Correcting for vignetting

[AtmosFearIC]

I notice that there is a lot of discussion about vignetting from some tube lens', some being better than others in that regard. So I thought I would go and lay out some of my astrophotography techniques that may be of interest to others.



What this image shows is the process. The image on the right is a raw NEF file out of camera. The image on the left is after correction. The image down the bottom is the image used for correction. With all of this, pay no attention to the different colour differences.

What you will notice is that the image on the left is perfectly corrected for vignetting compared to the original on the right. The bottom (middle) image is what we astrophotographers call "flat frames". They are used specifically for correcting with vignetting and removing dust motes and anything else within the optical train that isn't supposed to be there. Although not of importance for macro, this is also used for correcting for pixel to pixel intensity variations from one another, very important when you want to accurate measure star brightness however.

[billjanes1]

I notice that there is a lot of discussion about vignetting from some tube lens', some being better than others in that regard. So I thought I would go and lay out some of my astrophotography techniques that may be of interest to others.

What this image shows is the process. The image on the right is a raw NEF file out of camera. The image on the left is after correction. The image down the bottom is the image used for correction. With all of this, pay no attention to the different colour differences.

What you will notice is that the image on the left is perfectly corrected for vignetting compared to the original on the right. The bottom (middle) image is what we astrophotographers call "flat frames". They are used specifically for correcting with vignetting and removing dust motes and anything else within the optical train that isn't supposed to be there. Although not of importance for macro, this is also used for correcting for pixel to pixel intensity variations from one another, very important when you want to accurate measure star brightness however.

I would be interested in your methods for flat field correction. Of note, Lightroom has a flat field plugin to remove lens cast, but unfortunately It does not remove sensor dust.

http://labs.adobe.com/technologies/lightroomplugins/

Regards,

Bill

[AtmosFearIC]

I'm not actually sure if PS has a plugin for it, I don't use PS myself. Flat fielding is a normalised divisional process where the brightest pixel is given a value of 1. Lets say the centre is all very bright ~1 while the extreme corners have 0.5 (half as bright as the centre). The centre won't change as it is being divided by 1 but the corners are made twice as bright.

As dust motes cause vignetting they will also be corrected for. This is done on every frame prior to stacking. I personally use MaximDL and PixInsight to do this but if you want something free to play with I would suggest DeepSkyStacker (DSS).

[austrokiwi1]

The technique looks very much like the LCC method in Capture one Pro. I don't know how the LR version works to create a LCC. In Capture one pro( which also "removes" dust) you have to take a reference shot, by placing a white translucent sheet over the lens. Then in Capture one you use that reference shot to create the LCC which can then be applied to the pictures taken by that lens( in the same lighting conditions)

[AtmosFearIC]

The technique looks very much like the LCC method in Capture one Pro. I don't know how the LR version works to create a LCC. In Capture one pro( which also "removes" dust) you have to take a reference shot, by placing a white translucent sheet over the lens. Then in Capture one you use that reference shot to create the LCC which can then be applied to the pictures taken by that lens( in the same lighting conditions)

Sounds like the same thing as in Capture One. I use a light box and usually take 25 images which are then average stacked to get a decent signal to noise. This is then divided into the individual frames for correction.