Trying to Prevent Filter Glass Oxidation #1

[Cadmium]

If you want a safe coated filter like S8612. Just get a BG61. That stuff is coated and humidity resistant.

 

For all in UVA just get a Baader venus U filter. Mine has been excellent for nearly 12 years.

 

David, BG61 is like any other filter glass Schott makes, in the sense that it can only be coated after it has been machined (lapped) to a specified thickness.

BG61 has more surface durability, than S8612 or BG39 (which are both in humidity Group 3, two umbrella symbols),

but Schott classifies BG61 "resistance to humidity" in Group 2, (one umbrella symbol), which is the same group that BG40 is in when it comes to humidity.

Most importantly, to me anyway, BG61 doesn't come close to S8612 UV transmission, it doesn't even transmit UV as well as BG39.

So I see no reason to use it unless you have the need for IR suppression only in a situation that doesn't require good UV transmission,

then it might be a good alternative to BG39, but only if the IR transmission of BG61 is good enough for the application.

But I would not use BG61 for UV stacking, it cuts off too much UV bandwidth, peak performance and depth.

 

The thing about stacking filters is the versatility.

You can use the same S8612 2mm filter for various UV only stacks (UV-Pass, 320nm - 400nm range), various UV+Violet stacks, various UV+Blue+Green stacks (Bee Vision),

and any other kind of stack that might need the same transmission range of S8612 employed.

 

You can't do all those things with a Baader U, and you can't get a Baader U in larger sizes, and the cost of a U+S8612 stack is still less expensive than a Baader U even when glued.

So, there is all that also. But, I would not get BG61 in your head, anyone, it just isn't like S8612.

 

[dabateman]

Thanks Cadmium,

I mentioned BG61 as its a known glass with a good coating. In that we know its transmission based on thickness.

People here have started talking about coating their S8612's. You will have no idea how that will impact your UV transmission profile for your filter. Best to just buy a known one. Any of the ways to protect a glass filter from oxidation will clearly cut down its UV transmission.

We can see that with the BG60's series. So lets not all go too crazy looking for a wax or plastic spray to protect our filters. As we will be changing its character.

 

Yes I owned the BaaderU first and bought a 2mm S8612 just to play around with others. The B370 and U330 I bought from an excellent filter seller have been fun :)

[colinbm]

Not so fast Dave

I am gearing up for testing....

First can you tell me what the cheap protection / UV cut filters pass please ?

[Cadmium]

What do you mean by "known"?

BG61 has no coating.

Just to give you an idea of the difference, real Schott S8612 coated will cost just shy of 2 times the price of uncoated.

Price has part to do with quantity, and either coated or non-coated S8612 camera filters are specialty items that don't have a lot of demand other than a niche group,

so it is rather hard to justify a large enough quantity to acquire a lower cost. We do the best we can.

It is not hard to maintain an uncoated filter, you save a lot of money doing that,

you just need to clean them on a regular basis, or if you see something, don't wait years, examine them often, you will get the idea.

There are people reading these things, I don't want them to be misled and buy something they think is good when it is not.

If anything an AR coating should improve transmission slightly, on any of those glass types, you can even figure that into the calculations with the program.

It is not a coating that will cut down UV transmission, it is the glass. Here is a typical stack using U-360 2mm stacked with S8612, BG39, and BG61.

Coating S8612 or BG39 will not cut down on transmission. BG61 is simply a really poor idea for UV stacking, coated or not.

 

[dabateman]

Not so fast Dave

I am gearing up for testing....

First can you tell me what the cheap protection / UV cut filters pass please ?

 

I am not sure I understand your question.

If you are asking if you buy a bunch of $1 UV Chinese filters what does the transmission of the glass look like?

In that care it varies and by a lot. I have done that to get rings, spacers ete.

One of my 25mm UV filters passed everything above 250nm. Looked like some WG280 glass on my spectrometer. But some of my others only passed above 370nm.

Luck of the draw, whatever they may have wanted to get rid of.

 

Cadmium,

Ok that makes sense. I was just using BG61 as an example of a glass type I knew didn't oxide as quickly as S8612.

Then assume that reduced rate of oxidation related to the UV transmission. I always thought coated S8612 had reduced UV transmission than uncoated. Which is why I would use uncoated to maximize the UV transmission.

I don't know all the variables and didn't see coating options in the Schott spreadsheets.

[colinbm]

Thanks Dave, that is partly what I wanted to know, that the cheap UV cut filters don't cut UV.

I now have a spectrometer & the learning curve is vertical for me.....not coming from that sort of background.

But I will research these filters & coatings as best as I can, by posting results & having critical debate.

[Andrea B.]

People here have started talking about coating their S8612's.

 

Geez, people. How hard is it to clean a filter every couple of months? I'm so tired of this particular discussion, said the Editor.

[rfcurry]

Whoops, Andrea!

I guess I extended the coating discussion.

Sorry.

[Andrea B.]

Reed, I was referring to the tiring discussion about the fact that BG glass and some other types can become oxidized. I'm sorry I was not clear.

 

The topic of oxidizing BG glass was discussed here back in 2016, but I guess not everyone read it. So there have been complaints about oxidizing glass. (There might be earlier references?)