NGC3372 - Carina (yeah.. another one)

[whzzz28 (Nathan)]

Looking for some feedback on my Photoshop attempts.
So far i've been pushing out terrible stuff and i was trying to figure out what i was doing wrong.
Most amazingly, i knew what my problem was; i just never stopped to think about it. Wasted nights on this when it was easily fixed...
Stacked with darks+lights now... oh wow, processing just became that much easier and my images don't look like junk after a curves or level pass!

Subs:
10 x 160 @ 1600 and
4 x 300 @ 1600

50%: http://core-au.net/astro/ngc3372/ngc3372-50.jpg

100%: http://core-au.net/astro/ngc3372/ngc3372-100.jpg

(disregard left side of image - optics problem).

-edit-
Another go, more nebulosity this time.
http://core-au.net/astro/ngc3372/ngc3372-2.jpg
And the problem i was having before has re-appeared, the center of the image is lighter than the corners. May have to check my light frames.

[joecool (Mark)]

I must confess i don't like curves or levels. You will loose a lot of the data. Have a look at this lot. There is a lot of data still hidden here (look at all that nebulosity and so many more faint stars!!!). It now needs some artistic work to select the stars into another layer and dim them down a bit.

With the original darker stacked image before levels and curves I could pull the nebulosity out easier without blowing up the stars so much...

I will post this first and then write what I did over the next few minutes...

[joecool (Mark)]

When you adjust colours, use Auto Tone, Auto Contrast, Auto Colour. Then use Colour Balance to fix the rotten job Auto Colour does. Mostly you will only use the Midtones. Bring up the Histogram and set it to All Channels View. Adjust the balance until the RGB curves overlap each other. If you have any colour hanging over the right hand side then you can use Highlights adjustments, but I rarely touch this.

I've brightened this by making a copy of the image to a new layer and set the blend mode to screen. I flatten these 2 layers to 1 layer. I might do this again if I want it brighter, keeping each brightened image in separate windows.

When you screen 2 layers together you brighten faint stars and nebulae, but blow out the already bright areas. Now, by putting the brighter image into a mask in the layer above it you are replacing the blown out areas from the dimmer images.

I copy the images into layers in one image with the brightest image on the bottom of the layer stack. I copy this image to a mask in the layer above it (with the layers above that set to not visible) and Gaussian blur it by 50 pixels (for this resolution). I copy the resulting image on this layer to a mask in the layer above it and Gaussian blur this to 50 pixels, etc, etc, etc, and repeat until I've done it for all the layers above it.


Oh, and I have not applied any sharpening to this image either. That will selectively give the nebulosity a bit of punch and make it appear to be in the foreground, but this takes more than 5 minutes. The colour fix is fast so I did that first.

[bmitchell82 (Brendan)]

Sorry to say, Auto tone and that lot is one of the worst things you can do. it kills all your data something fierce.

If you understand what curves and levels do then you will know how to make it work correctly! Careful iterations and working your way into stretching the data will allow you to keep your stars from blowing (going white), minimise them from bloating (getting far bigger than they should) and give you absolute control on your white balance.

It is one of the most critical parts of learning how to process... is being able to stretch your data over a larger gamut.

The only difference with this image click here in processing terms is that i have worked with levels and curves to get my base image before moving onto more advanced techniques and no, its not taken by a high end telescope or camera either!

One night if you want we can set up a team viewer session so you can see how to do it right.

Oh on the point you made about the centre of the image being brighter than the outside. That is called vignetting. Flats take care of that abberation.

[whzzz28 (Nathan)]

Originally Posted by bmitchell82

One night if you want we can set up a team viewer session so you can see how to do it right.

That would be nice. I noticed a lot of my images at 100% res are rather blurry. Not sure if its a stacking issue, focus issue, dew problems or something else. Probably a mix of them all.

I played around with my M8 data a bit and got a lot more than i was expecting. Color is slightly off and vigenetting is obvious.

Oh and all of these images - the color looks to be a lot better on my screen in the photoshop (non jpeg compressed) images. In the Carina image the red is not as vibrant as in the jpegs. I am guessing its a 16bit to 8bit conversion issue.

Originally Posted by bmitchell82

Oh on the point you made about the centre of the image being brighter than the outside. That is called vignetting. Flats take care of that abberation.

I do have some flats (light frames as i keep referring to them), 8 in total @ 300seconds. It does help but doesn't seem to remove it completely. I think my flats are terrible though, so i am hitting up Steve to see if i can get a light box made.

And the scope is off on Monday to Bintel to see if they can correct the optics problem.

Thanks.