Mirror Cell

[Tom How]

I need to make a new mirror cell for my 200mm F5 Newtonian OTA.

Many years ago I constructed a very crude mirror cell to replace the standard fitting. The scope is a very early skywatcher circa 2003 back when they were branded Helios.

I did this mirror cell very crudely with some bits of metal and a hacksaw - the two main reasons: the desire to get rid of the mirror clips and to make collimation easier.

Although the cell works fine, it is very scruffy and considering my homemade mount I need to make a new cell to tidy it up!

You can see a bit of the existing cell on the photos on the above link.

I'm familiar with PLOP and so forth - I'm currently thinking a 6 point cell is more than enough for such a small mirror. I've heard 6 point cells are far more forgiving of machining tolerance than 9 point cells.

A six point cell consists of 3 arms which pivot - obviously on some kind of bearing. I have some suitable 10mm thick ally plate to cut the "arms" from.

I also have a good amount of 6mm stainless rod for the pivots, thus the bearing choice becomes something like

6262RSR Rubber Sealed Deep Groove Ball Bearing 6x19x6mm Simply Bearings Ltd

In my mind the load is entirely radial - so a simple ball bearing should do without the need for more expensive or complex types of bearing. In fact a simple bronze bush may be ok - or even better than bearings - less likely to corrode. After making the mount I have become a great fan of Oilite bushes - the mount used at least half a dozen.

AM0610-10 Oilite Plain Bush 6x10x10mm Simply Bearings Ltd

I know there is great debate about gluing mirrors, and I confess I'm a glue fan. The current mirror is silicone glued to the cell and I have no problems.

Still at the thinking stage: Just after any thoughts and words of wisdom

[Glasspusher]

Tom,

I have attached some pictures of the mirror cell I made for my 12 inch F4.2, it is all aluminium as you can see and made with readily available tools. You don't need bearings for the pivot supports...a simple design like mine will work well. As for gluing mirrors, I am not too sure, as a mirror maker it bothers me a little...probably get away with it with smaller mirrors.

John

[Astrokev]

Hi John

I love your cell, but am curious as to what radius you placed the supports. I've read recently that the optimum position is circa 40% out from centre, depending a bit on size of mirror and number of supports used, but yours look further out than this. It may of course be skewed perspective of the images.

Thanks

[Glasspusher]

AK, all the dimensions for the cell are here....

6-Point Mirror Cells

John

[Astrokev]

I now see that 6 point support points are further out than for 3 points (which was the figure I must have remembered). Cheers John.

[Tom How]

According to Plop the support radius for my mirror is about 60mm.

I've made a start on putting things together

Astronomy, Astrophotography and Telescope Blog » Telescope mirror cell stage one