Refractors- how tough are the lenses?

[mikeDnight]

12 hours ago, Andrew_B said:

I think what happens is that there's often a nugget of truth somewhere but people take an idea and run with it until it becomes nonsense.

Fluorite is quite a bit softer than normal window glass or pyrex and needs more care when handling, but that's mainly an issue when making lenses rather than using them. I just looked up some figures and fluorite is harder than limestone and similar hardness to some types of marble, which are not materials people think of as being very soft.

It's also true that it's soluble in water, apparently you can get 15 milligrams of the stuff to dissolve in a litre of water at room temperature. I wonder how many litres of water there are in a film of dew on a lens?

I bought a couple of pieces of natural fluorite a few years ago, and to test just how water affected the crystals, I placed them in a jar of water for a month. When I took them out of the water there appeared to be absolutely no discernible change to the crystals, which were each around 20mm in diameter.  I was quite disappointed  really as I'd expected them to have easily sheared or turned to mush.

Over 41 years I've owned 15 refractors, four of which were true fluorite doublets between 152mm to 100mm, and rather disappointingly never damaged a single one of them through cleaning, or through using them in moist conditions. I feel like I've let everyone down!

[Andrew_B]

37 minutes ago, mikeDnight said:

I bought a couple of pieces of natural fluorite a few years ago, and to test just how water affected the crystals, I placed them in a jar of water for a month. When I took them out of the water there appeared to be absolutely no discernible change to the crystals, which were each around 20mm in diameter.  I was quite disappointed  really as I'd expected them to have easily sheared or turned to mush.

Over 41 years I've owned 15 refractors, four of which were true fluorite doublets between 152mm to 100mm, and rather disappointingly never damaged a single one of them through cleaning, or through using them in moist conditions. I feel like I've let everyone down!

In failing to destroy a single one of those fine telescopes you haven't just let us down, you've let yourself down!

Good to know that they last so well though, even in our damp climate. The folks perpetuating these myths about fluorite or ED glass being incredibly fragile don't stop and think that if it was true we'd be hearing about telescope lenses being damaged all the time through normal use, but we don't.

[jetstream]

I think the responses in the thread point out that refractors are not fragile- they are in fact tough scopes.

[Shimrod]

22 hours ago, jetstream said:

How big was this chip? pretty small? Years ago I saw images of horrendous damage that didnt really affect the views. Did you get a new lens or whats happened with the scope?

It's not telescopes, but there was this article on damaged camera lens and the effect on images some years ago  Damaged elements - the cracks became more visible as the lens was stopped down, so possibly a slower telescope more susceptible to showing the damage. Would anyone with multiple 'scopes care to undertake an experiment and report back? 😀