Forbidden Lines

I wore out the threads to my OIII filter looking at the Veil Nebula over the years. Last week I couldn't understand why it wouldn't screw in securely into my 21mm Ethos eyepiece, even though my old UHC filter would. Then it dawned on me: 20 years of screwing it and unscrewing it certainly took its toll.

I was proud of that Lumicon filter: it was labeled as "Premium," boasting 95% light transmission at the 510 nm OIII band. I also have an 1.25" filter of the same era that wasn't "Premium," and it is "only" 91% at that band. I was going to buy a new filter housing when it occurred to me to ask if OIII designs had changed over 20 years. Duh!

Indeed, there are now competitors. Lumicon, now owned by Parks Optical, still labels their filters by hand, and their "Premium" filters boast a slightly higher transmission. However, I opted to try a German OIII filter by Astronomik. Their filter has similar transmission, but in a narrower band, so the overall contrast will be higher.

OIII filters are magical and I'm counting on stronger magic the next dark period. One can argue the virtues of the UHC and Deep-Sky filters, but it was the OIII that was worn out. Ultimately, I would have to say that it was memories of the detail in the Veil Nebula that drew me back to an 18" scope. I just can't stay away from those forbidden lines.