Can I rescue this?

[Ags]

I messed up my flats (I overexposed them by 10x) so my image has a nasty gradient. Is there any way in Gimp that I can rescue the image?

Taken with my ST80 on my SE mount. 60 lights (ISO3200, 20 sec) and 20 darks.

[Ags]

I seem to be unable to upload the image at the moment... EDIT: image uploaded now.

[Sparrow]

Take some more flats then. Just make sure you camera is as near to the original orientation in the scope as it was for your light frames. That should help dramatically with the vignetting in the image

Regards

John

[ollypenrice]

I'm sure John's right. Just get the orientation as close as you can. By comparison with the vignetting, which won't change, the dust bunnies, which might, are insignificant.

Olly

[great_bear]

--

[Ags]

OK Great Bear, what did you do?

More flats it is then!

[great_bear]

I painted out the galaxies, then blurred it like crazy, then subtracted it from the original picture and adjusted contrast back to normal again.

[garethmob]

have you tried these

gimp-plugin-astronomy | GIMP Plugin Registry

theres one there to create an artificial flat

[JimStan]

Great Bear is using a technique that produces really fantastic results. You just have to have a photo processing program that can work with layers.

Use the clone tool ( on the layer ) to "paint out" the galaxy, use the Gaussian Blur tool to eliminate the fine detail in the layer and select "difference" to make the layer work against the background color.

Back down the opaqueness of the layer so the background and negative background values cancel each other out. You will end up with an almost black sky and the gradient will be almost gone !

Hope you can figure it out from our discriptions. The technique is a GREAT one to learn !

Jim S.

[JimStan]

Here's my crack at it:

[Ags]

I took some new flats but they seemed to introduce a red gradient. This is my final go at this image - I desaturated it to make the color more natural.

[Smeg]

This is actually awesome.

Maybe we need a sticky on common "casualty corner" images and how to fix them.

[Smeg]

I painted out the galaxies, then blurred it like crazy, then subtracted it from the original picture and adjusted contrast back to normal again.

You mean masked out the galaxies?

What kind of blur?

I don't get it.

[Ags]

Great Bear edited the image so that only the sky background (minus stars and galaxies) was left, and then subtracted the sky background from the original image, yielding a flat background.

[Smeg]

ok thanks, gona practice

[Smeg]

I just duplicated a layer and on the top one, select the mode as MULTIPLY.

That reduced a lot.

Blend modes - Wikipedia, the free encyclopedia

Multiply

Multiply blend mode multiplies the numbers for each pixel of the top layer with the corresponding pixel for the bottom layer. As a simple multiplication for 8-bit/channel can get values as high as 65025 (255*255), which is far higher than the maximum allowed value, 255, the result is divided by 255.

The result is a darker picture.

Formula: Result Color = (Top Color) * (Bottom Color) /255

If the two layers contain the same picture, multiply blend mode is equivalent to an exponential curve.

If one layer contains a homogeneous colour, for example the gray color (200, 200, 200), multiply blend mode is equivalent to a curve which simply is a straight line.

For image editing it is sometimes more convenient to simply go to the curves dialog of the software, as it gives more flexibility in the shape of the curves.

Here is a multiply of two duplicated layers and the top one set to 80% opacity.

http://stargazerslounge.com/attachment.php?attachmentid=68571&d=1316731358

[JimStan]

Multiply seems to be too radical . You almost have the technique I use, except I use "subtract", and then vary the opacity of the subtraction layer until the background gets to be as dark as possible, without losing the midtones. You will notice that the smaller galaxies are almost gone, and a great amount of the arms of M31 have disappeared, as well, Smeg!

You can correct the color with the color settings. I like to use the one that gives multiple windows and works on the individual channels one at a time. I generally use the dark and the midtones to get a "neutral" black background. The arms in my process are possibly a bit dark, but if I tried to increase their intensity, the background started to get mottled. Probably a result of the extreme light pollution that was evident in the original photo.

Solution: more lights and darks ? That would probably have resulted in a need for less extreme processing. All in all, it IS AMAZING what you can accomplish with digital processing ! I am certainly no expert at it, but I am learning!

Jim S.

[Smeg]

I just did a quickie I didn't bother about the colour lol.

Just was playing with overlay modes

[Cloudwatcher]

The desaturated image shows a lot more detail,a great improvement on the original but on my screen it looks a little 'flat'.

I've slapped a blue then a sepia filter on the image,sharpened and flipped it which,on my screen,seems to enhance the view a bit........all a matter of taste I suppose.......

Don't quite know what to do about the stars though........

[Ags]

That's nice cloudwatcher...

I had one more go - and now I feel I have a satisfying image.

I dropped the red channel entirely as there was a strong red gradient. I would have dropped the blue channel as well given that the ST80 cannot focus it properly, but I needed it to suppress the remainder of the gradient.

I then desaturated the image using luminance, which gave tighter stars than a brightness-based desaturation. I did some careful stretching and then applied a strong lens distortion to correct the stretched stars in the corners (lens edge distortion of 100). Finally I applied a 2 pixel gaussian blur to smooth the stars and tone down the rough background.

I could have stretched the image more but I felt that made the image look too harsh. You can still see the LP in the image, which is I suppose true albeit not beautiful.

I think the real issue with the image is that I need to take much more data, use a lower ISO, get an LP filter, and learn how to take proper flats! Aside from that the image is fine... :-)

[JimStan]

The "stars" issue appears to me to be black circles around the brightest ones ? I suspect that is associated with over-sharpening them. Maybe not, after all, as I have said, I have a LOT to learn about astro-photography. Ags, I still think your processing needs some more contrast . What say ?

Jim S.

[Ags]

More contrast would be nice - but I think that needs more data...