8.5" f8 Dobsonian - restoration and rebuild.

[Moonshane]

Hi all

I bought an 8.5" f8 mirror off a fellow member - thanks Andy! - for £50 and he kindly let me have a few other bits for an extra £10. This included the original 'OTA' and secondary. Effectively enough to make a scope excluding the base.

The mirror was apparently made on a machine at Stafford University around 20-30 years ago and given to Andy by an older chap who is 'hanging up his scopes'.

It appears in good order with decent coatings and only the off surface scratch apparently from cleaning; there are a couple of areas where the mirror clips have rubbed off the coatings but no matter as they are hidden. The secondary is 35mm across the short axis and therefore provides an obstruction of 16%; with the 8.5" f8 mirror this could potentially be an excellent planetary scope. Even if I pay for new OOUK HILUX coatings (£120 approx for this mirror size) it's still a decent price.

The OTA is a boxy truss style and certainly usable for now. I will possibly make a bracket/adapter to enable me to hang this off my 12" OO dobsonian base to effect testing of the mirror including a star test etc.

True to form, I have modified it already. The secondary adjusters were simple set screws and I put in some cap head bolts (M4) to allow tool free adjustment of the secondary. As it happens, the two vane spider is not very stiff and there's a lot of vibration and movement in it. I'll be replacing this as soon as possible too.

The primary is on a plywood cell and one of the holes was in the wrong place. There were no ajusters but some bolts were in situ. I had some M6 rod connectors and bought some compression springs from the hardware shop and it's now sorted out. I added some tape around the mirror edge as the clips are not the best in the world/ All much better now.

I took the opportunity to mark a centre dot with a sharpie pen. I'll add a ring when I have got hold of one in due course.

The focuser is a basic 1.25" R&P with a good deal of travel. This will be good enough to test it.

Eventually and assuming the mirrors make it worthwhile, I'll be making a 'proper' obsession style truss dob for this as it will make an ideal travel scope for me or someone else in due course. I'll update this thread as I go.

here's some pics of things as they stand. the OTA is about 6" tall.

[Moonshane]

here's some more pics

[Moonshane]

last of the pics