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Hi Malcolm,
The information Alex has quoted from the DGM website pretty much explains things. Generally this occurs with "sandwich" or "laminated" filters, where the filter substrate is sandwiched between 2 flat highly polished pieces of glass. If all surfaces are not perfect, internal reflections occur within the filter. The reasons why newtonian telescope mirrors are "first surface coated" and not coated on the back with light passing through the glass, are similar. Some years ago this was considered the best way to manufacture these filters as it was felt this would make them more durable by sandwiching the filter substrate between 2 layers of glass and cementing it all together. At this time the filters which were externally coated were not that durable as the coatings in some cases could be prone to scratching. Consequently, most of the better filters were "sandwich" filters a decade or more ago. Unfortunately these sandwich filters did not last forever as they often delaminated after several years use being subjected to dew, moisture and dramatic temperature fluctuations. In addition they could sometimes exhibit minor internal reflections.
Turn the clock forward 10 years. Most of the better quality filters made today are externally coated using hard oxide dielectric coatings. These will last forever if treated carefully as the oxide coatings are very hard and scratch resistant. They also perform better optically than the older sandwich filters as the high quality ones eliminate the possibility of ghosting and other aberrations
I am not sure how the GSO filters are made, or how old your Lumicon filters are, but I would guess " a good few years".
What I can tell you is the newer Lumicon filters, the DGM Optics (made by Omega Optical) and the Astronomiks filters, all of which I own, are all dielectric surface coated filters of extremely high quality. None of them suffer internal reflections and they perform flawlessly.
I have the highest regard for the DGM filters manufactured by Omega Optical. They are superb mechanically and their passband characteristics are perfectly matched for their intended uses. I rate the DGM NPB filter as the best UHC filter currently available anywhere. The upside is they are very competitively priced with the 1.25" NPB costing about $85 landed in Australia and the 1.25" OIII costing about $80 landed in Australia. This is about the same price as what a Taiwanese made GSO UHC filter, of unknown quality and construction will cost you. Why delve into the unknown when you can buy the best for the same price? That's not downgrading GSO products BTW, I own a 10" GSO newt which is excellent after I made a lot of mods to it. GSO have put some very good quality products into the hands of amateur astronomers at very competitive prices but in this case you can buy the best filters (DGM) for the same price as a GSO filter. I am sure if you could buy a 12" SDM scope for the same price as a 12" GSO scope, GSO wouldn't sell a lot of 12" scopes.
Cheers,
John B
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