When Does a Mirror Need Recoating?
Jar-Jar Binks
the dumb question:
How often does a mirror in a Newtonian Telescope need to be recoated? I have
had the telescope since 2001 and it has a Zambuto Mirror. How much does
recoating cost and how much time is required? Do I need to send the mirror
back to Zambuto?
Thanks and Happy New Year,
Jar-Jar
Howard Lester
> How often does a mirror in a Newtonian Telescope need to be recoated? I
> have had the telescope since 2001 and it has a Zambuto Mirror. How much
> does recoating cost and how much time is required? Do I need to send the
> mirror back to Zambuto?
Much of it depends on where you live, meaning the quality of air that can
eat away at the surface. Other factors might be how well you keep it
covered, away from dust. Since mirror surfaces are overcoated with a
protective surface, I'd think you could go ten years without getting it
recoated. One way to find out is to take the mirror out of the tube and
examine its surface for tiny pits and holes. If there are lots of them, then
I'd say it's time. Find out where Carl gets his mirrors coated, and then
choose. My Spooner mirror was coated at Spectrum Coatings in Florida.
http://www.spectrum-coatings.com/
Jar-Jar Binks
"Howard Lester" <heyl...@dakotacom.net> wrote in message
news:13nj3m3...@corp.supernews.com...
Thanks for the information. How much did it cost you to get your mirror
recoated?
Regards,
Jar-Jar
(your favorite Star Wars character)
Shawn
Here's a start:
http://www.galaxyoptics.com/MirrorTestingAndRefiguring.html
Note where it says "refiguring includes C-1 coating".
Regarding when to recoat, my first thought was "When you can see the
collimation screws through the surface it's at least time." ;-)
Happy New Year,
Shawn
alli...@ignmail.com
> How often does a mirror in a Newtonian Telescope need to be recoated?
I happily used my 8" telescope for 25 years without ever having it re-
coated, so you can go at least that long. Finally I bought a bigger
telescope.
I am skeptical that re-coating is really necessary, though I do see a
lot of books saying that it should be done periodically.
Curtis Croulet
> I happily used my 8" telescope for 25 years without ever having it >
> bigger telescope.
>
> I am skeptical that re-coating is really necessary, though I do see a
> lot of books saying that it should be done periodically.
It depends upon the quality of the coating. My 1973 Cave 8-inch was
originally coated by Pancro, and the coating had lots of flaked-off patches
by 1985, when I had it recoated (by PNP, I think). AFAIK, neither of these
companies do telescope mirrors anymore, at least not for end users. I had
the mirror recoated in 2006 through Discovery Telescopes, who use LL Optical
Coatings in Santa Ana, CA. At that time the old coating, after 21 years,
was in about the same condition as the original coating after 12 years. The
optician for Discovery, a personal friend of mine, has high confidence in
LL. All that aside, a mirror can be surprisingly efficient with an old
coating or even no coating at all.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W
Edward Erbeck
Lester" <heyl...@dakotacom.net> wrote:
Last I'd heard Carl was using Spectrum, but it's been a while. And as
Howard said it really depends on how it was cared for and the environment
it's "lived" in. There's an Old Saying that works as a guideline rule of
thumb "If It Works Don't................." well you get the idea ;-).
Crazy Ed
Howard Lester
> Last I'd heard Carl was using Spectrum, but it's been a while. And as
> Howard said it really depends on how it was cared for and the environment
> it's "lived" in. There's an Old Saying that works as a guideline rule of
> thumb "If It Works Don't................." well you get the idea ;-).
Happy New Year, Ed!! Stay warm out there!
Howard Lester
>> http://www.spectrum-coatings.com/
>
> Thanks for the information. How much did it cost you to get your mirror
> recoated?
Mike Spooner sent it for coating, and it's still original (3 years), so I
don't know what they charge for re-coating. If you send them your mirror,
get the regular coating, not the enhanced. There are *rumors* the enhanced
coating does some funny things to the miror's figure. And then, maybe not. I
had Mike order the regular coating.
William R. Mattil
> It depends upon the quality of the coating. My 1973 Cave 8-inch was
> originally coated by Pancro, and the coating had lots of flaked-off patches
> by 1985, when I had it recoated (by PNP, I think). AFAIK, neither of these
> companies do telescope mirrors anymore, at least not for end users. I had
> the mirror recoated in 2006 through Discovery Telescopes, who use LL Optical
> Coatings in Santa Ana, CA. At that time the old coating, after 21 years,
> was in about the same condition as the original coating after 12 years. The
> optician for Discovery, a personal friend of mine, has high confidence in
> LL. All that aside, a mirror can be surprisingly efficient with an old
> coating or even no coating at all.
I surprised that no body mentioned this .... but the coatings will last
a whole lot longer if they are kept *dry*. Dust and minor pollutants,
tree sap etc if not cleaned off regularly can get damp with dew or rain
and the corrosion process starts. And I guess it could be argued that
high humidity might be enough. So I guess it depends a lot on local
conditions. Personally I clean mine approx every three years assuming
nothing catastrophic happens and I start checking the coatings after 10
years. A flashlight shining on the front surface and looking at the back
surface will reveal pinholes and generally thin coatings.
And yes ... I use Spectrum Coatings.
Bill
Sparky
like a rarity. Ask Carl where to get it recoated. Unless there
is some very good reason dont recoat at all. Again consult
Carl directly. You need someone 100% reliable who has
coated Zambuto mirrors before and knows what they are doing. Consult nobody but
Carl in his matter.
Jerry
Curtis Croulet
Howard Lester
> Tree sap?!
Sure! Don't you have trees growing inside your OTA? If trees can help fight
global warming, maybe they can help reduce air currents....
Edward Erbeck
When I lived in California they also reduced Light Pollution! GAWD I'm
glad to be outta there!
Crazy Ed
Martin
> All that aside, a mirror can be surprisingly efficient with an old
> coating or even no coating at all.
At age 12, I ground and polished my first mirror (only a 4 1/4") but could
not afford aluminizing. I was amazed that even uncoated, it delivered
wonderful views of the moon and planets and lots of stars.
The tube for the scope was made out of a 5" diameter rain gutter downspout
which I got from a nearby building that was being demolished. The mount
was a pipe fitting type which, if I remember correctly, was outlined in an
Edmund Scientific Sam Brown booklet. The eyepiece was fashioned from the
two little plano convex lenses that came with the mirror kit. There
couldn't have been more than 20 dollars spent on the finished scope and
that included the mirror kit.
God, how I loved that telescope. Those were the days.
--
Posted via a free Usenet account from http://www.teranews.com
William R. Mattil
> Tree sap?!
You bet your knickers ...... You'd be surprised at how far that can travel.
Bill
Yepper
Curtis Croulet wrote:
> Tree sap?!
>
Tree sap rains down in the spring here, floats in tiny droplets
on the wind especially at night. Spores, other bio stuff moving
around with the wind. You dont even have to be close to it,
just in the vicinity. We dont all live in deserts.
Martin Brown
> Curtis Croulet wrote:
> > Tree sap?!
>
> Tree sap rains down in the spring here, floats in tiny droplets
I am not sure if it makes it any better, but in most cases the tree
sap has been through the digestive tract of a sap sucking insect like
aphid or scale. Their exhudate waste is very sugary and sticky.
Several desert dwelling peoples recognise it as a sweet food when
dried and scraped off leaves.
Round here in the damp you just get black sooty moulds growing
wherever the stuff has landed.
> on the wind especially at night. Spores, other bio stuff moving
> around with the wind. You dont even have to be close to it,
> just in the vicinity. We dont all live in deserts.
Mostly wind borne pollen. The worst stuff I have come across was
pollen from forests of cryptomeria japonica which looks for all the
world like flowers of sulphur except that it sticks to everything and
was mildy corrosive. Huge amounts of it are released by every mature
tree and it is the main cause of hayfever in Japan. Other coniferous
trees in a forest with wind borne pollination can give similar
problems in the (fortunately short) flowering season.
http://www.blandon.co.uk/forestry/species/sugi.htm
They play down the hayfever problem, which admittedly only seems to
affect Japanese people badly.
Regards,
Martin Brown
Gary Honis
Please don't do another star wars please:)
I'm not a coatings expert, but in the process of needing my mirror recoated,
this is what I have learned.
My 20" mirror by Pegasus (John Hall) was coated in 1999. This past year
(2007) I noticed that when sunlight was hitting the back of the mirror box,
I could see right through the mirror from the front (reflective) side of the
mirror. With my Starmaster dob, the mirror box is open on the backside.
The mirror supports of the mirror cell were very visible. If I placed my
hand behind the mirror, from the front side of the mirror, others could see
the outline and movement of my hand. I did these same tests on other large
dobs, and could not see through the mirror. I also noticed when
illuminating the front of the mirror that there were many fine small
pinholes across the whole mirror. Again, comparing this to other mirrors on
the field gave me an idea of how much my coatings had degraded over 8 years.
The coatings were PAL coatings by Spectrum Coatings in 1999 and I have just
decided to have the mirror re-coated by Spectrum again but this time with
MaxR (98%) enhanced coatings.
My mirror saw almost continuous use over the years and I used it at star
parties across the country. I live in PA where it has seen the most use and
when on the observing field for a few days, the mirror would quite often be
completely dewed in the morning hours. We get acid rain in PA (thanks Ohio)
and I wonder if our dew here is acidic as well. Dewing was even more
pronounced when observing in Florida. I kept the scope/mirror in my van for
extended periods and even with the mirror being housed inside the van, when
I would remove the mirror cover I would often have dew on it's surface. I
did have times when the mirror was coated by pine tree sap, usually happened
in Fall when you could see clouds of green (sap) dust blowing off pine trees
near the observing field. I tried to clean my mirror as little as possible,
averaging about once a year.
My answer to your question about how often a newtonian telescope mirror
needs to be recoated would be to look closely at how the reflectivity of the
mirror has changed since you got it. I have seen some mirrors that have
coatings actually pealing off the mirror...that is certainly a sign of the
need for re-coating. If you can compare your mirror to others and see that
light is passsing right through your mirror as I saw with mine, I would
think it's time for re-coating. I always heard that most coatings last
about 10 years. I have a 10" Meade Classic LX200 that is much older than
my 20" and the primary coatings are fine, but the mirror is very protected
since it is in a closed tube.
Concerning the cost for re-coating, the coating companies have their prices
online and can tell you how long it may take for re-coating; here are two:
http://www.spectrum-coatings.com/Telescope-Mirror-Prices.htm
http://www.opticalmechanics.com
I don't think you need to send your mirror back to Zambuto. Instead, you
need to decide if re-coating is needed, then on a mirror coating company and
the level of coating to be applied. That's the rub..........from what I
have learned, there are widely differing views on the level of coatings that
are best.
Gary Honis
http://ghonis2.ho8.com/