The purpose of an Energy Rejection Filter is to prevent as much solar energy as possible from entering the telescope in the first place and thus avoid excessive heating of the etalon of an H-alpha filter.
This is done most effectively by a filter in front of the telescope. At the same time, this prevents the air in the tube from heating up - the telescope remains close to the ambient temperature, and there are no air turbulences inside of the OTA due to temperature differences. If we look at the solar spectrum, it becomes clear that such a filter must primarily block visible light. Ideally, it reflects the solar energy energy instead of absorbing it and and does not heat itself up in the process as well - that's why these filters are called Energy Rejection Filters (ERF for short).
Since it is occasionally asked: A Herschel wedge or a white-light front-filter cannot be used as an ERF because it also darkens the H-alpha line. In H-alpha, the sun is far less bright than in white light, so there would be practically no light left to observe.
There are usually three types of filters mentioned, when Energy Rejection...
