Skip to content
IMPORTANT: Our store is located in Canada. All orders that include products manufactured outside of Canada or USA will be subject to tariffs and duties, regardless of the order value. US customers are responsible for all applicable duties and tariffs, and those will be billed by the carrier, except for the Lacerta upgrade kit, for which we collect them at checkout.
IMPORTANT: Our store is located in Canada. All orders that include products manufactured outside of Canada or USA will be subject to tariffs and duties, regardless of the order value. US customers are responsible for all applicable duties and tariffs, and those will be billed by the carrier, except for the Lacerta upgrade kit, for which we collect them at checkout.
Daystar Energy Rejection Filter for SCT - 63-125mm Off-Axis ERF Glass

Daystar Energy Rejection Filter for SCT - 63-125mm Off-Axis ERF Glass

by Daystar Filters
SKU DAY-E-244F63
$550.00 - $1,195.00
$550.00
$550.00 - $1,195.00
$550.00
Price Match Policy!
  • Off-axis energy rejection filter (ERF) for Schmidt-Cassegrain Telescopes
  • Safely isolates a smaller aperture for high-contrast solar viewing
  • Available in apertures from 63mm to 125mm
  • Mounting cells fit telescope tubes from 244mm to 460mm in diameter
  • Required for using Daystar Quark filters on any SCT
Backordered
Request Stock Alert
Availability Info
FREE Shipping in CONUS
  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • Daystar Energy Rejection Filter for SCT - 63-125mm Off-Axis ERF Glass

    The Daystar Energy Rejection Filter (ERF) for SCTs provides a safe, high-contrast solution for hydrogen-alpha solar observation with Schmidt-Cassegrain telescopes. This system uses an off-axis glass filter, available in sizes from 63mm to 125mm, mounted in a cell and cap assembly designed to fit tubes with outer diameters from 244mm to 460mm. By rejecting harmful heat and unfocused light before it enters the optical tube, the Daystar ERF protects your telescope's secondary mirror and internal baffles while dramatically improving the view through a downstream Daystar Quark or other narrow-band solar filter.

    Off-Axis Energy Rejection: Safe Solar Imaging for SCTs

    Using an SCT for solar observation without an ERF concentrates immense heat on the secondary mirror and internal components, risking permanent damage. The Daystar Off-Axis ERF solves this by placing a high-quality filter over a sub-aperture of the corrector plate. This approach allows only the light from a 63mm to 125mm portion of the primary mirror to enter the telescope, effectively stopping the instrument down while keeping the vast majority of solar energy outside the tube entirely.

    Enhanced H-alpha Contrast: Protecting Your Quark from SCT Heat

    An ERF is not just for safety; it is critical for image contrast in H-alpha. Internal tube currents and heat build-up inside an unfiltered SCT degrade seeing and reduce the visibility of fine solar details like prominences and surface granulation. The Daystar ERF ensures the light reaching your Daystar Quark is cool and stable, allowing the etalon to perform optimally without thermal interference.

    Sizing Your ERF: Matching 63mm to 125mm Filters to Your SCT

    Choosing the correct model requires measuring the outer diameter (OD) of your telescope's front cell or tube. Daystar offers seven options with specific inside cap diameters (ID) ranging from 244mm to 460mm to ensure a secure fit. For example, the 244mm ID cap holds a 63mm ERF, while the 460mm ID cap is designed for a 125mm ERF, providing an appropriate sub-aperture for the largest SCTs.

    Off-Axis vs. Full-Aperture ERFs: A Critical Choice for SCT Owners

    While a full-aperture ERF offers the maximum possible resolution, it is often prohibitively expensive and heavy for large SCTs. The Daystar Off-Axis ERF provides a practical and effective alternative.

    • Off-Axis ERF (This Product): Cost-effective, lightweight, and significantly safer than using no front filter. It provides a generous aperture (e.g., 125mm on a 14-inch SCT) that delivers excellent resolution on solar details while protecting the entire optical system.
    • Full-Aperture ERF: Delivers the full resolution of your telescope's primary mirror but at a substantially higher cost, weight, and mounting complexity. For most visual and imaging applications with an SCT, the performance gains do not justify the trade-offs compared to a properly sized off-axis filter.

  • Why is a Daystar ERF required for my SCT with a Daystar Quark?

    For any Schmidt-Cassegrain telescope larger than 80mm, a front-mounted Energy Rejection Filter is mandatory for safety. The SCT design concentrates a huge amount of solar energy onto the secondary mirror, which can quickly overheat and be destroyed. The Daystar ERF blocks this energy before it enters the telescope, protecting your equipment and improving image contrast.

    How do I choose the right size Daystar Off-Axis ERF for my Celestron 8" SCT?

    You must physically measure the outer diameter of the front of your telescope tube, including the retaining ring, to determine the correct cap size. An 8" SCT typically has an outer diameter around 230-240mm. Based on the options, the 244mm ID Cap with the 63mm ERF would likely be the correct choice. Always measure your specific telescope before ordering to ensure a proper fit.

    Can I use this Daystar ERF for white light solar viewing by itself?

    No. The Daystar ERF is not a standalone solar filter and is not safe for visual or photographic use on its own. It is designed to be used only with a secondary, narrow-bandpass filter like a Daystar Quark or Quantum. For white light viewing, you need a dedicated white light solar filter.

    Will the off-axis design of this Daystar ERF affect resolution on solar granulation?

    The off-axis placement creates an unobstructed aperture, similar to a refractor, which can actually improve contrast on fine details like solar granulation by eliminating the diffraction effects of a central obstruction. While a full-aperture filter would use the entire mirror, a 100mm or 125mm off-axis ERF still provides more than enough resolution to clearly see granulation and spicules under good seeing conditions.

    What is the difference between this SCT-specific ERF and a regular ERF?

    This product is a complete assembly that includes the ERF glass, a mounting cell, and a friction-fit cap sized for the outer diameter of common SCTs. A regular or "OEM" ERF is typically just the glass filter itself, intended for custom mounting by the user in a refractor or other optical system.

    How do I install the Daystar ERF cell and cap on my telescope?

    Installation is straightforward. The cap is designed for a secure friction fit over the front of the telescope tube. Simply slide the entire assembly over the front of the SCT until it is snug. Ensure it is firmly in place before pointing the telescope at the Sun.

  • Filter Type Off-Axis Energy Rejection Filter (ERF) for SCTs
    Available ERF Apertures 63mm, 80mm, 90mm, 100mm, 115mm, 125mm
    Available Cap Inner Diameters 244mm, 281mm, 311mm, 356mm, 410mm, 460mm
    Application Required for H-alpha solar observing with Daystar Quark filters on SCTs over 80mm
  • Daystar Filters Warranty