Users will be pleased to know that the NSO/SP K-line filter is back in operation after a four-year downtime. In 1992, the filter had been sent to Halle for repair of a cracked calcite polarizer. It never worked properly after it was returned. Recently we found that the thinnest element in an "Evan's split element" was rotated 45° causing the central thick element to be totally off band. We had great difficulty in de-cementing the filter because Halle used RTV-based cement that didn't readily dissolve in a wide variety of solvents.
The filter is 0.3/0.6/1.2Å wide and is manually tunable. Videos of active regions show that the filter has excellent contrast. Its use can be requested by checking the "branch filter" box on the observing form.
Dick Dunn, Steve Hegwer