The HAS 12" Truss-Tube Dobsonian Project
PART III: The Mirror Cell:Cutting the basic shape
The purpose of the mirror cell is to support the huge 12" mirror of the telescope and to prevent it from distorting under its own weight. This seems highly unlikely since it is a very thick mirror (2") but, nevertheless, even if it sagged only slightly it would make the image deteriorate.
For the purpose of making a mirror cell, David Hughes from Balloch near Culloden was kind enough to provide us with a 6mm x 290mm diameter alloy disk. The challenge was to design a so called 9 point flotation cell which could be made entirely from this disk with a minimum of additional parts. Such a cell supports the mirror at 9 points, applying the same pressure at each of the points. The design on the drawing is the result.
The 9 support points can be clearly seen here and are grouped in 3 groups of 3. Each group of 3 points is situated on a triangular floating support. Each support is held in place by a bolt on which this support rests. The bolt is not fixed to the support at all, in order for the support to swivel to take up any undulations on the back of the mirror. The mirror is held in place by means of 3 steel L-shaped brackets and retaining blocks.
The entire cell can be collimated (aligned with the other optics of the telescope) by means of 3 bolts and wing nuts. The cell rests on 3 very stiff springs (valve springs from a diesel engine!).
In the picture to the left you can see the alloy disk before it was cut. A piece of the design drawing is put over the disk to show what it is going to look like once it is finished. To start the cutting process, 3 lines are drawn on the disk so that all 3 lines form angles of 120 degrees to each other.
The hole in the centre of the disk was already there. This is not a problem at all as there will be more than enough space between the hole and the edge of the final cell body to keep the whole construction rigid.
A friend of mine told me that the material of the disk was most likely of Duralumin, a strong aluminium alloy which is apparently hard to cut. However, this didn't turn out to be the case, so the material is possibly just another type of aluminium alloy, although it doesn't appear as light as ordinary aluminium.
On this image, the cutting has started. First, the three floating supports are cut from the edge of the disk, leaving a propeller shaped bit in the middle. The supports were drawn on the disk by using a template cut from paper.
The supports were cut using an ordinary hacksaw which was lubricated with candle wax. The small saw you see in the bottom left of the image was used to cut the last few millimetres as the hacksaw wasn't 'deep' enough to do it all.
Once the supports were done, unnecessary corners were cut from the cell body. Since 3 of those corners were going to be made into the retaining blocks for holding the mirror down onto the cell, these blocks were drawn onto the material first.
A hole was then drilled in each of the 'blocks' and threads cut into them.
Only then were the blocks cut from the main cell body. All sharp edges were then removed from all the parts using a file.
On the final photograph you can see all the main parts of the cell, with the exception of the L-shaped brackets. Although the design showed that the supports were triangular, I decided not to bother cutting the 'round bits' off the back of them as they were not in the way.
On top of the cell body you can see the three retaining blocks with a small bolt in each of them. These bolts will be used to fit the blocks to the brackets mentioned earlier.
