diagonal mirror coatings
Posted 21 December 2011 - 07:59 AM
Posted 21 December 2011 - 03:18 PM
The jury is still out on the long-term durability of dielectric coatings. They've only been available about 15 years or so.
But, though I would re-coat at about 10 year intervals for any normal aluminum coating (whether enhanced or not), the key to maintaining reflectivity is cleanliness.
Just like primary and secondary mirrors in a newtonian need frequent cleaning, so does the mirror in a star diagonal.
I would clean it (and do with my own) about once or twice a year or immediately after a star party in a dusty area (like OSP, RTMC, etc).
Here's a good video on cleaning a mirror:
Reflectivity stays high on clean mirrors, but declines fast when they are covered with dust and dirt.
Theoretically, dielectric coatings are harder and last longer. Given the nature of the coatings, they should. But, as I said, we don't know the truth of that yet. However, there is great lab work that shows they control scattered light better than standard or enhanced aluminum coatings, and they are definitely harder and less likely to scratch during cleaning.
To me, that says that they can be superior to normal coatings.
In the field, though, I have seen dielectric coatings with unusual reflectivity spectra, which impose a slightly tinted coloration to the images. I have not seen this with aluminum coatings.
I had both a dielectric-coated and enhanced aluminum-coated star diagonal to play with, and the aluminized mirror was a better mirror than the dielectric one, so mirror quality varies, and you cannot be guaranteed that the dielectric-coated one has a better optical figure. So caveat emptor.
Posted 21 December 2011 - 05:15 PM
Dielectric is very hard and should have exellent long term reflectivity however I know of at least ONE dielectric where the coating itself actually started to flake off of the glass. Someone on the forum posted pictures of this a couple of weeks ago.
That doesn't mean ALL dielectric will flake off, but what it does mean is that there is no guarentee that a cheap dielectric will outlast a high quality aluminum coating.
The vast majority of aluminzed diagonal mirrors are overcoated with Silicone Dioxide (SiO2) which is basically a fused quartz. This called "Protected Aluminum." Quartz is harder than the glass under the aluminum, so it is fairly scratch resistant. More importantly it protects the aluminum from the elements and in theory, should allow the coatings to survive for decades or perhaps longer.
Next is "Enhanced Aluminum." The protected aluminum can be made to VERY high quality, but the reflectivity is only about 92% (If I recall correctly). Enhanced aluminum is SiO2 overcoated, and a coating of tantalum pentoxide (Ta2O5) is put over that. This can boost the reflectivity to the point where it is VERY close to dielectric.
Once again, the Ta205/SiO2 seals the aluminum so that it should maintain its reflectance for many decades.
Most high quality diagonals (AP, Televue) have been Enhanced Aluminum for a very long time now. Again, these diagonals can offer performance on par with Dielectric, especially if the manufacturer cheats on the dielectric (which takes a LOT of coats to acheive 97% reflectivity).
Anyway, Enhanced Aluminum may last for a VERY long time. My bet is many decades, and perhaps it might be measured in centuries. Who knows.
Much will depend on the environment, but assuming normal amateur use, I would not worry about the lifespan of a high quality enhanced aluminum diagonal.
On the other hand, I might worry about the reflectivity and life expectancy of a cheap dielectric.
Posted 21 December 2011 - 07:18 PM
I know that over time newtonian optics reflect a lower percentage of light so that eventually they need to be recoated. Does this happen with star diagonals using a mirror as well? Or do dielectric coatings prevent this problem?
As others have said, dielectric coatings are much harder than the protected aluminum coatings and therefore can be cleaned without damaging the surface.
Dielectric coatings work on a different principle than standard metallic coatings, they are multiple layers of thin films and depend on interference between the layers for reflection.
As far as their durability goes, it is well established that the hard surface of a dielectric mirror can be cleaned using the same techniques one uses for lens without damaging the surface. In comparison, aluminized mirrors need to be treated gingerly and even then there will be micro scratches. I have a TeleVue Everbright diagonal that I have had for about 8 years, I clean it regularly, in fact I cleaned it last week, the surface still looks perfect to my eye.
This is important with refractor or SCT type diagonal because every time you insert or remove an eyepiece, it represents an opportunity for surface wear or small bits of dust or other foreign material to be bushed off and deposited on the mirror. That hole where the eyepieces goes is an open pit and so the diagonal surface needs to be cleaned more frequently than a Newtonian diagonal.
Posted 21 December 2011 - 08:20 PM
There are dielectric coatings and there are aluminized coatings.
Dielectric is very hard and should have exellent long term reflectivity however I know of at least ONE dielectric where the coating itself actually started to flake off of the glass. Someone on the forum posted pictures of this a couple of weeks ago. That doesn't mean ALL dielectric will flake off, but what it does mean is that there is no guarentee that a cheap dielectric will outlast a high quality aluminum coating.
You're right. Quality dielectrics cannot flake off, they bond to the glass and the glass has to be refigured to remove the coatings.
Most high quality diagonals (AP, Televue) have been Enhanced Aluminum for a very long time now.
AP and TeleVue are pure dielectric, no aluminum at all.
Posted 23 December 2011 - 09:30 AM
Current Telvue Aluminum Diagonals are Enhanced Aluminum, and have been for a very long time. In fact, I don't know that Televue has EVER made a SiO2 only coating, but perhaps in the 80s???
So, When I said Televue has been using Enhanced Aluminum, I meant that their ALUMINUM coated diagonals are Enhanced and have been for some time....
Televue still lists Enhanced Aluminum on their web page, so I can only assume that they still produce them.
Televue Enhanced Aluminum Diagonals
Of course they produce Dielectrics, and probably that is mostly what they sell, but anyone that purchases a Televue Aluminum diagonal, or any produced for a VERY long time now, were Enhanced ALuminum.
Posted 23 December 2011 - 10:53 PM
Posted 25 December 2011 - 11:43 AM
I'll probably upgrade to an AP Maxbright at some point.