What's causing this red gradient?

[Ags]

This is a shot with my 50mm straight up, with a UHC filter. I've stretched the red slightly to show the gradient. What could be causing it?

I bought a lens hood today on te assumption it might be peripheral illumination.

I don't think the gradient is in the sky because it is so symmetrical.

Any ideas?

[Rustmonkey]

Light pollution

[Digz]

It could be a number of things. Light pollution is a likely candidate, but it could also be amp glow - to me it looks like there is a red fringe it vignette on all four corners.

What camera was used and how long was the sub? Also what was the location?

[Ags]

Which sort of light pollution? I had hoped to minimize this with the UHC filter - but it looks like it is letting through a lot of red!

[Ags]

It could be a number of things. Light pollution is a likely candidate, but it could also be amp glow - to me it looks like there is a red fringe it vignette on all four corners.

What camera was used and how long was the sub? Also what was the location?

One minute sub at ISO 800 from a city. I've never noticed such a pronounced red glow from this location before.

[Digz]

Ah, I didn't see you we're using a UHC filter. I would do a test shot with same settings with and without the filter to see if its caused by the filter.

[JamesF]

And perhaps a dark with the same settings to see if the glow is external or internal.

James

[Ags]

No, it doesn't show on a dark. And the gradient was present without the filter. I expected a red gradient to be reduced by a UHC.

[JamesF]

I wonder if perhaps the UHC filter doesn't cut enough red to completely kill the wavelengths from the types of light in use near you?

James

[ollypenrice]

I'm sure it's just vignetting and is red because that's the dominant colour prior to colour balancing. Once balanced I think it would look like classic vignetting. OK, the top of the image is slightly more affected than the bottom but you see it at the two ends (top and bottom) on the long side of the chip. These are the ends reaching furthest into the light cone so are the most vignetted. I wouldn't call this a gradient, certainly not on my monitor. For me a gradient runs decisively from one side or corner to the other. Vignetting is rarely perfectly symmetrical. That would be too easy!!!

Flats should nail this provided they have a strong spectrum in the wavelengths of the vignetting, which they don't necessarily have to have if the light source is not similar to that of the sky.

Olly

[Ags]

I took flats but have not processed them yet as I messed up the darks. I notice what appears to be a color gradient in the flats too, now that you mention it - the center looks grey, but the corners look a little brown to my eyes.

[ollypenrice]

WIth my OSC I debayer the flats and throw away all colour information before applying them. I don't know how DSLR imagers deal with them, though.

WIth filters you can adjust the exposure of each colour when shooting flats. WIth OSC you need to be sure that you really are obtaining a flat in each channel. I once had a light source which gave out almost no blue. With filters this was obvious but with OSC I might have failed to realize that the blue channel was not, in fact, being flat fielded. I dare say you can check by looking at each colour channel of the flat separately.

Olly