The Altair 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 Ha Quark, or other Etalon filters. Because it has OD3 (0.1%) UVIR blocking and OD4 0.01% of unwanted visual wavelengths, the protection of your equipment is a priority.
The Altair MonoBand ERF filter transmits a narrow Hydrogen Alpha Ha band at the red end of the spectrum. This allows use of ultra narrowband Etalons such as Daystar Quark Chromosphere or Quantum PE, as well as the Lunt Solar modules or PST Mods. Specifications:
- ERF Cell fits telescopes with outer dewshield diameters of 173mm to 138mm.
- Clear aperture of 130mm / 5" (140mm ERF Filter 5mm edge for mounting)
- Unique Cold Cast Aluminium / Brass Cell - distortion free
- M3 Corp Silicon Rubber Cushions to protect optical tube paint
- Top quality substrate, parallelism and surface for sharp images
- Top quality Multilayer coatings with hard SIO2 protective overcoat
- 40nm FHWM* Bandpass at H-alpha (Hα) wavelength of 656.28 nm (*Full Width Half Minimum)
- Transmission: 95.5% T @ 656.28 nm
- UVIR blocking OD3 0.1% @400-1100nm
- Visible light blocking: OD4 0.01% @400-700nm to reduce reflection effects
- Diameter 140mm Thickness 12.0mm (required to achieve surface precision)
- Glass substrate is Russian K8 or German BK7 depending on availability (4x harder and less expansion than the glasses normally used in dye ERFs)
The Dichroic Energy Rejection Filter: Special Dichroic multilayer coatings on flat optical glass block Infrared and Ultraviolet light by reflection, as well as most of the unwanted emission spectrum of the sun, delivering only the useful Hydrogen Alpha wavelengths for solar imaging with your 3rd party Etalon. A D-ERF is far more efficient than the cheaper ERFs using coloured red dyed glass, because it works by reflecting heat, instead of absorbing it like dye filters. It therefore remains cool and stable. High optical quality and freedom from striae and internal tensions give the sharpest possible results.
Benefits of using a D-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 Etalon, prolonging life of the product.
- Reducers the impact of heat on your Etalon bandpass, keeping it on-band and allowing longer imaging sessions, as well as longer animation sequences. Example solar Hydrogen Alpha animation with Altair ERF (opens new window).
IMPORTANT SAFETY INFORMATION: An ERF is ONLY to be used for visual observing or imaging with a Daystar Quark, or other professionally made Etalon in place BEFORE the telescope 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 just the ERF in place - permanent eye injury will result. Never use a camera with just the ERF in place because the camera sensor can be damaged. Remember to cover your finder scope so you don't inadvertently look through it at the sun. We recommend the Altair Solar Finder which is the safest method of aligning your telescope to the sun. Always place the ERF on the scope and make sure all your equipment is installed correctly before pointing the telescope at the sun. If in doubt, consult the manufacturer.
D-ERF glass filter element: 140mm diameter with a clear aperture of 130mm diameter, 12mm thickness, using the periphery for mounting in a holding cell. 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 to maintain surface accuracy during the coating process. The coatings are ion beam dielectric coatings, and the glass annealed therefore free of Striae - internal tensions affecting image quality. Due to this process, there are usually a 2-3 very small bubbles inside the glass, usually about 0.2 to 0.5mm length, which cannot affect the image quality in any detectable way. Cleaning is the standard method. First excess dust is removed by blowing with a bulb (do not use compressed air). Then to remove stubborn greasy deposits or pollen, you can use Zeiss or other professional optical fluid and optical wipes. Wipe gently from centre to edge, while rolling the wipe to capture dust.
About the ERF Holder Cell Design and Construction:
The ERF Holder Cell is made from special "cold-cast" aluminium, and the retaining hand knobs are made from "cold cast" brass with silicon cushions to protect your optical tube. The ERF cell holds the filter glass securely by means of special pads, and is slightly loose in the holder. This is intentional, ensuring freedom from tensions which could distort the optical surface.
The ERF Holder Cell is quite special - a fusion of modern computer aided design, 3D printing technology, and old casting technologies. Cold casting is by Nick Brooks, a local British fine-art moulding expert. The initial production run is limited by the number of castings possible from the original mould, which due to heat build up, eventually disintegrates. We estimate 20-50 Cells can be made before the mould becomes useless.
When casting is complete, the ERF Holder Cell is subject to some machining operations, then hand polished and finished, giving it a unique high tech, yet timeless hand-worked quality. It's also very tough, UV resistant, and lighter than normal aluminium by volume. You can polish the surface more using Autoglym metal polish, or similar mild metal polishes and a soft cloth. We anticipate the Cell to last generations. We also sell the ERF filter itself, so you can construct your own holder if you wish.
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