Altair 115mm Aperture Hydrogen Alpha D-ERF for 4" 4.5" Refractors
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 special cell with 115mm clear aperture to fit your refractor. The ERF filter holder fits dew-shield diameters of 147mm to 125mm* which is compatible with most 4" / 102mm to 4.5" 115mm refractors*. The ERF cell is made from lightweight, tough UV resistant plastic with embedded aluminium flakes. The cell does not distort the optical surface of the filter, allowing for top quality images.
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.
- 40nm FHWM Bandpass at H-alpha (H) wavelength of 656.28 nm
- Transmission: 95.5% T @ 656.28 nm
- UVIR blocking OD3 0.1% @400-1100nm reducing heat by over 95% (any remaining energy is very low and does not contribute to heating. It is additionally blocked by your Daystar Quark).
- Visible light blocking: OD4 0.01% @400-700nm to reduce reflection effects and over 95% of heat.
- Diameter 120mm Thickness 12.0mm, required to achieve the best surface accuracy.
- Multilayer coatings with SiO2 protective overcoat
- 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.
- Snap-on protective lens cap included.
Benefits of using an ERF on scopes of 4" or larger:
- 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 the internal structure of your telescope. 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 has an FHWM of 40nm. It has extremely efficient blocking of UV and IR light. 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 Filter technical information: ERF Filter element is 160mm diameter and 12.0mm thickness. When mounted on the Altair ERF Holder, it gives a clear aperture of 152mm / 6" diameter because the outer periphery is used to hold the filter safely in position. The glass substrate is Russian K8 or German BK7 depending on availability. Glass blank specification is 8th wave surface accuracy optically polished with 30 arc second parallelism prior to coating. The filter is 12mm thick, in order to maintain surface accuracy during the coating process. The multilayer dielectric 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 the coatings.
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 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 do this over a towel or similar soft surface. It's also easy to clean without removing and the housing is impervious to alcohol based cleaning fluids such as Baader or Zeiss lens fluid.
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) or Zeiss, or other professional optical fluid, and optical wipes. Wipe gently from centre to edge, while rolling the wipe to capture dust.
*We can make custom threaded knobs if you have an optical tube which is narrower than this - contact us for more info.
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