Just got my first UHC filter from astronomik. I have a modded canon 77D. I usually set the white balance to an image taken in broad daylight with a grey card. I tried doing the same with the UHC but the images still appear mostly, red/green. Am I wasting my time doing a custom white balance with this filter? Should I just put it on auto white balance and if so how do I make the correction in photoshop when I start processing the image?
I think the UHC filter is really made for visual not photography.
It is an extreme light-pollution filter that cuts out most of the spectrum, so you don't have much color there to work with. It only passes some blue-green around OIII and Hb, and then some deep red for Ha.
Shooting a CWB with the filter in place and mainatining a color-managed workflow throughout processing will get you as close to accurate color as you can get.
Auto white balance, and, in fact, all of your white balance settings in the camera are no good anymore because you modified the camera. You have to shoot a CWB to get anything close to accurate color with a modded camera.
All serious images use gradient reduction, but it does not perform the same function as a filter, which actually reduces photon shot noise from light pollution. That's something advanced, knowledgeable imagers understand. Gradient reduction just fixes gradients. It can balance the color of the sky background too, but that is cosmetic. Gradient reduction in software is not the first step in fighting light pollution. It is the first step for reducing gradients which is why it is called "gradient reduction" and not a "light-pollution removal" software filter. It is not subtracting the shot noise from light pollution and you get no signal-to-noise gains like you do with a filter.
The UHC filter is approaching a narrowband filter, and even photographers who advocate for gradient removal use narrowband filters for light pollution. They don't expect accurate color with a narrowband filter though, because only one wavelength gets through. Really, with a narrowband filter, all you get is monochrome.
The UHC filter as two bandpasses (although the red doesn't have a cut off that I can see on the long IR side so you would need an IR filter with refractive optics) at OII and Ha, so essentially you could try to do bi-color with it.
Edited by Jerry Lodriguss, 17 August 2019 - 04:09 PM.