Altair 120mm Hydrogen Alpha D-ERF Solar Filter

Product code: AA120DERF-HA40NM
in stock
£332.50 (ex. VAT)
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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.

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.

This is the filter glass element only. Please contact us for mounting options including holders for the Altair 150EDF Refractor.

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 (*Full Width Half Minimum)
  • Transmission: 95.5% T @ 656.28 nm
  • UVIR blocking OD3 0.1% @400-1100nm (remainder is very low and blocked by 3rd party Etalon at eyepiece end).
  • Visible light blocking: OD4 0.01% @400-700nm to reduce reflection effects and 95% of heat.
  • Diameter 120mm Thickness 12.0mm (required to achieve surface precision)
  • Multilayer coatings with SiO2 protective overcoat

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.
  • 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 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 115mm / 4.5" 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 made from cold cast aluminium with cold cast brass screws to fit telescopes with outer dewshield diameters of 173mm to 138mm. 3x thumbscrews have special soft pads to grip the telescope tube firmly without scratching it. The housings are individually cast and processed by hand, and have slight variations in the surface, making them unique on-off collector's items. The ERF is mounted in a strain free manner within the cell with soft pads, and may feel slightly loose in it's mounting. This is intentional and ensures the glass free of any strain which could affect image quality, though at 12mm thickness, this is unlikely. 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
Product Code AA120DERF-HA40NM
Delivery Weight Package