The purpose of using an energy rejection filter (ERF) is to block as much solar energy as possible from entering the telescope, thus preventing excessive heating of the H-alpha filter etalon.
This effect can be effectively achieved by attaching an ERF filter to the front of the telescope.
At the same time, the air inside the tube is prevented from heating up - the telescope remains close to room temperature and there is no air turbulence inside the tube due to the temperature difference. If we look at the solar spectrum, we see that this filter mainly blocks visible light. Ideally, it reflects the sun's energy instead of absorbing it and does not heat up in the process - which is why these filters are also called Energy Rejection Filters (ERF).
The ERF filters from Artesky are unique in the world; their dielectric coating provides a beam of pure light with an FWHM of 60 nm!
The glass substrate in this case is not just any soft, coloured absorbent with streaks, but transparent of very high homogeneity, finely polished to the quality level of the lens surfaces and with all the powerful reflective layers on one side.
These reflect the entire desired spectrum from 300 nm to 1100 nm, leaving only the 60 nm wide spectral window around H-alpha (656.26 nm) open.
If these coating systems are not properly executed on both sides of the plate to equalise the stresses induced by the coating layers, the surfaces parallel to the plane can be easily distorted, meaning that the flat 1/10 wave plate deforms into an irregular substrate curve that acts like a (bad) lens.
The filter layers of the Artesky ERF are developed with the utmost care and applied using the most advanced optical coating machines in the world,
This care preserves the resolving power of the telescope's entire objective aperture and drastically reduces the thermal energy that passes through the telescope and reaches the solar spectrum filter.
This is not only the main reason for the significant reduction in thermal stress inside the telescope, but also greatly reduces heat-related visual effects.
Important note:- Artesky ERF energy rejection filters are only intended for use as pre-filters for H-alpha filters in the solar spectrum.
- The ERF filter must always be present and mounted in front of the telescope objective.
- The filter must be orientated so that the surface of the main cover faces the sun (an arrow on the edge of the filter points to this side). The ERF must be used together with the H-alpha Daystar filter or a similar filter.
- Using the ERF alone for visual observation will lead to irreparable eye damage.
Scope of delivery:Note: This filter is sold without cell.
Products specifications
| Type of filter | ERF Energy Rejection Filter |
| Thickness | 12mm |
| Diameter | 200mm |
