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Attacking Sir Isaac.

ollypenrice

By ollypenrice
in DIY Astronomer

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ollypenrice

ollypenrice

Posted
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My guest and friend Dr X is an optics expert and has agreed to supervise the overhaul of Sir Isaac's vision. (Sir Isaac is our 20 inch Dob.)

So first we took out the secondary and Dr X pronounced it too solidly glued to its support. Expansion in the support transmitted to the mirror = astigmatism. I was ordered to saw it off the support with a hacksaw. This was not an amusing task.

Once off, Dr X had the foresight to check the support was truly cut at 45 degrees. It wasn't, it was miles out. Dr X told me to hand over the hacksaw because my eye was lousy and the 45 degrees needed to be right. Miraculously DR X got it bang on.

Then we went to the builders' yard for some secondary mirror mounting silicone (also used in swimming pools, it seems.)

This time the secondary was attached by means of a small circle of thick silicone so as to minimise the expansion effects from the support on the mirror. It would be nice to report a total success as of this stage but it seems that the silicone is rather slow to cure. We need to give it another day and then maybe go back to try a different make of builders'yard secondary mirror fitting silicone...

Watching Dr X collimate a telescope is interesting. No Cheshire, no laser, just the Mk1 eyeball. Does it work? Oh yes indeed.

I'll let you know if the silicone ever does go off...

Wish me luck.

Olly

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ollypenrice

ollypenrice

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If it helps, humidity helps some silicones dry faster, so even a bucket of water, with the secondary close by in a sealed 'ish container should help.

Must be nice to have an expert on hand!

Aha, thanks for that. Humidity is not a routine commodity down here!

Olly

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Peter Drew

Peter Drew

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Ollie. A slight caveat re the generally ideal use of silicone sealant, some brands, in the small print, state that their's are not suitable for bonding to aluminium, a common metal for secondary mounts. Although rather more expensive than commonly sourced sealants, I could recommend Dow Corning products.

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Doley68

Doley68

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Ollie. A slight caveat re the generally ideal use of silicone sealant, some brands, in the small print, state that their's are not suitable for bonding to aluminium, a common metal for secondary mounts. Although rather more expensive than commonly sourced sealants, I could recommend Dow Corning products.

I would second that. I use it at work and it is the best.

It holds my 8.5 inch primary in place! :)

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ollypenrice

ollypenrice

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Thanks guys. It seems the stuff we bought only cures under water. Doh!! Shopping trip required.

My support turns out to be PVC, not aluminium, and Dr X insists on sillicone in order to give as much flexibility as possible in the attachment, expansion-wise. I dare not argue with this...

Olly

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Merlin

Merlin

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Black BOSTIK glue used to be recommended for flats. This stuff never sets completely hard.

Personally, I prefer clips to hold the flat. There have been a good many cases of glued flats unexpectedly coming free and falling onto the primary mirror.

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ollypenrice

ollypenrice

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Black BOSTIK glue used to be recommended for flats. This stuff never sets completely hard.

Personally, I prefer clips to hold the flat. There have been a good many cases of glued flats unexpectedly coming free and falling onto the primary mirror.

Indeed. Using Araldite to attach a second thin wire attachment is one insurance. Above all we are trying to eliminate astigmatism caused by distortion of the flat.

Olly

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MikeWilson

MikeWilson

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Hello chaps,

Not meaning to come across as the village idiot, but what is "the flat" with respect to something that may come loose and fall? Are you talking about the spider, or the surface to which the secondary mirror is attached?

:)

Clear skies,

Mike

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Andrew*

Andrew*

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They're talking about the actual glass of the mirror, the back of which is often glued to a holder.

This thought terrifies me! I just wonder what are the chances of your average scope losign its flat spontaneously, and what conditions increases this.

Andrew

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ollypenrice

ollypenrice

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It is a bit scary but if Dr X says it is OK... What we did, though, was stick a pin through the solidified silicone ring that holds the secondary so that pressure can get out that way.

Once back in Sir Isaac the adjustments have done wonders. Collimation is far easier with the secondary now at the right angle. Finding a daytime target is tricky because Dobs cant look low but there is a phone mast on the hillside 300 metres away. We focused on a Yale lock at 200X and resloved fine detail on the slot into which the little key fits... I have never, ever, seen Sir Isaac perform like this. Needless to say we are promised a thunderstorm tonight so the stars will have to wait.

Dr X says some astigmatism remains and we are discussing his refiguring the primary to perfect it, but the transformation is truly spectacular already.

Olly

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