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UHC filters graduations?

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Can someone please help me with understanding what is meant by a UHC filter with a -1 or a -3? 

Such as with https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F382412136463



Thanks beforehand. 



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They don't have a type, 1, and 3 UHC filters on Optolong's website. Perhaps these numbers are just auction numbers to identify a batch of similar filters.





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Thanks for replying :icon_biggrin: I'm curious because the plastic filter box has the numbers and it looks like it was put there at factory.


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Those numbers don't form part of the specification of the UHC filters as far as I can see. As has been said, they may be stock, batch or item numbers.

From the light transmission graphs in the filter descriptions, they seem to be fairly conventional UHC type filters with a reasonably wide band pass width. The key O-III and H-Beta lines are admitted at over 90%.





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