The Altair Dichroic Solar Energy Rejection Filter or D-ERF protects your telescope and solar imaging equipment from excessive heat build-up when solar imaging or observing with your Daystar Hydrogen Alpha Quark, or other Hydrogen Alpha Etalon filters.

The 120mm D-ERF filter element is mounted in a holder cell with 115mm clear aperture. The holder cell fits refractors with dew shield outer diameters of 147mm to 125mm* so it will fit most 4-4.5" refractors including the Altair 102mm and 115mm refractor range. The cell is made from lightweight, tough UV resistant plastic with embedded aluminium flakes. The holder cell does not put any pressure on the optics and will not distort them. The cell is extremely tough, and if dropped, has a high likelihood of protecting the filter glass from breakage, absorbing impact.

NOTE: This D-ERF filter cannot be used on it's own. It can ONLY be used in conjunction with a Daystar Quark, or other similar product, which has it's own complete blocking filters built-in. Serious eye damage will result if this product is used on a telescope without a professional Etalon product (such as the Quark) with it's own blocking filters at the eyepiece end of the telescope. If in doubt, contact Altair Astro or your dealer for clarification before ordering.

Special coatings block Infrared and Ultraviolet light, as well as most of the unwanted emission spectrum of the sun, delivering only the desired Hydrogen Alpha wavelengths, for solar imaging with your 3rd party Etalon filter**.

• Reduces approx. 95-98% of heat reaching the optics to keep Etalon from drifting off-band.
• Extra narrow 30nm FHWM Bandpass at H-alpha (H) wavelength of 656.28 nm Tolerance ~1.5%
• Transmission: 94% Transmission @ CWL 656.28 nm Tolerance ~1.5%
• UVIR blocking Average OD5 0.001% Transmission @200-1100nm reducing heat by over 98% (any remaining energy is very low and does not contribute to heating. It is additionally blocked by your Daystar Quark). OD measurement excludes 60nm of visual spectrum centred on CWL.
• Visible light blocking: Average OD5 0.001% @400-700nm to reduce reflection effects and over 95% of heat. OD measurement excludes 60nm of visual spectrum centred on CWL.
• Diameter 120mm Thickness 14.0mm, required to achieve the best surface accuracy.
• BK7 Glass with multilayer dichroic coatings and SiO2 super hard protective overcoat to seal against moisture and protect against scratching.
• ERF Holder cell included with soft tip threaded knobs. Tough and very light, easy to disassemble for cleaning the filter with optical wipes. Filter cell does not put any tension on the ERF optics.

Benefits of using an Energy Rejection Filter:

Reduces heat build-up by reflecting heat before it enters the telescope, leading to a sharper image and no heat haze from air currents inside the telescope tube.
Reduces the impact of heat on your equipment and Etalon prolonging life of the product.
Protects your telescope and prevents it from catching fire. No more burnt baffles inside the telescope tube, or cracked or melted UVIR cut filters.
Reduces heating of your Etalon which reduces the chance of bandpass shifting. Keeping your Etalon on-band allows longer imaging sessions, as well as longer animation sequences with more consistent appearance between frames, so all you have to worry about is clouds no temperature drift. Example solar Hydrogen Alpha animation with Altair ERF (opens new window).
The Altair Hydrogen Alpha D-ERF filter transmits Hydrogen Alpha or "Ha" light and have some of the narrowest bands on the market due to use of modern coating methods. They are extremely efficient at blocking of UV and IR light but permitting the desired wavelengths to pass. This allows use of ultra narrowband Etalons such as Daystar Quark Chromosphere and others for longer life. 

** IMPORTANT SAFETY INFORMATION: An ERF is a pre-filter and is ONLY to be used with a Daystar Quark, or other professionally made Solar Etalon Filter in place BETWEEN the telescope focuser and eyepiece. Any Etalon must incorporate full blocking filters. If in doubt, ask a professional. An ERF cannot be used with a standard Hydrogen Alpha filter such as that used for deepsky imaging. Never look at the sun with only the ERF in place - permanent eye injury will result. Never use a camera with only the ERF in place because the camera sensor can be damaged by intense light. Remember to cover your finder scope so you don't inadvertently look through it at the sun. We recommend the Altair Solar Finder or similar solar finder without lenses, which is the safest method of aligning your telescope to the sun. Always place the ERF on the scope first, cover it, and make sure all your equipment after the Etalon is installed correctly before pointing the telescope at the sun. If in doubt, consult the manufacturer of your Etalon.

D-ERF Pre-Filter coatings and structure:

ERF filter elements are typically a little larger than the clear aperture provided by a mounting cell, because the outer periphery is used to hold the filter safely in position. The glass substrate BK7. Glass blank specification is 8th wave surface accuracy optically polished with 30 arc second parallelism prior to coating. Coatings are baked to release tension after they are applied. Altair ERF filters are usually quite thick, in order to maintain surface accuracy after the coating process, and during use. The multilayer dielectric hard-coatings are protected by a tough silicon SiO2 overcoat and the glass annealed to be free of Striae, which are internal tensions affecting image quality. Edges are ground and chamfered to protect them.

ERF Cell / housing:

The outer housing is made from a tough durable, UV stable plastic with embedded aluminium flakes to reflect heat and light away from the telescope and optics. The inner surfaces of large ERF holders are conical to reduce the chance of boundary layer air currents developing near the aperture. 3x large thumbscrews with special soft pads grip the telescope tube firmly without scratching it. The housings are 3D printed and the ERF is mounted in a strain free manner within the cell with soft pads. The ERF glass may feel slightly loose and may rattle slightly in it's mounting. This is intentional, and ensures the glass is free of any strain, which could affect image quality. Should you want to remove the ERF glass from the holder for cleaning, this is easily achieved by unscrewing the stainless steel retaining screws. The optical element will drop out if inverted, so please do this over a towel or similar soft surface. It's also easy to clean the glass without removing it from the housing.

Cleaning your D-ERF:

Use the same method for cleaning any astronomy filter. First, excess dust is removed by blowing with a bulb (do not use compressed air). Then to remove stubborn greasy deposits or pollen spots, you can use Baader Wonder fluid (ordered separately), Zeiss lens wipes or fluid, or other professional optical fluid, and optical wipes. You can also use isopropyl alcohol. We recommend spraying cleaning fluid on an optical wipe, not on the glass directly. Do not use microfibre cleaning cloths as they tend to grab particles of sand and so-on, dragging them over the surface of the optic and causing scratches. We recommend using purpose made natural fibre optical cleaning tissue or wipes. Wipe gently from centre to edge, while rolling the wipe to capture dust. Wipe from centre to edge, not in a circle, lifting and rolling the wipe to prevent particles being picked up and causing scratches. Use a fresh part of the wipe for each stroke. The ERF housing is impervious to alcohol based cleaning fluids such as Baader or Zeiss lens fluid. However, do not use acetone or any chemical which can dissolve plastic, because it can damage the housing or dissolve the glue which holds the soft pads to the housing.

Scope of Delivery:

• Altair 115mm Aperture Hydrogen Alpha D-ERF (120mm filter with housing cell)