Teleskop Service on their webpage describe simply as "prevents heat from entering the telescope" This describes perfectly what the filter does, but personally feel does not do it full justice. The following is taken directly from their website and gives further technical details about the filter.
The D-ERF filters by Baader are unique - and made in Germany! The filter glass is, in opposite to other makes, plane-parallel polished with a precision of 1/10 λ. The have an effective infrared reflective coating protecting the tube's internal parts from heat. Tube seeing is reduces and expensive filters near the focus are protrected from thermal stress. This is a prerequisite to use H-α narrow band filters.The Baader D-ERF main advantages:-- High quality plane-parallel polished substrate with a surface precision of 1/10 λ.
-- Dielectric interference layer coating system for IR reflection
-- Multi-layer anti-reflective coatings to prevent reflections
-- Stress-relieved substrate
-- Diameter: 75mm
-- IR and UV reflecting
-- Dielectrical coatings
ERFs or Energy Rejection Filters like this are used in Hydrogen Alpha solar scopes to reject all but a narrow band of energy centred on the Ha line at 656.28nm, this can be seen in the graph above. If not rejected, the IR and UV light can damage delicate coatings in etalons and blocking filters, not to mention protecting our eyesight. Intially designed for the Solar Spectrum and Daystar rear mounted Hydrogen Alpha filters, these ERFs have found favour with amatuers doing Coronado PST mods - removing the etalon from a PST and incorporating this in a much larger refractor with suitable filtration - ie. a Baader D-ERF.
I have purchased several items from Teleskop Service in the past and have never ceased to be impressed with their level of service and communication, and their dealings with them in this instance lived up to my expectations. The filter was shipped by UPS and arrived in a small shoe box sized cardboard box.
Upon opening this I found wrapped in copious amounts of bubble wrap another smaller and flatter cardboard box. Within this was more bubble wrap, and inside this a bubble wrap bag that contained a small jiffy bag, which, within this was the ERF which was wrapped in a tissue paper. I have to say I am completely impressed with the care and attention that Teleskop Service have gone to with their packaging, and short of the box being run over by a truck it is highly unlikely that any damage to it's contents are ever likely to occur. Very impressive!