumbing seal that was the perfect fit.

Image 2

This is the dial insitu on the scope.

Image 3

This method uses the wire to measure the difference in RA by timing the components as they cross the wire. I have ordered (and now received) a screw in cross-hair eyepiece (£11) from Sherwoods photos.

Before this arrived I cut a thin strip of card from the waste of the laminated card and placed it across the eyepiece.

This is my 10mm Plossl that is now my experimental eyepiece. With a Barlow the card is about 40” in width but as you are only using it as an occulting bar to measure when a star crosses it this is not a problem.

I have made some initial measurements using this system and they are fairly good.

The Process of Taking Measurements.

Using this system measuring double stars is much faster and easier than the ring micrometer method.

First you find a star near the equator and align the internal wire until the star runs along it. This gives you West (270) on the dial. You rotate the dial so it is showing 270 aligned with the wire.

Centre the double star you wish to measure and rotate the wire until it is parallel with axis of the star. If you have a motor drive just turn the wire until the wire covers both stars. If you don’t you rotate the wire until the stars are occulted at the same time by the wire as they cross the FOV.

Rotate the external pointer to align with the wire and this gives you the PA.

Rotate the wire to a North-South orientation and turn off the drive make a note of the time difference as the stars cross the wire.

The dial has given you the PA and to work out the separation you use the following formula

(15.0411xt(time difference)xcosd (declination in decimal)/sin PA

Here is my example of SHJ 40 AB (52 Tau)

In 1999 it was measured at 49.2” and a PA of 256

The declination is 27.2103 which in decimal form is 27.3508

The PA I calculated was 257 and the time difference was 3.7 seconds.

The calculation is 15.0411x3.7xcos27.3508/sin257 = -50.73. The minus sign shows the secondary is to the West of the primary.

As an initial attempt I am pleased to be within 1 degree and 1.5” of the last know figure. Getting much more accuracy is I believe going too be far more difficult.

I am already working on the Mk2 version using a larger protractor.

Cheers

Ian

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