Coated Primary Mirrors.

[Crossway]

I know that the efficiency of reflector mirrors deteriorates  over time owing to tarnish or perhaps some other chemical action but have never been aware as to the time scale of this problem although I would think that it is a matter of years.

The reason that I mention this is because  just before Christmas  my wife and I were browsing through a "bric a brac shop and came across a solid brass refractor telescope  mounted on a very substantial wooden tripod.  The object lens was probably around the 4 inch mark and focal length 900 mm.  There were two eyepieces  and optically the scope appeared to be first class condition with clean unblemished lenses.....unfortunately there did not appear to be a manufacturers  identity.  The proprietor " thought" that the instrument was perhaps  80 years old  but  it could have been anything !!.   The price tag said  £80 !!  ( EIGHTY)  Ah, well, being that time of year and a large family  any  ideas of a quick buy were stamped on immediately !!.

I got to wondering later what my trusty reflector ( a young one) would perhaps look like in say 80 years .  Or to be exact, the mirror.

Pete.

 

[John]

Are you comparing a refractor (the old brass one you saw in the bric a brac shop) with a reflector ?

Obviously the mirror has reflective coatings which are usually overcoated to protect them wheras the refractor is likely (as it's old) to have uncoated lenses.

Mirror coatings (aluminising and over coatings) do have a finite life before they need to be renewed. The coatings on my 12" newtonian are currently 9 years old and look unblemished. They might have lost some reflectivity though, over the past 9 years.

 

[Peter Drew]

In general, the smaller the mirror the longer the aluminising seems to last. All larger ones seem to lose their lustre after a few years but perform well despite this for several years more.    ?

[Crossway]

Thank you for replies and encouraged that mirrors do have a decent life ......my reflector will  doubtless see  me out !!

No John, I wasn't  comparing the  lens of the refractor with  the reflectors mirror ....just the life span.  Coated or not the lens with be set to retain its integrity  indefinitely probably.

Pete.

 

[John]

34 minutes ago, Crossway said:

....  Coated or not the lens with be set to retain its integrity  indefinitely probably.

Pete.

 

That was one of the reasons that I used to prefer refractors over all other designs. I've learned more since then though and I've been happy to own quite a few reflecting scopes. I currently have 6 refractors and just the one newtonian so I guess I'm still rather partial to the 'fracs

[Louis D]

It depends on the conditions the mirror has been stored and used under.  Highly acidic/salty moisture in the air tends to be the worst on coatings.  Dry desert air tends to be the best for coatings.  It also depends on the quality of the transparent overcoat on the reflective coating.  I'm kind of in the middle where I live in Texas, and my coatings look fine after 20 years on my Dobs.

As far as the brass scope, beware.  Most are Chinese made replicas of vintage scopes, especially at that price.  Most vintage brass scopes will have the manufacturer clearly engraved somewhere and the seller will want real money for it.  Even at estate sales, multiple seasoned buyers will recognize the value of a true vintage scope and not it go for cheap.

[Crossway]

John, you leave no doubt as to where your preference lies....... and no collimating to worry about either. 

[Crossway]

Louis  D,   Your comment re salty  moisture in the air  is interesting and certainly food for thought.......I frequently use my reflector  for terrestrial observing  on  or near the beaches of  North East England where the  atmosphere can at times be  like sea water.  

I admit to having never seen a Chinese replica telescope  but  I take your point  and from here on will always  ponder just what this exhibit was .  If it was , in fact, a replica then they made a very good job of it

Pete.