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Post Info TOPIC: UV-IR CUT filter


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UV-IR CUT filter
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 Revelation Infra Red Blocking Filter 1.25"

This standard size 1.25" filter will thread directly in to the majority of standard webcam adaptors as well as eyepieces, barlows and accessories threaded for filters.
The IR blocking filter is an interference filter with a steep, precisely controlled filter curve. For visual use the filter seems fully transparent since the human eye doesn't perceive the IR and UV light.

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Price £16.00 (Includes post & packaging for the UK)

Ed ~ Preliminary testing indicates that it has a steep and effective IR-cutoff. The price is a fraction over ebay video camrecorder filters from Hong Kong, but has a correct sized filter cell (1.25").

Revelation Infra Red Blocking Filter Test.

 80mm refractor and Vesta pro webcam used.



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IRCUT.gif



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Ultraviolet filters
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UV filters are used to reduce haziness created by ultraviolet light, to which photographic film and sensors are sensitive, but not the human eye. A UV filter passes all or most of the visual spectrum, and blocks ultraviolet radiation.
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Achromatic refractor telescopes will bring ultraviolet light to a slightly different focal point relative to other wavelengths; and many colour films are sensitive to Ultraviolet light which will result in a blue cast and image blur or reduced contrast.

A UV filter should pass most of the visual spectrum, and block ultraviolet radiation, lessening the bluish haze seen in distant subjects.
UV filters should look clear and neutral to the naked eye, and can protect the front lens or digital sensor chip from physical damage and dust.

However, digital camera sensors are not as sensitive* to UV light as photographic film, and IR-cut filters are equally good at keeping the digital sensors dust free.
And it is debatable if adding a filter in the light path just to protect the front lens outweighs the slight reduction in image quality caused by the filter itself.

*CCDs are sensitive down to ~250nm, but the protective glass and coating over the digital chip already blocks the ultraviolet light



The bottom line is that with modern digital sensors UV filtration is no longer necessary.



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UV-IR CUT filter
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Schott Glass-Short Pass-IR Blocking-KG3 Filter

PART CODEDIAMETERCENTRE THICKNESSMATERIALPRICE
SPF-KG3-06016mm1.00mmSchott KG3
£8.00
SPF-KG3-120312mm3.00mmSchott KG3
£11.00
SPF-KG3-250325mm3.00mmSchott KG3
£15.00
SPF-KG5-250325mm3.00mmSchott KG5
£15.00
SPF-KG1-31.40531.4mm5.00mmSchott KG1
£20.00
SPF-KG3-5050033.00mm50mm x 50mmSchott KG3
£28.00


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irglass.gif



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Achromatic refractors form an out-of-focus IR image on webcam sensors, so it is important to filter out those wavelengths (less than 700nm).

Those on a tight budget may wish to reuse the IR filter from the web camera lens. The tiny IR-cut filter is usually mounted on the rear but sometimes can be found on the front of the webcam lens. The fiddly job of removing the delicate filter and mounting it (perhaps between two metal spacers?), and then fitting that into the telescope/webcam adaptor, is not for those with shaky hands.
But it is possible...

A less surgical procedure is to reuse a green/blueish heat-absorbing filter from an old slide or cine projector. Light bluish coloured infrared filtering glass can also be found in photocopier machines. These filters will effectively remove the excessive IR.



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IR-Cut Filter
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Tanaka Engineering IR Cut Filter

IR-Cut Filter



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Altair IR cut-off filter
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Altair-EX 1.25" IR Cut Blocking Filter

The IR cut-off filter blocks the transmission of Infrared light, while passing visible light. Webcam chips are very sensitive to IR. Because it has a longer wavelength than visible light, when IR passes through optics, it comes to focus at a different point. This causes a blurring effect in Monochrome images with loss of fine detail. When you put a webcam adapter (available separately) on your webcam, the factory IR-Blocking filter is removed (it's usually part of the lens assembly and pretty low quality anyway). This filter fits on to the front of your webcam adapter to make sure that no Infrared light lands on the chip. Please note, this filter cannot be used to image the sun on it's own - it should only be used with the appropriate solar filter on the front of the telescope.

£14.99

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Ed ~ this will not block Ultraviolet frequencies.



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Infrared cut-off filter
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Heat absorbing glass filters Schott® KG-1, KG-2, KG-3 and KG-5 or Hoya® HA-30, HA-50 are used to reduce the amount of heat transmitted through illumination optical systems.
KG glass filters absorbs the infrared radiation and then dissipates the heat into the air around the glass.

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irglass.gif

Infrared cut-off filters, sometimes called IR filters or heat-absorbing filters, are designed to reflect or block mid-infrared wavelengths while passing visible light.
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UV-IR CUT filter
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UV-IR CUT filter will block undesirable ultraviolet and infrared rays that may interfere with the image. The filter will remove the IR chromatic aberration in lens imaging systems, and give pinpoint star images with digital and CCD-equipment.
This is especially important where the digital image sensors (webcams) have had the IR protection filter removed.
ie. Without the filter, IR wavelengths will blur fine detail in digital images.

It should be noted that reflector telescopes do not require this filter.

Baader UV/IR Cut Filters

 1.25" £ 38.00   
 2"      £ 65.00
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This filter is an 'must have' for digital imaging through a refractor. But there is no need to buy an 'specialist' astronomy filter costing £30-40, when you can use a cheaper, and equally effective, video camera filter.

28mm Optical UV-IR CUT filter

£14.00
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Ed ~ these filters can be removed from their holders and inserted into a spare eyepiece filter, or glued directly into a webcam adaptor.

 

1.25" IR Filter / infra red blocking filter

£14.50
Read more

 

For those on a tight budget, a filter from a broken digital camera will do the same job.



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