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Category: Configuring Your DayStar Filter
SCT Telescopes - ERF and Aperture Reduction
Posted on Apr 20, 2011, 4:48 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.


The DayStar Filter can be afixed to the rear of the SCT telescope with an adapter typical to photo applications from SCT to T-thread. For those who don't own this part, we stock it as part number:  GF2SCTT.   It is a small snout with male T-threads on one end and a female "jingly ring" on one end which allows you to orient the DayStar (or camera) on the back of the SCT thread.  See our adapters and snouts page for more information about the GF2SCTT https://www.daystarfilters.com/AdapterSnouts.shtml

The DayStar Filter will thread to the GF2SCTT via female T-threads on its "sun side" plate.

There is one small effect we need to consider in the off-axis application.  That light is exiting the SCT telescope off-axis in relationship to the position of the reduced aperture hole on the front of the telescope.    The angle is about 1.2° off-axis.  This may not seem like much, but it will cause issues with the filter performance if we don't compensate for that slight tilt.  We simply provide a front plate for the DayStar which has a slight 1.2° wedge machined into the thickness of the plate.  See part number:  Q3GWTF on our adapter plates web page: https://www.daystarfilters.com/AdapterPlates.shtml   When you order your DayStar Filter, we will ask what telescope it is to be mounted on.  If the answer is SCT, then a Wedge plate is supplied at no extra cost with the filter order - mounted to your DayStar Filter at our factory.  

If a customer has acquired a DayStar Filter indirectly, then the required wedge plate can be ordered and installed by the filter owner.    Note that each DayStar Quantum, ION, ATM and UNI model filter has TWO levels of plates on each end.   The owner/user may remove the outer, metal plates safely without any ha rm to the filter.  The inner, delrin plastic plates on the other hand should never be opened or removed outside of the DayStar Filters laboratory.  They hold tension on the glass inside the filter.  To loosen screws on the inner, plastic end plates could cause damage to the optical assembly.   We are shipping all new Quantum and ION filters with Phillips screws on user servicable plates and allen head screws on non-servicable plates. 
Again:  Outer Metal Plates may be removed by the customer.
             Inner, Plastic Plates should never be removed by the customer.

Once the DayStar Filter is attached to the Telescope, the orientation of the filter must be aligned to the orientation of the ERF opening.   With the LCD readout UP at 12:00 position, the ERF hole should go DOWN at the 6:00 position. 

The rear plate on the DayStar Filter has a male SCT thread just like your SCT telescope.   You may use all of the same accessories, diagonals, visual backs or camera adapters that you did before installation of your DayStar Filter. 

Tagged Keywords: telescope, SCT, DayStar Filter, Off axis, Aperture, Reduction, LX200, reflector, DayStar, Hydrogen Alpha, Halpha

Next »  How does the DayStar Mount on an LX-200

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