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Category: Energy Rejection Filters
SCT Energy Rejection Filters
Posted on Apr 20, 2011, 10:23 pm Author: Jen Winter

SCT Telescopes typically employ a front mounted Energy Rejection Filter (ERF) with an off-axis opening of reduced aperture.  

By reducing the aperture, the focal ratio of your SCT will be increased from the native F/6 to F/10 to an ~F/30 beam.   Because the SCT has a central obstruction from the secondary, we need to move the opening off-axis.   Now the beam that exits the SCT telescope will be smooth and parallel when it reaches the rear mounted DayStar Filter.   Aperture reduction does not change your overall focal length.  Your image size will remain the same as before the aperture was reduced.  Only resolution will be reduced.   Typical apertures of SCT telescopes reduced for use with DayStar Filters are 90mm or greater.  Please see our ERF wizard pages for the correct ERF for your SCT Telescope.


Once the DayStar Filter is attached to the Telescope, the orientation of the filter must be aligned to the orientation of the ERF opening.  

On a Quantum Filter,  the LCD readout UP at 12:00 position, the ERF hole should go DOWN at the 6:00 position.

On older ATM Filters,  the cord goes DOWN  at 6:00 position, the ERF hole should go UP at the 12:00 position.


While some refractors are able to use a rear mounted UV/IR Cut filter for energy rejection, SCT telescopes and reflectors cannot.   The rear mount application would allow concentrated sunlight to hit the secondary, heat up and compromise the adhesive of the mirror.   DayStar does not recommend use of UV/IR Cut filter ERF options on an SCT Telescope.


Tagged Keywords: Schmitt, Energy Rejection, Filter, ERF, SCT, LX 200, C5, C14, DayStar, Filters

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