Filter Questions

[WolfieGlos]

To date, the only filter I’ve used is the optolong lenhance for emission nebula, but I often see people using UV/IR cut filters for broadband imaging. In daytime photography they are used  essentially as lens/dust protectors, so I’m wondering what these do for Astro imaging, and if I should be using one?

I use one of two cameras, and aim to upgrade to an Astro cam (probably a 533mc) in the next 12 months. 

1) Canon 800d DSLR (astro-modded) so the filter was removed as part of the mod.

2) Canon 77d DSLR (stock), so the inbuilt uv filter remains.

Another thing I’m confused by is luminance, and LRGB images. All I can work out for luminance is that it appears to be a filter that is used for mono cameras?

[Elp]

UV/IR cuts out the bloat (UV/IR shifted light) so you get sharper images. A luminence filter is similar and is used as a luminence layer over RGB to sharpen up details in mono, also hence why it's useful to concentrate most of your mono imaging (or OSC) in Lum. Your RGB can be blurry to a degree and just used for colour values, the luminence layered on top then provides the shaper detail and correct luminence levels.

A DSLR astro mod usually comes in two stages, stage 1 removes the IR filter making the sensor more sensitive to hydrogen alpha wavelength, stage two also removes the UV filter. Most astro cams have anti reflective (AR) coated windows only so are effectively UV/IR sensitive based on the QE response of the sensor chip used, so people use UV/IR filters to get the detail back.

IR imaging does have its uses, for planetary for example you can get detail which is minimally affected by atmospheric disturbances and overlay it on top of RGB data to reveal fine planet surface detail. It can be used in deep sky also but requires a magnitude higher exposure time compared to any other bandpass.

Edited May 6, 2023 by Elp

[WolfieGlos]

40 minutes ago, Elp said:

UV/IR cuts out the bloat (UV/IR shifted light) so you get sharper images. A luminence filter is similar and is used as a luminence layer over RGB to sharpen up details in mono, also hence why it's useful to concentrate most of your mono imaging (or OSC) in Lum. Your RGB can be blurry to a degree and just used for colour values, the luminence layered on top then provides the shaper detail and correct luminence levels.

A DSLR astro mod usually comes in two stages, stage 1 removes the IR filter making the sensor more sensitive to hydrogen alpha wavelength, stage two also removes the UV filter. Most astro cams have anti reflective (AR) coated windows only so are effectively UV/IR sensitive based on the QE response of the sensor chip used, so people use UV/IR filters to get the detail back.

IR imaging does have its uses, for planetary for example you can get detail which is minimally affected by atmospheric disturbances and overlay it on top of RGB data to reveal fine planet surface detail. It can be used in deep sky also but requires a magnitude higher exposure time compared to any other bandpass.

Thanks Elp. Apparently the DSLR Astromod removed the UV/IR and colour correcting filter (which allows it to see Ha), but an antialiasing and UV/IR filter remains which reportedly blocks about 97% of UV and IR http://www.astronomiser.co.uk/filters.htm .

I haven’t noticed “star bloating” like Astronomiser mentions (which is where I got the camera from), but then I don’t think I’d know it if I saw if.

I do notice CA on stars, a lot less on my new scope (in the brief chance I’ve had thanks to clouds) which made me wonder if this filter would help? I also noticed that it’s made a lot worse using Asinh during processing, something I’m going to be a bit more careful of, but that may be going off topic.

[Louis D]

Here's a great photo from a CN post showing the difference a UV/IR filter can made for DSO imaging:

And here is another image showing how Mars can benefit from an IR-pass filter:

Edited May 6, 2023 by Louis D

[Elp]

Your lens optics makes the most difference, I've used vintage lenses which provide some star halos but that's more to do with the optics not focusing RGB at the same rate so one side of focus gives me one colour, and the other another. The Samyang 135 is crisp as is my apo refractor. Using a UV/IR cut filter or luminence filter makes it even more tighter.