Help with levels and curves please?

[kirkster501]

Hello all,

Got 15 x 3 mins of M31 in this with darks and BIAS frames. It looked quite good on the DSS preview screen!

So now I am working with Photoshop - I only have CS3 but should be OK?

So I do step one where I cut up to the peak to its left.

Then I do stage 2 with the curves tool.

I get that posterised look. But I am confused what I do to remove that look and get it looking like a astro picture...Is it continued iterations of levels and curves?

Tips and guidance much appreciated guys please? I can see teh data is in there somewhere!

I am not worried about that line on the picture at this point.

Thanks for looking and Regards, Steve

[ollypenrice]

The first iteration of Levels should only be to bring the black and white point sliders in to meet the start of the data line. This may not even be necessary. It is too soon to do any clipping of the black point because you risk losing data.

You Curve is broadly right but has one fatal error. It has a negative bend in it at the top. Now let's think about what that means; it means that you have taken something brighter and told the image that it is really something dimmer, you've reversed some brightness readings from the camera. Curves must always be positive. Making a good Curve is, in part, down to craftsmanship with the cursor, something which some imagers like (that's me!) and some dislike, preferring an all-mathematical system like Pixinsight. Your Curve is maybe just a tad too aggressive as well. I'd roll it off just a little lower then make sure it goes in a flat line to the finish.

To make this curve start with the bottom marker and make the big lift. The next one up is just a pin on the first line. The one above that bends the curve round and the last one is there to get the straight line to the finish. If you have more than four markers you are likely to introduce wiggles and the deadly negatives!

In the image above the posterized look vanishes once the Curve is applied, though on a slow computer or with a very big image this make take a while so give it time.

Next cut back in Levels but not too far!!

You'll need room to manoeuvre when working on local contrasts later so keep the image overly light till right at the end.

The next curve will be much less aggressive, rolling off lower down. Another careful clip in Levels and then stop when your image starts to get to its noise limit.

For Andromeda I'd forget about the core in this stretch and let it burn out. Save this main stretch then do a 'Core Control Curve' where you look only at the core and get that right. Then apply it as a layer underneath and erase the over exposed top layer.

Oops, there's a bit of a negative dip to this example. Do as I say, not as I do!!! You can see what this kind of control curve does. It simply brightens the faint stuff but flattens too soon to do much brightening of the bright stuff.

Core control is often best done to the RGB only layer and used at the end to control over-exposed parts of the LRGB image. The colour filters have helped dim the bright regions at the capture stage.

If you really want to combine layers the pukka way follow Lodigruss, but a simple eraser will often work fine with a good feather and partial opacity of the brush.

http://www.astropix....GIT/LAYMASK.HTM

Olly

[pete_l]

I expect you know this already, but I find it helpful sometimes to do "layers" on the individual colours separately. That lets me get the white balance to a pleasing state.

So far as CS3 is concerned. Yes that should be fine. Mostly the processing I do is based on the "less is more" principle and most of the new versions of PS have some really funky functions that go far beyond the needs of astronomical processing.

[steppenwolf]

I confirm CS3 is absolutely fine for astro-imaging

[kirkster501]

Cool - thanks guys for this and on my M57 thread the other night. I am going to have another crack again with PS tonight after the kids' fire works party is done!

Glad to here I am OK on CS3 Steve .....

[kirkster501]

BTW... You guys should get paid a salary for the free help you dish out on this forum! I'd be happy to pay!

[Steve Ward]

Check out Doug's tutorials here . . . http://www.youtube.com/user/namregd?feature=g-user-c

Steve.

[ollypenrice]

Another CS3 user here...

Olly