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Old 23-09-2021, 10:24 AM
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Merlin66 (Ken)
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IR blocking filter

Why is it so difficult to find an IR blocking filter (say>700nm) with NO UV restriction (ie <380nm)???
They all seem to be variations on the UV-IR blockers.
I'm looking for a IR blocker which still allows access to CaK/H at 393nm
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  #2  
Old 23-09-2021, 10:28 AM
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multiweb (Marc)
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Quote:
Originally Posted by Merlin66 View Post
Why is it so difficult to find an IR blocking filter (say>700nm) with NO UV restriction (ie <380nm)???
They all seem to be variations on the UV-IR blockers.
I'm looking for a IR blocker which still allows access to CaK/H at 393nm
Have you tried Edmund Optics? They're not cheap but they have a good range of band pass filters.
https://www.edmundoptics.com/c/optical-filters/610/
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Old 23-09-2021, 10:47 AM
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Merlin66 (Ken)
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I looked at the Edmund and Thorlabs filters.
Not much in the IR blocking and narrowband filters don't cover the critical wavelengths.

>$300 for a 25mm diameter filter!
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Old 23-09-2021, 12:26 PM
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Quote:
Originally Posted by Merlin66 View Post
Why is it so difficult to find an IR blocking filter (say>700nm) with NO UV restriction (ie <380nm)???
They all seem to be variations on the UV-IR blockers.
I'm looking for a IR blocker which still allows access to CaK/H at 393nm
Hi Ken,

If you are using a colour camera with sufficient UV response for your purposes in the Blue channel when naked without any filter, then you could try using the camera that way (naked) and simply discard the Red Channel containing the IR, depending of course on how much IR bleed through there is in the other Green & Blue channels. Sometimes there is some IR pickup in the green and blue channels above 800nm or so, from what I recall of manufacturer spectral response curves and therefore may effect the suitability of employing this "trick".

Of course if it's a mono camera application, then keep looking
EDIT: there may be another "trick" possible in this case

Best
JA

Last edited by JA; 23-09-2021 at 12:43 PM.
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Old 23-09-2021, 02:57 PM
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Merlin66 (Ken)
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JA,
Life for me is never easy at times.....
I'm actually trying to construct an ERF filter to use with the MiniSHG.


I have a good Baader 7nm H alpha filter with blocks all other wavelengths and is ideal for H alpha imaging.


For CaK, I need something with a "good" transmission at 390nm and effectively blocks everything else including the NIR.
(I have various narrowband filters which can do this job but they are assembled into my "DIY CaK" filter)


I've been using a Baader 0.9 ND filter plus a UV-IR cut filter for all other wavelengths down to 400nm and then just removing the UV-IR filter.
(Having said that I'm about to trial an old "York Optical" IR blocker plus the 0.9ND for CaK)
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Old 23-09-2021, 04:18 PM
JA
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Quote:
Originally Posted by Merlin66 View Post
JA,
Life for me is never easy at times.....
Maybe it just got easier for you..... (hopefully)
ZB1 B390 UV Pass Filter in 25mm Diameter unmounted size
https://www.ebay.com.au/itm/26428845...0AAOSwFs1cFQQd

ZB1 B390 UV Pass Filter in 49, 50, 55, 64 & 75mm Diameter unmounted sizes
https://www.ebay.com.au/itm/27439485...YAAOSwUixe4u~H

ZB1 B390UV Pass Filter in M52, M58, M67 & M77 Screw in filter sizes
https://www.ebay.com.au/itm/27439485...EAAOSwocFe3zUg

These ZB1 filters show reasonable ~75% transmission at 390nm -ish with not much ~0, in the visible range.There is some minor 0-10% transmission between 700nm to 1000nm and more beyond that , but the camera's response should be falling heavily by then. See the ZB1 filter curve below in red ....
Click image for larger version

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Even I might have to get 1

Best
JA

Last edited by JA; 23-09-2021 at 05:02 PM.
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Old 23-09-2021, 08:51 PM
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Camelopardalis (Dunk)
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Ken, I ordered an Astronomik IR block recently, it's on its way here, but it was relatively inexpensive (under $100 shipped from Europe).
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Old 24-09-2021, 09:40 AM
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JA,
It's not very clear from the illustrations exactly what the transmission at 390nm would be.
One of them would probably do......
Dunk,
Yes, the Astronomik UV-IR looked promising. I sent a message asking the transmission at 390nm but so far have received no reply.
Revisiting: The L1 version seems to cover the 390nm region. Which version did you order?

Last edited by Merlin66; 24-09-2021 at 10:55 AM.
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Old 24-09-2021, 10:25 AM
JA
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Quote:
Originally Posted by Merlin66 View Post
JA,
It's not very clear from the illustrations exactly what the transmission at 390nm would be.
One of them would probably do......
Hi Ken,
the ZB1 filter is the RED coloured transmission curve I posted above, there being 3 curves shown on the same graph. reading from the graph, I would make the ZB1 (red curve) transmission as 72 to 73% at 390nm.

Best
JA

Last edited by JA; 24-09-2021 at 07:10 PM. Reason: typo
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Old 24-09-2021, 09:31 PM
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Camelopardalis (Dunk)
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Quote:
Originally Posted by Merlin66 View Post
JA,
Dunk,
Yes, the Astronomik UV-IR looked promising. I sent a message asking the transmission at 390nm but so far have received no reply.
Revisiting: The L1 version seems to cover the 390nm region. Which version did you order?
Ken, I was referring to just this one…arrived today (7 calendar days after ordering, from a certain vendor in Exeter )

https://www.astronomik.com/en/uv-und...ck-filter.html

At 390nm the plot suggests 90% transmission. I bought this to experiment with a #47 for Venus… might be a little too interesting on a multi-coated SCT, but was reasonably priced.
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Old 25-09-2021, 09:16 AM
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Merlin66 (Ken)
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OK,
I've gone ahead and ordered an Astronomik UV-IR filter.
I'll test the transmission on the spectrograph......
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Old 25-09-2021, 03:04 PM
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Originally Posted by Merlin66 View Post
OK,
I've gone ahead and ordered an Astronomik UV-IR filter.
I'll test the transmission on the spectrograph......
That should an interesting experiment. I’ve got the L2 for deep sky imaging but I think it is supposed to cut off at ~400nm.
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Old 30-09-2021, 04:57 AM
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Quote:
Originally Posted by Merlin66 View Post
JA,
It's not very clear from the illustrations exactly what the transmission at 390nm would be.
One of them would probably do......
Dunk,
Yes, the Astronomik UV-IR looked promising. I sent a message asking the transmission at 390nm but so far have received no reply.
Revisiting: The L1 version seems to cover the 390nm region. Which version did you order?
Maybe this curve will help:
Looks like 50% cutoff at about 360-370nm.
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