First Centaurus A ...

[midnight (Darrin)]

Well done Carl.

You should be very happy with that image. Centaurus A is a target I would like to have a go at one day - if only I can get the time...

Darrin...

[Nugeorge (George)]

That's an awesome looking galaxy.. Well done Carl, looking forward to my first shot at it.

[CarlJoseph (Carl)]

Thanks for the feedback folks. I'm quite chuffed at the response. I know I can get a better shot so will keep practicing at this one for a while once the skies clear again.

[bmitchell82 (Brendan)]

Ill pitch in my 2 bobs worth on the processing side of it. Although I agree with Mark, with the levels and curves you should try not to use the curves feature till you have fully stretched your data.

What are you trying to do?

Dynamic range. Bright to dark areas, M42 Orion nebula has a massive dynamic range from faint whispy's to bright enough for the naked eye to see.... Stars are a large dynamic range

Linear Vs Non Linear stretching. Levels = Linear, Curves = Non Linear
This is how a lot of more advanced images keep star colour (that and having a big pixel well depth more photons before the well floods (turns white).

If your using Photoshop I know you can work with the RGB or individual R G B.

Step 1. Levels

Iteratevely clip the black point to the start of the histogram (left hand side like doug said. My own process i don't take it right to the edge as you may loose some really faint data a small tail is desirable in my eyes), Leave the white point alone for most case's, and the middle point slide about 1/3rd closer to the start of the histogram from the right hand side. As a general rule i will do this iterations 4-5 times but you know when your done, the histogram will not change any more. You have now streched your data to fill the full gamut (range of colours).

Step 2. Curves

Use sparingly this is non linear, eg you can hold one section while changing the other. Highlights, Mid tones, shadows, RGB or R G B, this is a good way to boost certain colours while keeping the others the same. play with it get a feel for it and then use it like a feather not a black smiths hammer.

Black point and White point, If you change your black point (clipping) then you will change the value of the pixel under that point to zero. eg, the pixel has a value of say 100 ADU (it escapes me right now but its basically the count of pixels in the pixel well) if you change the black point to clip everything under 900 ADU, the pixel with 100 will be zero essentially turning it black.

Be aware of the colour of the back ground, HLVG is great for removing Green from a image, it is a photoshop plug in, though it only works with 32bit.

Once you master the levels and curves you will find your white balancing and colour reproduction will be very good and your star colours will stay really nice!

Hope that helps
Brendan

[irwjager (Ivo)]

That's a wicked shot! Honestly, a data set and object like this has no place in the beginners section anymore

Many, many great tips here already. Once you're comfortable with all these great pointers, you may want to try & get your head around masks/layers and working on your data selectively. It's what the greats like Ken Crawford do.

When it comes to color correction; trust your data. Try to understand exactly why things look the way they look. If your colors are off (which is almost always the case), chances are there is something polluting the signal (light pollution, secondary reflections, IR or UV fuzz, etc.) Methodically find the causes and simply remove the isolated pollution from the signal. Pixel math will help you here. Once you have subtracted all polluting influences, chances are you will be left with something close to the real colors of the object. A lot of people seem to struggle with color correction; it's not magic, just science! One last tip on color correction is that some objects in the sky are perfect samples to calibrate your white balance against (notably a lot of galaxies).

If you use an OSC or DSLR and you have oversampled your data (e.g. the CCD resolution was higher than the scope could resolve under the seeing conditions at the time - or in laymans terms, your image looks blurry due to seeing), then use software binning to reclaim signal fidelity by trading in resolution. Many DSLRs and compact cameras these days have the exact same option. They call it something like 'low-light' mode and it is the reason why they can attain these high ISO ratings; they trade in resolution.

I took you dataset and put it through StarTools. Here are some of the steps;

• Binned it to 71% (2x noise reduction)
• Had the CPU have its way with the levels (manual Curve/histogram manipulation is sooo last decade ).
• Modeled the light pollution (at least I assume it was light pollution - a signature orange glow) and subtracted it. After that no color correction was necessary (a good sign), although I did bump up the saturation a bit.
• Then applied a lens correction model to the image. This sorted out the worst of the distortion.
• Cropped the image to center Centaurus A.
• Created a mask with just Centaurus A, minus halo and applied a tiny bit of deconvolution to it.
• Created another mask to preserve Centaurus A + halo.
• Applied a median filter, combined with a luminance mask to modify only the darkest parts (deep space noise reduction).
• Selected all the stars and put them through the Repair module to warp them into perfectly round shapes.
• Applied a touch of Synth (8" refractor - I wish!) to the stars to soften them up a bit.
• I'm sure I forgot some steps here.

I didn't bother with the star colors too much - I like how it keeps the focus on Centaurus A as the only distinctly colored object.

Great image and I hope to see more in the Deep Space section!