Flamsteed Astronomy Society

“Webs and bubbles” by David Westcott

— Apr 21, 2008

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Flamsteed member and ROG Tour Guide Extraordinaire, David Westcott, is a retired Scientific Instrument Maker.  When Jane found out about this, Dave offered to enlighten us on the strange practices he once employed to make graticules and levels for the instruments ...Yes Madam, it is legal ...   and quite fascinating.

Firstly ‘Graticules’ — these are the ‘sights’ or ‘cross-hairs’ you see when you look through the eye-piece of an instrument or telescope fitted with measuring scales such as micrometer screws.   Graticules were made from ...   spider’s web !!    How, pray?  (or should that be ‘prey’ ?)

1.  First catch your spider !  This type isn’t common in Greenwich but can be found in shipments of bananas.  A common or garden spider will suffice.

2.  Prepare the ‘Former’ — take a postcard sized card and cut out the middle rectangle.

3.  Coat the top and bottom with thinned glue (Secotine)

4.  Get the spider to spin the web.  How?  Drop it from your finger!  (Apparently, a job for the ladies).

5.  The spider and its web are transferred to the prepared card.  The web is wound on to the card letting the spider drop at an even rate to get constant thickness of web.

6.  Applying the web.   The graticule mount is marked out accurately.  The web is the laid across as shown.  A little thinned lacquer is applied where the web touches the mount, and the ends are cut with a razorblade.

7.  Finally ...  the graticule mount is mounted into the telescope eyepiece tube and centralised and positioned in the focal plane of the eyepiece.



Read on about bubbles ...