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Old 17-12-2016, 04:40 PM
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New filter technique- NIR

Seen this everyone? Looks impressive and very different. Serious hours required it seems. Will try this year hopefully.

https://www.astrobin.com/gear/15177/astrodon-sloan-i

http://www.cloudynights.com/index.ph...and-Hyperstar?

Just when you thought you had enough filters!
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Old 17-12-2016, 04:58 PM
glend (Glen)
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In terms of the question asked by the CN original poster, the ASI1600MM-C does not block IR with the sensor window. Bandpass will be determined by the filter you place in front of it. ZWO does have a spectrum chart for the glass sensor window - not to be confused with the sensor cover glass which is typically clear. So NIR can be captured.
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Old 18-12-2016, 09:14 PM
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Yeah that's pretty interesting...I've got a couple of filters for planetary that are up in the IR so I'll have to check their bandpass
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Old 18-12-2016, 10:04 PM
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SimmoW (SIMON)
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Great Dunk, post when you check it out!

Ive found some SLR lens filters that only pass above 720, wondering if that might be worth trying before going the Astrodon route, at least for wider field work.
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Old 18-12-2016, 11:09 PM
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You can try a HOYA R72 ..... for 720nm +

See attached

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Old 19-12-2016, 11:28 AM
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Quote:
Originally Posted by SimmoW View Post
Great Dunk, post when you check it out!

Ive found some SLR lens filters that only pass above 720, wondering if that might be worth trying before going the Astrodon route, at least for wider field work.
Sure thing, just need the clouds to shift

I've got both 742 and 807 filters from Astronomik ProPlanet series that claim >96% transmittance.

I don't see why a SLR lens wouldn't work well...surely much of a muchness?
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Old 19-12-2016, 01:52 PM
glend (Glen)
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Gents can i just raise the issue of "focusing" a filter that does not pass visual light. Astronomik comment on this in their literature, pointing out the 642mm IR Pass facilitates focusing as it has a component of visible spectrum. Since they claim these filters are parafocal one might assume if you can focus one that passes visible light then you might assume the 742 or 807 are also in focus. I would like to know how people shooting with the deep ir pass filters achieve focus, do you pick a bright object for focus then move to your target, assuming parafocus will be ok, or are test subs required to check?
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Old 19-12-2016, 02:09 PM
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Quote:
Originally Posted by glend View Post
Gents can i just raise the issue of "focusing" a filter that does not pass visual light. Astronomik comment on this in their literature, pointing out the 642mm IR Pass facilitates focusing as it has a component of visible spectrum. Since they claim these filters are parafocal one might assume if you can focus one that passes visible light then you might assume the 742 or 807 are also in focus. I would like to know how people shooting with the deep ir pass filters achieve focus, do you pick a bright object for focus then move to your target, assuming parafocus will be ok, or are test subs required to check?
IF using a camera lens .....Some camera lenses are marked for infinity focus at infra red as well as the normal infinity focus (visible light) and the trick is normally to focus in visible light (without filter installed) then apply the difference marked between infinity IR focus and infinity visible light focus to your current focus setting to obtain what should be the correct setting on the same object with the Infra red filter fitted. This works better for focus near infinity, which is where you want it for astro, but not necessarily always perfectly for terrestrial photography at the other close (non-infinity) end of the focus scale.

Perhaps it would be possible to do something along the same lines, by calibrating a telescope focuser to find this IR to visible light focus difference/compensation, by using an infra-red LED and visible LED at a significant distance from the telescope, obtaining the different focus points (the camera needs to be able to see the IR light - modified is better) and then determine and apply this compensation to the normal visible light focus in all your infra red work with that telescope.

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JA
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Old 19-12-2016, 02:39 PM
glend (Glen)
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Originally Posted by JA View Post
IF using a camera lens .....Some camera lenses are marked for infinity focus at infra red as well as the normal infinity focus (visible light) and the trick is normally to focus in visible light (without filter installed) then apply the difference marked between infinity IR focus and infinity visible light focus to your current focus setting to obtain what should be the correct setting on the same object with the Infra red filter fitted. This works better for focus near infinity, which is where you want it for astro, but not necessarily always perfectly for terrestrial photography at the other close (non-infinity) end of the focus scale.

Perhaps it would be possible to do something along the same lines, by calibrating a telescope focuser to find this IR to visible light focus difference/compensation, by using an infra-red LED and visible LED at a significant distance from the telescope, obtaining the different focus points (the camera needs to be able to see the IR light - modified is better) and then determine and apply this compensation to the normal visible light focus in all your infra red work with that telescope.

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JA
I believe that techique is required because you are adding a glass filter to the lense thus slightly altering the focal length. With an astro camera, no lense, the image is focused on the sensor, if the focus is conducted with a parafocal filter you shoukd be ok in ir, would be my guess.
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Old 19-12-2016, 06:03 PM
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Quote:
Originally Posted by glend View Post
I believe that techique is required because you are adding a glass filter to the lense thus slightly altering the focal length. With an astro camera, no lense, the image is focused on the sensor, if the focus is conducted with a parafocal filter you shoukd be ok in ir, would be my guess.
Hi Glen,

Whether the focus is different for visible light or infra red really depends on how well the optical system (Camera lens, Telescope, etc..) is corrected for chromatic aberration, especially axially. If the correction is poor or not wideband enough (far enough in to the infra red) then the focal points will be different. If the focal points are so different that they are out of focus compared with visible light focus, then some workaround/correction/adjustment is needed to improvise some sort of method for focus. That's why I suggested the LEDs as a possible means to determine such.

The situation would be different for reflectors, but for the images that were linked/posted refractors were listed as used to produce the images - at least the ones I looked at. Who knows what their degree of chromatic correction into the near infra red or deep infra red would be? It is certainly not something we could easily judge with our eyes, like we can with normal chromatic aberration in the visible light range.

Best
JA
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Old 19-12-2016, 07:28 PM
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Surely many stars would emit in the NIR? If so, just take a short sub with a Bahtinov mask on the scope
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Old 19-12-2016, 11:41 PM
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I have a Hoya R72 and two other IR filters. One 850nm (black as, you can't see through it) and another around the 600's. Sounds like a bit of fun to experiment with once these clouds buzz off.
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