Altair 120mm Hydrogen Alpha D-ERF Filter (Glass Only)

Product code: AA120DERF-HA30-BK7-GLASSONLY
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£450.00 UK Price
£375.00 (ex. VAT) International Price
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Product information

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. Click here to see example images with Altair ERF Energy Rejection Filters (opens new window).

NOTE: This is the filter glass element only. Please contact us for mounting options including holders for Altair 125mm and 130mm Refractors.

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*.

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.

  • 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.

Benefits of using an ERF on scopes of 4" or larger:

  • Reduces heat build-up by by approx. 95-98% 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.
  • 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 and OD4 blocking of visible wavelengths outside the desired target. This allows use of ultra narrowband Etalons such as Daystar Quark Chromosphere and others for longer life. 

* 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 first and make sure all your equipment is installed correctly before pointing the telescope at the sun. If in doubt, consult the manufacturer.

D-ERF Filter technical information: 120mm diameter ERF glass is 12.0mm thickness, which when mounted in an optional ERF Holder, usually gives a clear aperture of 152mm / 6" diameter because the outer periphery is used to hold the filter. 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. Due to this process, there can sometimes be 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. Edges are ground and chamfered to protect the coatings.

Optional housing: An optional art quality housing can be purchased for your Altair D-ERF please contact us with your dew shield diameter in mm. You can also construct your own housing if you wish using various materials.

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.

Product Info

Brand Altair
Delivery Weight Package