Processing M42 with flats, gradient help!

[VigdisVZ]

Hello SGL

Here is an example of a problem im not sure about what to do with.

The animated gif shows two sets of images.

THe one where the gradient is in the middle is using NO flats.

The one where the gradient is mostly on the right hand side but not in the middle is with a flat.

Does this mean that the flat is bad?

I tried playing with a flat tiff in photoshop but DSS refused to stack it for some reason.

Tearing my hair now.

[andrewluck]

It looks as if the flat is correcting the vignetting but you have an additional gradient as well. You could try something like GradientXterminator or PixInsight's Dynamic Background Extraction tool to remove the residual gradient.

Regards

Andrew

[VigdisVZ]

I tried gradient xterminator and that was just making it even worse on all settings

My best try is a manual gradient removal, but I was hoping that I'd have the flats take care of that. However, I think my flats were a bit bogus, next time I'll make sure to try again, with a few different exposure settings and light box solutions, that way I should be able to rule out the flats as the problem if I'm not lucky and get a clean image.

Thanks for reading

[andrewluck]

The flats will only take care of the vignetting. Gradients require separate processing. Essentially, flats are a division process and gradient removal a subtraction. There's another trick to flat calibration. If you're subtracting darks from the lighs then you need to subtract bias from the flats. This ensures that the numbers work out OK. Of course, if your flat has a gradient on it...

If you like, upload a light and a flat to Dropbox and I'll take a look.

Andrew

[VigdisVZ]

Ok in that case... but it's strange how the gradient starts to appear when the vignetting is gone

I forgot to take bias files this time. I'll remember it next time, they're pretty quick anyhow.

Andrew, I sent you a PM with a link to the flat.

[andrewluck]

Carl

Looking at the flat I'd say it is the cause of your gradient problem. If you stretch it you'll see that the illumination is uneven with the field noticeably darker down the lefthand edge. This will have the effect of lightening your images in the same place.

Andrew

[VigdisVZ]

Carl

Looking at the flat I'd say it is the cause of your gradient problem. If you stretch it you'll see that the illumination is uneven with the field noticeably darker down the lefthand edge. This will have the effect of lightening your images in the same place.

Andrew

Thanks for taking time to look into this Andrew. Now I know that I have to create a better lightbox.

Cheers

[ollypenrice]

I'm sure Anfdrew's right and this is a common issue with flats. I've just had a set like this, in fact.

Light can get in through manual filterwheels, the source can be uneven, light can get under one side of the source, etc etc etc. Frequently it is just voodoo and don't let anyobody kid you otherwise!

What you have is well within the range of Pixinsight's DBE, the best gradient remover of them all.

Olly

PS Your lightbox may not be the culprit. I see this problem farly often even with a panel. Sometimes if the source is too bright I think it can generate reflections so slowing it down by using a white sheet or something can help. Users of cameras with shutters must take care to expose for long enough to make the wipe of the shutter insignificant. Also, I take flats in the dark. I find them consistently more reliable, though others don't always find this.