C8 primary coating resilience

ariefm71 · Post by **ariefm71** » Sat Aug 11, 2007 2:20 pm

Hi,

Quote:I think this whole thing is a hoax, same as the comments about modern coatings degrade faster. Wait, degrade? i thought coating performance should increase over time?

Why coating performance should increase over time?? Coating will degrade over time, so the coating performance should also decrease over time.

That's why I'm confused. Rich's statement is contradictory.

Post by **Airconvent** » Sat Aug 11, 2007 8:51 pm

weixing wrote: You mean corrector plate performance will increase over time?? Hmm... Logic tell me that this should not be true: The coating on lens and corrector plate is basically use to reduce reflectivity and scattering. Therefore increase the transmission (uncoated lens(2 surface) loss about 8%, single layer coating lens(2 surface) loss about 4% and multilayers coating lens(2 surface) loss about 1%). Over the time as the coating degrade, the transmission will decrease.

Anyway, if this is true, why put the coating in the first place... this doesn't make sense.

Have a nice day.

Hi weixing
Alamak...you are another sotong lah..
if you see my earlier post, I am talking about reflectivity, not transmission.
Hence it should increase with time as the coating degrades. I think Arief also similarly misunderstood also... hope this clears everything up

kayheem · Post by **kayheem** » Sat Aug 11, 2007 9:17 pm

Err...I think I am another sotong. If coating degrades with time, why should reflectivity increase?

To word it in another way, if it becomes less shiny('degrades'), shouldn't reflectivity decrease?

Post by **weixing** » Sat Aug 11, 2007 9:55 pm

Hi,

Hi weixing
Alamak...you are another sotong lah..
if you see my earlier post, I am talking about reflectivity, not transmission.
Hence it should increase with time as the coating degrades.

Oh... You mean "reflectivity increases with time" is referring to the corrector plate?? If yes, then it's your sentence "Some guy did some study previously and discovered the reflectivity increases with time." make us all blur la... because I think that's already been known long ago and your that sentence sound like it's just been discovered.

Also, when talk about coating of lens and corrector plate, most of us are used to "coating degrade, transmission of lens/corrector plate decrease" instead of "coating degrade, reflectivity of lens/corrector plate increase", because transmission will include the effect of reflectivity and also include the effect of scattering.

If coating degrades with time, why should reflectivity increase?

I think Airconvent refer that to the corrector plate: Coating degrade-> corrector plate reflectivity increase-> transmission decrease.

Have a nice day.

kayheem · Post by **kayheem** » Sat Aug 11, 2007 10:22 pm

OIC :oops:

Post by **Airconvent** » Sat Aug 11, 2007 11:13 pm

haha...weixing finally gets it. I think my posts sometimes too long and when readers skim through it, they miss the key words. And its not helping I was also not clear too in the beginning....sorry.. sorry..

In anycase, this post was originally about mirror coating, so I guess should drop the corrector plate and revert to the mirror

rlow · Post by **rlow** » Sun Aug 12, 2007 12:31 pm

Haha!... interesting turn of events.

Standard coatings degrade much more slowly than exotic ones.

There might be a grain of truth to this. I agree that the Starbright XLT anti-reflective multicoatings on the corrector plate has improved light transmission, however it may not be as durable as the simple MgF2 coating on older corrector plates. I noticed in one case that the Starbright XLT multicoating on the corrector plate is more susceptible to permanent damage from mildew. Does any one here have a similar experience?

mrngbss · Post by **mrngbss** » Sun Aug 12, 2007 3:53 pm

Do coatings on mirrors here apply to coatings on eyepieces too? Are MgF2 coatings on eyepiece are more durable than multicoated eyepieces?

hmm.. am i OT?

Post by **weixing** » Sun Aug 12, 2007 4:19 pm

Hi,

Do coatings on mirrors here apply to coatings on eyepieces too?

From what I know, mirror and lens use different coating:
1) Mirror is usually coated with a reflective metallic coatings to increase the reflectivity... a standard Aluminum coating which give will reflect about 86%-88% of the light. After the reflective coating, protective layer(s) of overcoat will be apply to protect that reflective metallic coating, such as silicon monoxide, silicon dioxide or titanium dioxide.

2) Lens (include corrector plate) is usually apply layer(s) of anti-reflective material, such as magnesium fluoride, to increase the light transmission.

Are MgF2 coatings on eyepiece are more durable than multicoated eyepieces?

The main purpose of multicoating is to increase the the light transmission, but I'm not sure does it make it more durable.

Have a nice day.

rlow · Post by **rlow** » Sun Aug 12, 2007 8:22 pm

Do coatings on mirrors here apply to coatings on eyepieces too? Are MgF2 coatings on eyepiece are more durable than multicoated eyepieces?

Firstly, I was refering to the XLT coating on the corrector plate, not the XLT coating on the primary mirror.

Secondly, I would not extrapolate my observation of one XLT coating on the corrector plate to correlate to multicoated eyepieces. Celestron applies layers of MgF2 (Magnesium Fluoride) and HfO2 (Hafnium Dioxide), using the special 'exotic' rare element (that Celestron advertises as costing US$2000 per kg) onto the corrector plate.

Btw, my experience with some of my earlier eyepieces seems to indicate that simple MgF2-coated eyepiece may not be as durable as multicoated eyepieces, but this observation is not conclusive.

Wxg is generally correct that lenses' AR (anti-reflection) coatings and the primary mirror's usual coating, which is the high-reflection metallic coatings, are different.

However multilayered broadband dielectric AR coatings for lenses and multilayered high-reflection dielectric coatings for mirrors (eg, diagonals) do work on the same principles of destructive interference and constructive interference of reflections respectively.