dlastro

I have struggled with this problem with red dot finders for years. As has been mentioned, most RDFs are repurposed gun sights. As such they have an interference coating on the concave lens at the front that both enhances the reflection of the red LED light and dims the light from the target star. This is exactly what we don't need in an astronomical finder. The problem could be solved if the importers of RDFs stipulated that the lens NOT be coated and that a higher resistance potentiometer be used. My first RDF did not have a coated lens and the dimmest setting was very dim. If you want to dim the light of the LED either of the two methods suggested will work, introducing another resistor into the dimming circuit or putting a transparent cyan (minus red) filter in the LED light path. Red filters are transmit red light and will not dim it. Photographic color correction filters such as a CC50 or CP50 filter are ideal. More than one layer may be necessary and requires a bit of experimentation. The filter should be cut to fit and placed in the housing at the back of the finder where the red LED light is emitted so that it interrupts the beam. I folded mine into a "U" shape and inserted it into the channel in the forward part of the rear housing. A piece of tape holds it in place. This should work with other RDF designs proved the filter can be placed in the LED beam. Unfortunately nothing can be done about the coating on the lens short of the importers stipulating RDFs without coated lenses. I have not experimented with the resistor modification since inserting a filter is much easier.