AASC/AOS wood primary mounts

[torana68 (Roger)]

Hi all, does anyone have a photo of the wood primary mounts as supplied by AOS/AASC?. Just dismantled my 10" and it looks like an artful bodger has been in there and I would like it restored to "as new'

[Wavytone]

What vintage ?

I recall one for my 8" circa 1975... it was quite roughly made from ordinary ply, and clearly a product of hand tools. The worst part was it was constantly shifting to the point the scope had to be collimated for every use.

Made my own to replace it out of aluminium plates and bingo scope stayed aligned pretty much indefinitely.

[torana68 (Roger)]

This is a '71 Ill post a photo tomorrow and see if you recognise the bits?

[torana68 (Roger)]

Ok, attached are picks of the hardware, plywood is 31mm (1 1/4?) and 33mm ( 1 5/16?) from a quick Google these sizes seem to be unusual today so they may be original.
The mirror holders maybe not original?
The three supports for the mirror look like the back of a coach bolt?
who remembers these things

[Wavytone]

Yup same as the one I had. Frankly it should be replaced, not fiddled with.

Easy to do: remove metalwork and paint it all matte black, then reassemble.

1. The 3 clamps that stop the mirror falling out probably don't stop it moving sideways. Secondly they are so grossly made that they have three fingers protruding over the front surface and I have seen mirrors chipped by these. In addition they cause nasty diffraction spikes on bright objects.

So think about getting rid of them.

2. Mirror sits on top of the adjustments bolts. Also may fracture the mirror.

3. Mirror cell can shift laterally (side to side) relative to the OTA because the bolts are just cheap rubbish ones in sloppy holes in wood, with the result being misalignment.

I'm guessing your cell is spring-loaded with wing nuts on the back as mine was, that's also hopelessly prone to shifting.

Really should be using decent metric cap screws in a push-pull arrangement.

4. The base plate and the board under the mirror are ply, which changes dimension as often as the weather does. Another source of trouble. Really should be metal or a composite of some sort.

Mirror cells are a good candidate for using aluminium composite panels or glass/carbon composites, all of which are far lighter than ply and 10x stiffer for the same thickness. Draw up what you want and you could get a price from several suppliers in Sydney.

I'd also investigate using silicone pads under the mirror to attach to the plate without the ugly clips.

[torana68 (Roger)]

nice info those mirror holders are what they supplied these with ? really? , its a restoration so it will go back to new condition, even if that's not so good (by the standards of today). Thanks

[glend (Glen)]

No disrespect Wavy, but in this case your wrong. I have built seven composite boats and all of them had components of marine ply (not cheap exterior stuff, it must be structural). First off, good marine ply is one of the most dimensionally stable things you can use. Which is why it is used in high stress load applications in boat building. You want to use at least 18mm when building a mirror base. It does need to be sealed with a good boat building epoxy but it is a good mirror base. Epoxy encapsulated Marine Ply is a composite. Plenty of scopes have been built with the Stellafane mirror cell plans.

https://stellafane.org/tm/dob/ota/cell2.html

I used a variation of this design to build the cell for my 10" imaging newt, which supports my full thickness mirror perfectly.

Side 90 degree L brackets are fine, leave a gap and inject silicon through the bracket hole so it just touches the mirror side, once cured no movement possible and it won't fall out. Pretty sure the photos from my Newt build are still in the ATM forum here.

[torana68 (Roger)]

unfortunately this stuff doesnt look to be high grade or sealed in anyway, it will be when it goes back together