Do you always use an IR cut filter with an IR sensitive camera?

[PeterC65]

I have an IMX585 based camera (Player One Uranus-C) which is sensitive in the IR band. I use it (mostly for EEVA) with a filter wheel which contains, amongst other filters, an Astronomik Clear filter and an Astronomik L2 filter. I am finding increasingly that I prefer the L2 to no filter and wondered whether there are circumstances when others find the unfiltered view useful?

At the moment my filter wheel is full and removing the Clear filter would free up a slot for something else. I already have the Astronomik L2, UHC, OIII and IR Pass (742nm)  filters and was thinking I might add the Hα (12nm) filter.

 

[Stuart1971]

Well, on a one shot colour camera, without an IR filter you will get ugly bloated stars, and probably colour fringed stars, so in pretty much all cases you need an IR filter with these cameras…there may well be some types of imaging where you don’t need one, but that is not a subject I know anything about…

[Stuart1971]

Most one shot colour cameras, have an IR window built into them, the sensor cover glass window is normally an IR cut glass….

Edited February 11, 2023 by Stuart1971

[geoflewis]

The Astronomic L2 is an IR/UV cut filter, essentially all luminace filers cut IR+UV, that's what they do. Check the specs on the Astronomic website - L2 is the general purpose one of the 3 versions, L3 the most agressive cut filter.... https://www.astronomik.com/en/uv-und-ir-block-filter/luminance-filter-l-1-l-2-l-3.html

FWIW I use the old Astonomik Type 2 Lum as my UV/IR cut for planetary imaging.

[PeterC65]

10 minutes ago, Stuart1971 said:

Most one shot colour cameras, have an IR window built into them, the sensor cover glass window is normally an IR cut glass….

My camera has a clear window so that it's possible to take advantage of the IR band, using an IR Pass filter (which I find useful for observing the Moon).

 

[PeterC65]

10 minutes ago, geoflewis said:

The Astronomic L2 is an IR/UV cut filter, essentially all luminace filers cut IR+UV

Yes, that's how I use it, to restrict images to the visible spectrum. What I'm interested in knowing is whether there are times when no filter (or a Clear filter, to keep things parfocal) is useful.

 

[geoflewis]

45 minutes ago, PeterC65 said:

Yes, that's how I use it, to restrict images to the visible spectrum. What I'm interested in knowing is whether there are times when no filter (or a Clear filter, to keep things parfocal) is useful.

Not sure what you mean by 'keep things parfocal', i.e. what is a clear filter parfocal with? Personally I wouldn't use the camera without any filter though, at least I can't think of any time when I would do that.

[PeterC65]

1 minute ago, geoflewis said:

Not sure what you mean by 'keep things parfocal', i.e. what is a clear filter parfocal with?

All my filters are Astronomik filters so that they are all the same thickness and I can change filter without adjusting the focus. In the no filter slot I have an Astronomik Clear filter (just a piece of glass) so that this too is parfocal with the other filter.

To be fair the IR Pass filter is not quite parfocal with the other, I think because it's passing wavelengths that are quite a bit longer than the other filters.

 

[geoflewis]

10 minutes ago, PeterC65 said:

In the no filter slot I have an Astronomik Clear filter (just a piece of glass) so that this too is parfocal with the other filter.

Ok, now I understand what you mean. I think I'd use that slot for something more useful.

[Elp]

I find for colour imaging (for hue), without UV/IR cut filter seems better, otherwise why would they make cameras with windows which allow IR light through? Dslr astro mods also involve removing the IR filter at the most basic level, so I don't really understand why IR cut filters are recommended for OSC imaging, other than to use that later as a luminence layer for detail and more accurate colour saturation.

[PeterC65]

1 hour ago, Elp said:

I find for colour imaging (for hue), without UV/IR cut filter seems better, otherwise why would they make cameras with windows which allow IR light through? Dslr astro mods also involve removing the IR filter at the most basic level, so I don't really understand why IR cut filters are recommended for OSC imaging, other than to use that later as a luminence layer for detail and more accurate colour saturation.

This was my thinking too originally, but as I understand it, the focus position of a scope varies with wavelength, so getting a good focus is harder the wider the wavelength range that is captured. IR / UV cut filters are recommended to restrict the wavelength range to just the visible spectrum which reduces the variation in focus position. This is why it's often said that using an IR / UV cut filter reduces star bloat.

From my practical observations I think that star bloat is reduced a little when using the L2 filter, but more noticeable is that the focus seems sharper and the colours are less blown out (more natural looking). To be fair, I always focus using the L2 filter so that may be why the focus looks sharper, but I always check the focus with the Clear filter (no filter) and it is always spot on (with a Bahtinov mask).

 

[PeterC65]

I've asked this question on CN too, and the response there was mixed. Some people use no filtering quite a bit, sometimes with specific scopes, sometimes with specific types of targets. Quite a few people mentioned that when observing galaxies which are wideband emitters the view with no filter is better than with an IR / UV cut filter. Last night I had a chance to test this ...

M82 with no filter (Clear filter)...

M82 with IR / UV filter (L2 filter) ...

I prefer the view with the L2 filter because this target has a significant colour component, but when observing a faint spiral galaxy where it's all about seeing the structure things are different.

NGC3938 with the Clear filter ...

NGC3938 with the L2 filter ...

This time I prefer the Clear filter as the view is brighter and I can see more detail.

So the Clear filter is staying in the filter wheel which means no space for a new Hα filter.

 

Edited February 14, 2023 by PeterC65