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The need for IR/UV cut filters with reflectors.

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#1 mattproulx86

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Posted 23 February 2020 - 03:01 PM

I have a ZWO ASI294MC camera which does not have a built in IR/UV filter. I see a lot of people recommending using the ZWO IR/UV block filter with refractors to reduce star bloating. My question is, is this a problem with reflectors? Currently I use a Celestron UHC/LPR and Optolong L-Enhance, I would like to try imaging galaxies without any filters if possible. I should mention that I have bortle 4 skies. 


Edited by mattproulx86, 23 February 2020 - 03:07 PM.


#2 DrGomer

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Posted 23 February 2020 - 03:19 PM

All reflective coatings we use in the visible will reflect a significant amount of UV and IR, so I'd say, yes, you still need it.   UV/IR filters shouldn't impact imaging at all in a negative way unless they are scratched or have junk on them. 



#3 sg6

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Posted 23 February 2020 - 03:20 PM

The Optolong appears to only pass Hb+OIII the Ha and blocks wavelengths below 470nm (UV) and above 700nm (IR).

So the Optolong acts as a UV/IR block.

 

The filter has no data for below 470 and above 700 so the presumption is that nothing is passed. Which may be a little questionable. But the filter curves they give say nothing less the 470nm (UV) and nothing greaterr the 700nm (IR).


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#4 TOMDEY

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Posted 23 February 2020 - 03:39 PM

All-reflective telescopes are free of chromatic aberrtions... so will not cause stars to bloat.    Tom


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#5 DSOs4Me

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Posted 23 February 2020 - 03:54 PM

If you have light pollution around you would want to use one. For me it's the Baader UV/IR cut.



#6 OldManSky

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Posted 23 February 2020 - 04:04 PM

All-reflective telescopes are free of chromatic aberrtions... so will not cause stars to bloat.    Tom

True...unless you use a coma corrector (a refractive set of elements), in which case...not so much :)


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#7 nic35

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Posted 23 February 2020 - 04:05 PM

+1 on what TOMDEY says,  Plus, galaxies are strong IR emitters, IIRC.  So you don't want to filter out the IR range - you are throwing away photons..  Bortle 4 skies shouldn't cause light pollution problems.  If you feel a need for a light pollution one, then try the Astronomik CLS non-ccd.  It passes more of the IR than does the "regular" CLS.

 

john


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#8 TOMDEY

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Posted 23 February 2020 - 04:27 PM

+1 on what TOMDEY says,  Plus, galaxies are strong IR emitters, IIRC.  So you don't want to filter out the IR range - you are throwing away photons..  Bortle 4 skies shouldn't cause light pollution problems.  If you feel a need for a light pollution one, then try the Astronomik CLS non-ccd.  It passes more of the IR than does the "regular" CLS.

 

john

Yes! I also use a Night Vision Eyepiece that is sensitive into the Near Infrared... and galaxies are quite spectacular using that... can actually see the NIR!    Tom



#9 evan9162

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Posted 23 February 2020 - 05:02 PM

All-reflective telescopes are free of chromatic aberrtions... so will not cause stars to bloat.    Tom

 

Actually, they will.  For a given aperture, the spot size of the airy disk of a star is larger in IR than it is in visible light.  The resolving power of an optic is dependent on the wavelength of light as well as the aperture size.  



#10 TOMDEY

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Posted 23 February 2020 - 06:45 PM

Actually, they will.  For a given aperture, the spot size of the airy disk of a star is larger in IR than it is in visible light.  The resolving power of an optic is dependent on the wavelength of light as well as the aperture size.  

Meh... of course... but that's not chromatic aberration; it's the Heisenberg uncertainty principle. Say, here's a brain-teaser puzzle that I submitted to our lens design class back in the 1970s >>>

 

Design a telescope whose Airy Disc is wavelength-invariant.

 

Hint: The solutions are deceptively simple, but have some subtle nuances! I'll present a couple later, if there is any interest (and if no one comes up with any aforehand).

 

Bonus puzzle: Design a white rainbow.  Tom



#11 KLWalsh

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Posted 23 February 2020 - 11:22 PM

Diffraction is a property of waves, not necessarily Heisenberg Uncertainty. Macroscopic waves (water waves) also exhibit diffraction.
While it’s true that IR energy will have a larger diffraction pattern than visible light, that also means the energy is spread over a larger area. Thus the intensity at a given spot would be lower, all else being equal.
The intense blue usually seen around the Class B stars in photos of the Pleiades is due to the smaller diffraction pattern for blue wavelengths.


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