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Category: Energy Rejection Filters
UV/IR Cut Filter application for Refractors
Posted on Apr 20, 2011, 10:40 pm Author: Jen Winter

UV/IR Cut Filter application for Energy Rejection on Refractor Telescopes

Certain refractor telescope owners may choose to employ a UV/IR Cut filter for means of energy rejection INSTEAD of a front mounted colored glass ERF filter.    Not all refractor telescopes may use this option.  The UV/IR cut filter is a dielectric coated filter that reflects UV light and IR light like a mirror.   It does not absorb light and therefore is not succeptable to heat gain or expansion like colored glass ERF's are. 

Applied correctly, the UV/IR cut filter acts in a way similar to a Herschel Wedge, reflecting much of the spectrum back out of the telescope, while passing a smaller percentage of light.  In the case of the UV/IR cut filter, the light which is allowed to pass is the entire visible spectrum.   

UV/IR Cut filter shown here threaded on the end of the 4X powermate


Because the UV/IR Cut filter acts as a mirror, it is best inserted in the light path prior to any other optical element which might encounter concentrated light.   For example, the UV/IR cut filter may be threaded onto the front snout of the forward section of a diagonal mirror.  Most diagonals offer filter threading in their front end.    If a powermate or barlow encounters concentrated light before the diagonal, then the UV/IR cut filter should be threaded onto the lead end of that element. 

Unwanted light which hits the UV/IR cut filter is then reflected back out the front of the telescope, arriving at the objective lens at nearly the same unfocused beam as it does coming through the front side.  

UV/IR cut filters are available at many telescope dealerships in a variety of brands and manufacturers in 1.25" or 2" sizes.  
the size of the UV/IR cut filter should match the element on which it is to be screwed.   Most often, this is a 2" diagonal filter or a 2" TeleVue 4X Powermate.

Instances where the UV/IR cut filter should not be applied include: 
- Oil Spaced Triplets.   We are uncertain of the effects of high amounts of solar energy on the oil used in spacing these triplets.
- Ptezval Refractors.  Any telescope with lenses positioned in the light path where the UV/IR cut filter may not be inserted before are unsafe applications of this technology.
- SCT or reflector telescopes.   SCT and reflector telescopes encounter concentrated light on the secondary optics.  Concentrated light on a secondary would cause overheating of that element, cracking and/or possible failure of its adhesive.

In general, if a telescope owner is uncertain or uncomfortable with the application of UV/IR cut filters on the rear of the telescope, then it is best to revert to the standard DayStar Colored glass front mount Energy Rejection pre-filter.

Tagged Keywords: refractor, prefilter, ERF, UV/IR, filter, cut, daystar, energy rejection

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