Dividing this would have been an interesting exercise !!

This job would be very simple on a large rigid jig borer (Loewe, Brown and Sharpe etc). Any watch, clock, gauge or instrument factory in the US in the 1920's / 1930's making their own precision dies for stamped parts would have such a jig borer. Builders of the telescope discussed would likely know of, or be able to find a shop to have the part made. Hole positioning accuracy on the Loewe jig borers was +/-.0002" or better, to XY coordinates, not rotational dividing. (position tolerance was often closer than .0002" on one in a factory toolroom I worked in)

I don't see it as a circular dividing problem at all, in fact you could not do it by dividing since the pins are not radially equi-distant as dividing would give.

The only consideration is that the wires are equi-distant linearly - the pins are only off-set radially due to space constraints. A jig bore would make easy meat of it, or a mill and modern DRO.

EDIT: as a matter of fact the easiest way to set the wires at equal pitches is to use a slitting saw of the same thickness as the wire diameter and cut alignment grooves in the raised annulus. The pins are then inconsequential.

Edited By Pete Rimmer on 15/08/2021 15:29:09

As Pete Rimmer says, it is the parallel spacing that matters, the radial aspect could have been taken care of simply with a scratch made with dividers at two radii.

.

I think that’s pretty-much where we came in, Pete

MichaelG.

At the risk of exhibiting my lack of understanding, I think that making a template in a rectangular frame then transferring the spacing to the circular frame might work.

David

I've just tried drawing it and confirm the small pins aren't spaced by equal angles. On my metric version, the angle between the top two pins in a quarter is 1.01069° stretching out to 1.63147° apart at the bottom. So as Pete says, not a dividing problem. The same applies to the adjusting pins, which are offset as Michael said, and staggered on opposite sides to allow space for the screwdriver.

Looking closely, each pair of adjusting pins controls three spans between two small pins. If I were making it, I'd have two adjusters, one length of wire and more small spacing pins. I guess that layout was found wanting because the wires sag. Breaking the grating wires into spans of three allows a lot scope for equalising the tension.

Also, as Michael suggested, seems considerable trouble has been taken to put the pins neatly on circles. As only the height spacing matters, perhaps it's been done to emphasise the staggered adjusting pairs. Must have been annoying to accidentally twiddle adjusters on different spans.

Another mystery is the detector originally used with the grating? The spacing suggests infra-red or lower, which is invisible to the human eye. Sensitive semiconductor IR detectors make the job easy today, but weren't available when the grating was made. Considering the technical limitations of a hundred years ago, a lot of very clever experimental science was being done.

Dave.

Scribing the circles and drilling on an X Y table would do it. As has been mentioned onlt the Y spacing is critical to the grating the X can be picked up on the scribed circles.

Our workshop used to make Area Detectors for X Ray Crystalography diffractometers at one time. These consisted of a large number of gold wires (around 1 to 2 thou thick) soldered to cct board in a frame. The first end was attached and then the wire tensioned by a small weight and the other end soldered on. Alignment to the centre of the pads was by eye + micriscope. They were used in pairs at right angles to one another.

I have made wire diffraction gratings for telescope mirror testing by the following method.

Take 2 rectangular pieces of blank cct board and fix together with small screws and nuts in the corners. Cut an aperture in the middle of the blanks. File the edges so they are flush. Affix 2 wires of the chosen gauge to one edge close to a corner. Wind the wires around the blanks so that each wire is closly touching the other and that they don't cross at any point. Fix off the ends. Now unwind one of the wires. Araldite the remaining wire to the edges of the PCB's and allow to set. File the edges to separate the frames and you will find you have two diffraction gratings that are surprisingly accurate.

regards Martin

I notice that bare copper wire is specified and that the device goes over the end of a telescope.

I have observed that bare copper oxidises gradually would said oxidation affect the long term operation as a Spectrograph?

Edited By V8Eng on 15/08/2021 18:44:11

I am pretty certain that this is not a spectrograph grating. There are too few wires, which would give a very dim image; and the spacing is too large for optical wavelengths which would make the first order spectrum very close to boresight. As Martin and another previous poster have suggested this is most likely to be an aid to mirror or lens testing - Martin maybe you could describe how the gratings you made were used?

Maybe like this:

**LINK**

Superb practical demonstration of diffraction [teacher’s version] , here: **LINK**

https://youtu.be/71Rp-jG6Eek

MichaelG.

.

Edit: and here’s the student version: https://youtu.be/MZktgCWvHlE

Edited By Michael Gilligan on 16/08/2021 10:04:13

Blimy, now you are asking, its a while ago. Essentially its a Ronchi test. From memory I used a green LED through a pinhole with the grating the other side of the pinhole. The device is placed at or near the centre of curvature of the mirror and viewed through the grating which etends above the LED mount. Interference generates a pattern of light and dark stripes across the surface of the mirror. For a perfectly spherical mirror the lines will be straight. It's quite good at detecting turned edges and hollow centres. For better deternination of shape the knife edge test with an appropriate mask works well.

The thing that delighted me when I made my 12" mirror was that you could generate and refine a surface to a specific shape to better than 50nm just using what is essentially finely graded mud, your hands and very simple devices made from wire, razer blades and a few other odds and ends.

regards Martin

Rewinding to 1913 … which pre-dates the object in question: **LINK**

http://articles.adsabs.harvard.edu/pdf/1913MNRAS..74...50C

9 pages :

On the Application of Parallel Wire Diffraction Gratings to Photographic- Photometry.

MichaelG.

.

Edit: although made for a much larger telescope, the description of the wire tensioning [section 5] seems relevant.

Edit: and here is the cross-referenced article about 262 stars:

http://articles.adsabs.harvard.edu/pdf/1913MNRAS..74...40C

Edited By Michael Gilligan on 16/08/2021 11:00:09

Not my field at all but the function section in Michaels original link describes it as follows:-

This form of diffraction grating would be put on the end of telescope and produce a spectrograph of whatever was observed with the telescope.

The distance between the wires (about 1.5 lines per mm) suggests it was used for microwave or infrared radiation.

Edited By V8Eng on 16/08/2021 10:33:18

Thanks Martin. The Wiki article also mentions that it can be used for lens testing. The device seems to meet the requirements for a Ronchi test grating pretty well.

Found it

.

MichaelG.

.

Ref. **LINK**

https://books.google.co.uk/books?id=oCoDAAAAMBAJ&pg=PA71&lpg=PA71&dq=harvard+observatory,+wire+diffraction+grating&source=bl&ots=YkPLjWwgZh&sig=ACfU3U0dH5d3o5aFV8F_QrWzgrQkUwm5DQ&hl=en&sa=X&ved=2ahUKEwjorabF9bXyAhUPiFwKHXwNDBkQ6AF6BAgTEAM#v=onepage&q=harvard%20observatory%2C%20wire%20diffraction%20grating&f=false