Large Magellanic cloud --- image processing skills

[LT]

Hi,

Taken in Siding Spring in Australia with Pentax 175mm F2.8 medium format lens set at F2.8. 26 min exposure on Fuji Provia 400F slide, and +1 stop push in developing. No flat field correction or gradient removal and so quite serious vignetting in this image. Is there anyone can tell me how to do the correction?

Originally planned to shoot at 30 min but the rolling-in clouds stopped exposure. Also, there was a very thin layer of dew formed on the lens surface. So, image clarity not so good. Moreover, overprocessing happened on the cloud center.

Those result in this image just fair. Happy to see no elongation on star images. This GP mount can do unguiding task well with a ~200mm lens.

But the colour cast on 400F seems not so pleasing than that on E200.

Any comment or suggestion on my processing are most welcome.

Thanks for looking.

LT

[sn-10]

artificial flattening by gradient mask, just a quick try so not perfect, hope you don't mind me playing with your pic without permission.

[opa]

Sorry, LT 前輩, 我又試下玩..

[universe24]

我也来玩玩...鲜艳版的...我所用的方式是局部leveling来去除vignetting。

[LT]

artificial flattening by gradient mask, just a quick try so not perfect, hope you don't mind me playing with your pic without permission. 8)

Never mind. By the way, could you post some printscreen image to show us how you generate such an artificial gradient mask to correct the vignetting ?

Thanks.

LT

[LT]

Welcome to play with my image, and if possible, please state your steps to share us with your processing skills so that we can benefit from you.

Thanks.

LT

LT

[sn-10]

Welcome to play with my image, and if possible, please state your steps to share us with your processing skills so that we can benefit from you.

Thanks.

LT

LT

There are many ways to do it. One of the easiest approach can be found here:

http://www.astropix.com/HTML/J_DIGIT/VIGNET.HTM

My approach is similar - subtracting a mask containing all vignetting and illumination information. However, the mask I generated isn't a radial gradient as described in that link, but is rather a "real" background generated from the original. The method is to use blurring and dust and scratches techniques to remove all stars and nebulosity while leaving the background behind. This is the "mask" you want for adjustment.

[sn-10]

As per request, I have prepared a few screen shots to demonstrate what I have done.

1, duplicate image, select the copy
2, dust and scratches - amount depends, trial and errors issue
3, g blur - same as 2 to get a smoother background
4, paste background layer mask on top
5, subtract mask
6, DONE!

[sn-10]

The "DONE" photo is here:

I wish these steps are clear enough and more people know one of the many ways for removing gradient. Good luck.

[PaulNg]

http://www.pleiades-astrophoto.com/tuto ... /en01.html

This is one of the methods i used for flatfielding. However, with my 2 latest images, I used that software to generate flatfield, but then use photoshop to do the subtraction.

The guide and menu of that software is very comprehensive.

[sn-10]

http://www.pleiades-astrophoto.com/tuto ... /en01.html

This is one of the methods i used for flatfielding. However, with my 2 latest images, I used that software to generate flatfield, but then use photoshop to do the subtraction.

The guide and menu of that software is very comprehensive.

Paul, the nonlinearity as shown in page 1 was caused by direct substraction only.

Anyway, "why not?" is what I would say if the software is free of charge. Thanks for sharing. (though I don't know much about film photography)

[LT]

sn-10,

Thanks for your demonstration. That definitely helps me a lot. I will try following your steps to learn this technique.

Paul,

Thanks for the link. Could you show us your reworking?

Clear skies,

LT