SW Newtonian Mirror Washing

[sophiecentaur]

I am half way through the process of washing the primary and secondary mirrors on an ancient SW 250mm Newtonian. It only cost me £60 so the risk is low!!

The job has gone well except when removing the 4mm machine screws that hold the mirror clamps (six pairs) most of them were very tight, several were stuck and broke off. My mole grips coped with most of those but I was left with two which had broken off right where they go into the casting. Always a nasty business but I managed to drill them out and re-tap the holes. I also ran a tap through the remaining ten holes as the screws that came out were a bit stretched and graunched due to over tightening, I think.

I don't know how the could have got like that - except that someone could have used a power tool to screw them in and didn't use the right torque setting. Is this what to expect from SW or was the mirror removed once before and replaced by King Kong?

The design of the cell and fitting is pretty good; it's a lot nicer than my original 200P Skywatcher Dobsonian.

The results of my cleaning are pretty stunning. The heavy primary mirror is much easier to handle than I ever imagined. At first, the surfaces looked dodgy after the wash but they dried out and are very good. They both looked like a chip shop ceiling to start with.

Colimating is not hard as I bought a cheap and cheerful laser collimator, which works well now I have removed the rubber filler from the adjustment screws and collimated the collimator. "Quis custodiet ipsos custodes" or "Who takes care of the caretaker's daughter when the caretaker's busy taking care? (Bobby Darin, late 50s)

[Doc]

I have had quite a few dobs in the past and have always regulary washed the mirrors. I have never removed them from the cell though. A few had fans on, so I removed the electrics and just washed the mirror connected to the cell in one go, and left it to dry.

It most likely doesn't improve the view that much but it looks better.

[bottletopburly]

screws will have corroded 

 

[Peter Drew]

The screws that attach the clips that retain the primary, I suspect, are somewhat overtightened by the manufacturer in order to prevent the mirror jiggling about in transit to the customer. This is usually the cause of reported image distortion but thankfully easily remedied. Steel screws don't get along with aluminium long term and eventually both metals corrode making the screws very difficult to extract. Best to lubricate before replacing. 

[triton1]

I found mine had locktite on the threads so was probably applied at the factory I replaced them with Allen bolts and re applied some studlock it was obviously there for a reason.

[Skipper Billy]

Its probably  too late now but there is a product called Duralac.

https://www.amazon.co.uk/Duralac-Anti-Corrosive-Jointing-Compound/dp/B00LB6EUEO

Its for isolating dissimilar metals - especially stainless and mild steel from alloys - we use it extensively on yachts where there are hundreds of stainless bolts into alloy castings which are regularly soaked in salt water - not a great recipe!

Just a wee smear on the shank and it will not seize. I have used it on just about every fastener when I rebuilt my AZ EQ6 GT and my ES 208MM f3.9

[Beulah]

1 hour ago, Peter Drew said:

Steel screws don't get along with aluminium long term and eventually both metals corrode making the screws very difficult to extract. Best to lubricate before replacing. 

Learn something new every day. Thanks.

[Peter Drew]

3 minutes ago, Beulah said:

Learn something new every day. Thanks.

You certainly do!,  I've never heard of "Duralac" before. Good idea to get some. 

[JamesF]

I wish whoever manufactured our gas hob had heard of it.  I needed to replace two of the thermocouples recently which required removing the stainless screws holding the top onto the cast aluminium body.  Two screws had corroded themselves to the point where I couldn't remove them and eventually when I resorted to an extractor the heads snapped off

I might get some and put it on the rest of the screw threads now.

James

[sophiecentaur]

3 hours ago, Skipper Billy said:

Its probably  too late now but there is a product called Duralac.

Yes. I've used it on my boat (when I had it). Messed up a perfectly good new jacket with yellow smears all over the sleeve from it!

I may be able to find it before the new screws arrive. But, dammit, we're not in a marine environment (It's not getting hot like a cooker hob, either) and a bit of ordinary grease should be enough. Actually, the threads were mostly silver coloured and clean from inside the hole - the screws were just a bit knackered looking - so I go for the over-tightened solution.

Anyway, thanks for the responses. The horrors of mirror removal and cleaning are a bit overstated, I think. I am looking forward to resolving stuff that I have never seen before, once it's all back together.