ERFs are used for 2 main reasons.
- Improved image quality, by reducing heat buildup and hot air currents inside your telescope and by keeping your Hydrogen Alpha or Calcium filter at a stable constant temperature, it keeps it “on-band” – Etalons of all types are very sensitive to heat, and some even use heat for tuning the bandwidth.
- To protect your equipment. It’s quite easy to burn the interior of your telescope if the sun is off-centre even for a short period of time. This is far more likely at 4″ aperture and above, and it works just like a magnifying glass in the sun. This is a common cause of telescopes being sent for servicing.
Therefore, for ANY scope, using an ERF is always sharper due to less heat currents, and your equipment stays “on-band” better for longer. However most solar imagers consider an ERF as optional for apertures of 4″ or less if the Hydrogen Alpha equipment you are using has built-in safety pre-filtering such as the Quark. (Even then we recommend a 2″ (not 1.25″) UVIR cut filter on the front of the diagonal where the spot size is still relatively large and less concentrated, being further forward of the focal plane. For 4″ aperture or more, ERFs are recommended to prevent damage and improve image quality.