Box Of Interference Filters

[lonesome_dave]

I recently acquired a large cache of interference filters across the spectrum from 330nm to 1064nm in mainly the 1 inch size. They are about 20 years old and most are in their original sealed envelopes. There are about 150 of them and I can see some problems on about a quarter of them in two ways. Some have pitting/oxidation visible in the metallic layer on one side while a few that seem OK on the surface have a cloudy appearance when looking through them.

 

I see Andrea's 'UV/Vis/IR Filters' sticky from 2014 mentions:

1. Clean with hydrogen peroxide.
2. Clean/polish with cerium oxide.
3. Store with a dessicant to keep them dry.

 

Some later posts mention using acetone or even Diet Coke (for the phosphoric acid) but I didn't see much support for those methods. I have had some success cleaning surface cloudiness but not the problems cited above. I have about a dozen new interference filters I've purchased in recent years and haven't found any problems with those. I see Andrea is recommending cleaning/polishing them occasionally but I'm afraid to mess with them when they still look perfect. I do store them in a dry place.

 

Are these still the best practices for maintaining filters?  Is there something more that would apply to interference filters specifically?

[Stefano]

I think interference filters are more delicate, as they have very thin layers of materials deposited on the glass substrate which make constructive/destructive interference at different wavelengths. Those layers could be scratched off, I think. I never touch my BrightLine filters.

[Andrea B.]

I second that. Those dichroic surfaces can be scratched more easily than manufacturers mention. I clean dichroic filters only with something like MC lens cleaner: LINK

 

Hydrogen peroxide and glass wax (cerium oxide) are only for plain, uncoated, non-dichroic glass filters.

 

Acetone can trash lens coatings and plastic. I don't know what it would do to dichroic coatings? 

 

I acquired a box of old filters a few years ago. A couple of them were cloudy like Dave mentions, but were not cleanable. If oxidation sits too long without being tended to, it damages the surface permanently.

 

I'm not sure what it means if the filter is cloudy when looking through it? Maybe there was some deterioration under the interference coating?? Or if there are glued layers, then maybe the glue began to deteriorate?

 

Good luck with the cleanup, Dave !

 

 

 

[lonesome_dave]

Thanks Andrea.

Those that aren't good enough for imaging/viewing might still work for light source filtering in educational exercises. Many are laser line narrowbands that will be used for that anyway.

 

The foggy ones are color dye - interference combos. I think the colored glass portion is where the foggyness exists. Or maybe in sandwich glue like you say.

 

There were several H-alpha (656nm) narrowbands that look good for astro use. Just need to work them into 1.25" holders.

[Andy Perrin]

Sounds like a nice haul despite the issues! 

[Unscenerie]

I thought I could use hydrogen peroxide on any filter. Should I be more cautious, @Andrea B., since you mention that hydrogen peroxide should be used "only for plain, uncoated, non-dichroic glass filters"?

Could it damage some of the "common" filters that forum members might use, besides interference filters?

 

Interesting collection you've got, @lonesome_dave!

 

[Andrea B.]

In the Filter Sticky and other cleaning topics, we discuss using hydrogen peroxide on uncoated filter glass to dissolve any contaminating oxides from leached ions.  For coated filters or dichroic filters, it is best to use a standard filter or lens cleaner. I don't really know from experience what H2O2 would do to dichroic or coated filters.