Altair 120mm TRIBAND D-ERF Filter (Glass Only)

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. Now you can save money by combining Hydrogen Alpha, Calcium K-Line and Calcium H-Line in one TRIBAND pre-filter. Click here to see example images with Altair ERF Energy Rejection Filters (opens new window).

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, Calcium K-Line, and H-Line wavelengths, for solar imaging with your 3rd party Etalon filter**.

Bandpass is 30nm for Ha side, and 30nm for the Calcium side best contrast and blocking. Transmission is unprecedented at approx. 95% for Ha and 90% for Calcium peak. 

An individual spectrographic test report corresponding to the serial number is included with each filter. 

Please note: This product is the 120mm D-ERF filter glass element only. You will need to make a holder cell to place it in front of the objective lens of your telescope. Alternatively you can order product code: AA120DERF-TRIBAND-115CA-147OD which includes a 115mm Aperture Holder Cell, with the outer 5mm margin being used to mount the 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. WARNING: 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.

Product Features:

• Reduces approx. 98% of heat reaching the optics to keep Etalon from drifting off-band and protect your equipment.
• 30nm FHWM Bandpass at H-alpha (H) wavelength of 656.28 nm (Full Width Half Minimum) Transmission: ~95.5%
• 40nm FHWM Bandpass with CWL at 395nm covering Calcium H-Line and Calcium K-Line wavelengths. Transmission: ~90.5%
• UVIR blocking OD3 0.1% @400-1100nm blocking over 98% of heat to your 3rd party Etalon at eyepiece or camera end, E.g. Daystar Quark). Please note it's your responsibility to use a proper commercial Etalon with this product, and to follow the manufacturer's instructions.
• Visible light blocking: OD4 0.01% @400-700nm to reduce reflection effects and over 98% of heat.
• Diameter 120mm Thickness 14.0mm 
• Multilayer coatings with SiO2 protective overcoat.

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.

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.