mirror deterioration

[paler31]

I read the other day that a telescope with a mirror i.e newtonian or sc loose efficiency by 1.5% a year due to mirror deterioration and need to be recoated every 5/6 years. Is this true? I have a celestron sct but when i bought it I did not know about mirror deteriation. Does anybody know How much it costs say to get a 6" mirror re-coated. Might spend a we bit more to get a refractor next time if its expensive.

John

[johninderby]

The life of the mirror coatings on a modern SCT is probably over 30 years. The mirrored surfaces are protected inside an essentially closed tube with only a small amount of outside air getting in through the back of the OTA when there's no diagonal/eyepiece inserted.

So in other words it's not really something to get concerned about.

John

[Peter Drew]

I completely agree with John. :)

[solo]

Hi John,

I've had my newt almost ten years now and there's no noticeable deterioration in performance so far. As long as you let any dew evaporate before you put the covers on and look after the optics there's not much to worry about.

[Moonshane]

I agree with other comments but it's about £50-100 depending which coatings you go for so not madly expensive.

[johninderby]

I forgot to mention that newts with plain aluminized coatings do deteriorate every year though. Particularly if used near the sea. Modern coatings though have changed this.

John

[brianb]

I forgot to mention that newts with plain aluminized coatings do deteriorate every year though. Particularly if used near the sea. Modern coatings though have changed this.

Yes. It's always worth paying extra for premium coatings rather than plain aluminimum / silicon dioxide overcoat ... the reflectivity is 99% instead of 80% so with two mirrors you're gaining ~40% in effective light grasp (20% in aperture) when the coatings are fresh, the image contrast is higher and the dielectric coatings last at least five times as long.

[paler31]

Thanks guys for putting my mind at ease. I was starting to get worried.

John

[AndyH]

Again, agree with whats been said above. I've got a few well used Tal newts, with a few from the 90's and their mirrors are in perfect nick.

Andy.