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When to use IR filter??

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#1 James Cunningham

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Posted 25 March 2013 - 08:29 AM

What DSO's are best viewed with an IR filter? Thanks.
Jim

#2 core

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Posted 25 March 2013 - 09:16 AM

An unfiltered sensor can be quite sensitive to IR. When an image comes into focus onto the sensor in the visible spectrum, the IR portion may not be in focus (think CA). This unfocused IR (eg, pin-point visible stars become bloated stars in IR) gets picked up by the sensor and manifest itself in the output. Then again iirc many have reported here in this forum that they've seen no difference.

#3 GlennLeDrew

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Posted 25 March 2013 - 09:30 AM

Extending the spectral response out to the 1000-1100nm range should result in no particularly notable difference in the overall view of DSOs. As noted, the more notable difference is the difficulty to maintain focus with refractive optics, hence the usual practice of filtering out the IR.

Allowing the IR to pass does better record the emission from very cool stars whose peak emission is in this region. A filter which blocks blue and green (perhaps red as well) light but passes IR will show carbon stars, red giants, etc., as relatively brighter with respect to hotter stars, in some cases markedly so. And spiral galaxies would show a fairly different aspect, with the narrower spiral arms both diminished in intensity and diffused.

#4 mclewis1

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Posted 25 March 2013 - 09:53 AM

Jim, First are we talking about an IR cut filter or the less popular IR pass filter?

IR pass filters are primarily used for planetary imaging with monochrome sensors to help reduce some scintillation effects from our atmosphere. These filters are pretty rare and generally have no place in viewing DSOs with video cameras ... although having said that I'll bet that someone will come along and describe something cool then can do with one of these filters ... lol.

IR cut filters on the other hand are much more popular in the imaging and video/live viewing world. In general many filters (LP for example) include some IR cut capability (cutting or blocking near IR and longer wavelengths), and there are also individual IR and IR/UV cut filters that do this by themselves. The primary reason for IR cut or blocking in any filter is to make stars tighter when you have refractive elements (glass) in your optical path.

In a telescope IR light doesn't come to focus in the same place as shorter visible light lengths and the more glass you have in your optical path the more pronounced this difference becomes (and the larger the bloating of the stars). Some CCD sensors have more sensitivity in the near IR region than others which can also make the bloating more pronounced.

So if you have no glass (Newtonian reflector) or very small amounts (thin like the corrector in an SCT) you don't need any IR cut/blocking. Most folks with SCTs however also use focal reducers and sometimes find a bit of IR blocking useful. So you'll likely see all kinds of comments about how useful IR blocking is ... it's very dependent on your hardware.

As for DSOs there's an IR component in many galaxies especially the irregular ones as well as in some extended nebulae (think star forming regions). I don't know of anyone in the video/near live viewing mode who really wants to block any of this additional information ... everyone usually wants as much image or signal as possible. Using an IR blocking filter makes some objects look a little different but that seems to be more of a curiosity rather than something desirable. If you are imaging/video viewing through a lot of glass (refractors or in particular telephoto lenses) then some IR blocking is very desirable.

So I don't think there's anything that's "best viewed" or enhanced with IR blocking, just those DSOs that change a little bit and have nicer looking stars around them. Since most of use use some form of LP filtering we're getting some IR blocking whether we want it or not. You'll often see folks talking about one particular filter or another that passes more IR light and is therefore somewhat better on galaxies.

If you don't use any other type of filtering and are using a refractor or telephoto lens and want tighter stars then a separate IR cut filter would be a good choice. Beyond that I'm not sure what use a separate IR cut filter would be.






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