KG3 IR Absorption vs IR Blocking filters? Same?

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MalVeauX
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KG3 IR Absorption vs IR Blocking filters? Same?

Post by MalVeauX » Wed Apr 11, 2018 4:59 pm

Hey all,

I broke a KG3 today. Looking to replace it for a DIY kit (the Skybender) that I use to image in 393nm and 540nm. Dunno how I broke it, but when I was tilt-tuning with the Skybender I heard a sound and looked in there and found the IR filter (KG3) with a total crack down the center. Only looking for 1.25" size.

Is a KG3 IR absorption filter the same as an IR blocking filter? I can readily find IR blocking filters. But IR absorption, like the KG3, is different maybe? It's only purpose is to reject heat and allow 393nm and 500~540nm pass.

Thanks!

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by Merlin66 » Thu Apr 12, 2018 5:10 am

If you check out the KG3 transmission curve, you'll see it acts as a UV-IR blocker.
https://www.google.com.au/search?q=Kg3+filter&rlz
It looks a good match for 393nm (whereas "normal" UV-IR filters cut at around 400nm and effectively block the CaK.)
You probably should replace like with like. Adding just an IR filter won't cut the UV side.
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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by marktownley » Thu Apr 12, 2018 6:55 am

MalVeauX wrote:
Wed Apr 11, 2018 4:59 pm
Is a KG3 IR absorption filter the same as an IR blocking filter? I can readily find IR blocking filters. But IR absorption, like the KG3, is different maybe?
Hi Marty, KG3 blocks a lot further into the IR than a IR Blocker filter. I use KG3 in my CaK filter, also in a skybender (thanks Apollo!) I find it helps increase contrast when imaging,

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by MalVeauX » Thu Apr 12, 2018 10:28 am

Thanks guys,

I just ordered a new KG3 from NewPort (same exact filter). Just was curious if there was a way to make it even better compared to absorption versus reflection, etc, regarding IR.

I'm mostly interested in 393nm with it, as an inexpensive way to play with near Calcium with a Skybender for now, rather than shell out for a Quark CA or Lunt CA (can't do the Lunt CA, if its limited to 100mm apertures, I don't want something that has a hard limit). Rarely see the Quark CA's used, or I'd get one. So until then, I'll fool with the Skybender. It does seem to work decently to show something significantly different from just white light. And I have a set of 532nm filters to use with it too, along with a 540nm filter, to play around in white light more and try to see if I can do better convection cells that way, as soon as the KG3 arrives to test it out.

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by Bob Yoesle » Thu Apr 12, 2018 1:41 pm

Remember that at 393 nm that 100 mm aperture is equivalent to about 166 mm of aperture at 656 nm. You can also go larger with more sophisticated ERF/blocking filter implementations.

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by MalVeauX » Sat Apr 14, 2018 3:32 am

Thanks Bob,

Experimenting with some older tech and older information, simply new to me! :)

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by MalVeauX » Mon Apr 16, 2018 4:22 pm

Hey guys,

Can't find it in any of the documentation, but does it matter which side of the KG3 is facing the heat source (sun)? I got it in today, but it's just the glass, and I have to put it in filter cell which I have, but just wanted to make sure it's not directional. One side has the KG3 printed on it plainly to see. But again, the documentation on the filter, and on the website, doesn't really cover whether it matters what direction or face the filter is used.

Ideas?

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by marktownley » Mon Apr 16, 2018 4:52 pm

No it doesn't matter Marty
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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by MalVeauX » Mon Apr 16, 2018 8:07 pm

Thanks! Did the trick and took care of the heat, back in business.

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by marktownley » Tue Apr 17, 2018 5:45 am

If you are using that with a 120mm scope on the nosepiece like that my money is that the heat will crack it.
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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by MalVeauX » Thu Apr 19, 2018 3:55 pm

I've been imaging this way for months and so far, no cracking. Granted, I image for maybe 30 minutes in the morning. I don't go for hours and hours all day with it.

What would your recommend?

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by christian viladrich » Thu May 03, 2018 6:55 pm

A KG3 filter absorbs energy from the infrared.
In other words, the energy received is accumulated in the KG3. So the temperature of the filter increases untill an equilibrum is reached when the energy radiated by the filter equals the energy received.
But .. because an increase of temperature means dilation of the glass, and because the temperature is not uniform accros the filter, the dilations are not uniform accord the filter.... which means ... the filter breaks...

A dielectric (multicoated) IR blocking filter is a different beast. The substrate is transparent, which means it doesn't accumulate heat. This is the work of the coating to reflect IR. This is done with some energy losses, which means a very small amount of energy is transfered to the subtrate. This is very minimal. So, no dilation and no filter breaking (unless you use a large scope).

So, a dielectric filter is the way to go. But ... dielectric filters do not cut IR beyond about 1100 nm. This might be an issue or not, depending on the size of the scope.

Depending on the optical setup, the use of a red multicoated filter could also be a good option.
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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by TheSkyBurner » Sun May 06, 2018 12:10 am

beloptik sells an IR cut dielectric coated kg3 glass... I use it with a 120mm objective similar to yours, near the focus and it has yet to break :) I think sometimes it is just a case of bad luck with humidity from your area adding to the equation.

That skybender is a great piece of hardware, very useful ( i have seen several people posting images with it lately with admirable results) I have one too but cannot thank the person who gave them out for free, he doesnt answer questions anymore.

https://beloptik.de/de/uv-ircut-on-kg3-sperrfilter/

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Re: KG3 IR Absorption vs IR Blocking filters? Same?

Post by christian viladrich » Sun May 06, 2018 8:52 pm

As you said, the Beloptik filter is a dielectric filter. The coating reduces the heat load on the substrate. That would be a different story with a KG3 with no coating.
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