The Altair Solar Energy Rejection Filter or ERF protects your telescope and solar imaging equipment from excessive heat build-up when solar imaging or observing with your Daystar Ha or Calcium Quark, or other Etalon filters.
Special dichroic coatings block Infrared and Ultraviolet light, as well as most of the unwanted emission spectrum of the sun, delivering only the useful Calcium and Hydrogen Alpha wavelengths, for solar imaging with your 3rd party Etalon.
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 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).
The Altair TriBand ERF filter transmits three narrow bands: Calcium Ca-K and Ca-H at the blue violet end of the spectrum, and at the red end of the spectrum Hydrogen Alpha Ha. This allows use of ultra narrowband Etalons such as Daystar Quark Chromosphere and Calcium Quark, as well as the Lunt Calcium modules. It can also be used with wider band interference filters for observing/imaging the solar continuum near the above lines, and at the G-band (430nm) and TiO band (706nm), with the Altair Solar Wedge.
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.
ERF glass is 140mm diameter with a clear aperture of 130mm diameter, 12mm thickness, using the periphery for mounting in a housing. 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.
The ERF housing fits telescopes with outer dewshield diameters of 173mm to 138mm. The 3x thumbscrews have pads to grip the telescope tube firmly without scratching it. The ERF is mounted in a strain free way within the cell with soft pads, and may feel slightly loose in it's mounting. This is intentional to keep the glass free of any strain which could affect image quality, though at 12mm thickness this is unlikely. The housing has soft tipped screws, so they do not scratch or mar your telescope tube. The housing is made from UV resistant PETG (which is tougher and more UV resistant than the usual ABS used in car plastics) with metal flakes in it to deflect heat. We also sell the glass filter on it's own for the same price, so you can construct your own housing if you wish.
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.
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