Mirror re-coating?

[Dave9281]

Greetings, all!

 

I've been doing a lot of research on buying a new telescope, but in the mean time, I've been using my 80's era 8" f/6 Meade Starfinder (or maybe 826? I've lost the documentation...). It's in reasonably good condition, and even with my clumsy attempts at collimation, provides a surprisingly good view! In fact, I've been so happy with it (nostalgia playing a big part, as this 'scope was the first major purchase I'd ever made using my own saved money as a bus boy when I was 16) that I've decided to refurbish it, and maybe postpone any new purchase. But as part of it's ongoing restoration, I'm wondering if I should have the mirrors re-coated. They are dusty, but haven't been subject to very much weathering, and the telescope has been handled gingerly, but can time alone damage the mirrors?

 

How do you know if it's necessary or not? Can you tell by visual inspection, or does it require an interferometer?

 

Any advice would be greatly appreciated, and many thanks!

[Peter Drew]

If the mirror is really dusty just give it a clean in the first instance, lots of advice on how to do this elsewhere on this forum.  After cleaning, hold the mirror in front of a bright light and see if it shows through.  In general, if you can see your face clearly in the mirror it's still serviceable.    🙂

[Dave9281]

Thank you, Peter. Simple and helpful!

[Pixies]

Hi Dave.

Does your mirror have a centre spot? If not (as plenty didn't back then), this would be a good opportunity to add one.

[bosun21]

Shine a torch into the rear of the mirror and if you can see light coming through the other side then it needs re-coatinng.

[Carbon Brush]

To add to @bosun21 comment.
A lot of mirrors pass a bit of light through. Whether it is a problem is a matter of extent.
Unless you have experience, or the mirror has lost lots of aluminium, you may find it difficult to interpret the result.
It is amazing just how much a mirror can lose and still give good results.

[bosun21]

Yes I should have mentioned that the amount of light passing through is an indication of the condition of the coating. A small amount of pass through is acceptable however a lot will have a detrimental effect on the views produced.

[Gonariu]

I also have the Meade Starfinder 8", bought in November 1992 and which I had transformed into a Dobson years ago because its mount and tripod were too heavy, the instrument as it was sold is for a fixed position. A few years ago I washed the mirror and I should completely re-collimate it, which in 1992 I did not find easy at all. Before washing it it seemed to me that it had lost a bit of its aluminum coating, after which I changed my mind. From a review I read that it has an obstruction of 0.2, so it should be fine for the planetarium; it also says that on the planets it can reach X400, I would be more cautious and not exceed X200, X300 generally gave a poor view with a Vixen LV 4 mm except a few times in one of which I saw the internal Gassendi rims very well but the collimation should have been done better in any case.