Is an IR cut filter for one shot colour camera necessary?

[Ouroboros]

I’m thinking of buying a cooled one-shot colour cmos camera.  But I’m confused about the use of IR cut filters. On the one hand I understood the advantage of using a OSC camera or modded DSLR was that it meant the sensor also captured the near infrared. But I keep reading mention of the use of IR cut filters to reduce the amount of IR to the camera. I understand  that in a refractor the IR may not focus in the same focal position to the visible light and thus result in bloated stars. On the other hand it looks as though one has to throw away an interesting part of the spectrum.  What is the correct way to proceed - an IR cut filter or not and in which circumstances? 

[old_eyes]

I can only speak about my own practice. An IR cut filter is recommended for my OSC camera (QHY168C). With the camera on my Equinox ED80 I used this filter https://www.firstlightoptics.com/idas-filters/idas-heuib-ii-uvir-blocking-filter.html. This both blocks UV & IR, and isolates the H-alpha line in the red. It gives good separation of emission nebulae from general background red glow. Useful, if like me, you have a fair bit of sky glow in various parts of the sky. I thought he combo worked very well. 

I am now doing some widefield imaging with a Canon 200mm lens and the adapter I am using does not support 2-inch filters, so while I am waiting for a new adapter to arrive, I am working without a filter. The images show bigger stars (higher HFR according to NINA) and a rather more murky image. I will have a direct comparison when the new adapter arrives and I can update then.

So, in my experience you do better with an IR cut filter, but I don't yet have a side-by-side comparison with and without on the same target.

[Cosmic Geoff]

You don't say what you want to use the camera for.  I have a planetary camera (ASI224MC) which has a significant response in the infrared.  When I tried it with a fisheye lens in the kitchen I found that it worked OK with the IR-cut filter, but without it the colours looked a bit off.  So if you want true colour, you need the IR-cut filter. If you don't then fine.

[david_taurus83]

Unlike a DSLR, astro cameras have no blocking filter so will pick up all visible wavelengths plus UV and IR. Mono cameras are used with filters to isolate specific wavelengths, ie RGB, Ha etc. Most of these filters already have IR/UV cut off. With a OSC you will typically be imaging RGB so its recommended to use an IR/UV cut filter to isolate the visible spectrum between 400/700nm. A good LP filter will have IR/UV cut already, like an IDAS or Astronomik CLS-CCD. Then there are the newer dual band filters which will isolate just the Ha and Oiii wavelengths whilst blocking put other wavelengths.

Edit: Also, I think some people get confused with the notion of removing the IR filter from a DSLR and then using an IR/UV cut filter again. The DSLR filter cuts IR (700nm+) and a lot of the Ha spectrum (656nm). The mod just opens up the DSLR to a lot more Ha and using a IR/UV (like a LP filter for example) gives a sharper cut off from 700nm.

Edited October 28, 2020 by david_taurus83

[vlaiv]

Depends on camera in question and optics used.

Some OSC cameras have integrated UV/IR cut filter and there is not much you can do about it. Some have AR coated protective window - and that means simply anti reflexive / clear glass.

These are usually sensitive beyond 400-700nm range.

Some sensors behave like monochromatic sensors above certain wavelength. For example new ASI462 (and some older models)

If you use ~820nm and above IR filter - response curve for each color is practically the same - you'll get monochrome sensor effectively.

If you want to exploit additional wavelengths, get model without integrated UV/IR cut filter, and use reflective optics. Even very good triplet refractor will struggle with IR and UV part of the spectrum and you'll end up with star bloat and blurry image.

For IR channel, use dedicated IR pass filter.

[Ouroboros]

6 hours ago, Cosmic Geoff said:

You don't say what you want to use the camera for.  I have a planetary camera (ASI224MC) which has a significant response in the infrared.  When I tried it with a fisheye lens in the kitchen I found that it worked OK with the IR-cut filter, but without it the colours looked a bit off.  So if you want true colour, you need the IR-cut filter. If you don't then fine.

Deep sky.  Basically I want to move up from the DSLR I am using to take advantage of cooling and to give me better access to the H alpha. I don’t want to mod my lovely old Canon 450D. I still use it for terrestrial photography too. 

[Ouroboros]

6 hours ago, vlaiv said:

Depends on camera in question and optics used.

Some OSC cameras have integrated UV/IR cut filter and there is not much you can do about it. Some have AR coated protective window - and that means simply anti reflexive / clear glass.

These are usually sensitive beyond 400-700nm range.

Some sensors behave like monochromatic sensors above certain wavelength. For example new ASI462 (and some older models)

If you use ~820nm and above IR filter - response curve for each color is practically the same - you'll get monochrome sensor effectively.

If you want to exploit additional wavelengths, get model without integrated UV/IR cut filter, and use reflective optics. Even very good triplet refractor will struggle with IR and UV part of the spectrum and you'll end up with star bloat and blurry image.

For IR channel, use dedicated IR pass filter.

Very clear thanks. Before I buy (if I can find one for sale) I need to check what filters are built in. 

[Ouroboros]

6 hours ago, david_taurus83 said:

Unlike a DSLR, astro cameras have no blocking filter so will pick up all visible wavelengths plus UV and IR. Mono cameras are used with filters to isolate specific wavelengths, ie RGB, Ha etc. Most of these filters already have IR/UV cut off. With a OSC you will typically be imaging RGB so its recommended to use an IR/UV cut filter to isolate the visible spectrum between 400/700nm. A good LP filter will have IR/UV cut already, like an IDAS or Astronomik CLS-CCD. Then there are the newer dual band filters which will isolate just the Ha and Oiii wavelengths whilst blocking put other wavelengths.

Edit: Also, I think some people get confused with the notion of removing the IR filter from a DSLR and then using an IR/UV cut filter again. The DSLR filter cuts IR (700nm+) and a lot of the Ha spectrum (656nm). The mod just opens up the DSLR to a lot more Ha and using a IR/UV (like a LP filter for example) gives a sharper cut off from 700nm.

Thanks for this. Your second paragraph describes me. But thanks to your post and others here I now think I have a clearer idea of the situation.  As I said above. I need to check the in built filters in any camera before I buy.