Sadr region Mosaic

[ian_bird]

A bit of an "Ooops" in the Mosaic planning!

13 x 30 minutes with an Astrodon 3nm HaII, Altair Astro 10" RCT, and Atik 4000. 4 Panel Mosaic. (OK - 3.5 panel mosaic).

Guiding left a lot to work on - but as a first result I was quite pleased.

Incredible local light pollution, and some of the subs were shot through clouds! Is there nothing 3nm HaII can't cut through? ;-)

All in all - a bit messy. But I'm impressed with what this filter can do!

[Gina]

Nice start but I think you've lost black level.

[ian_bird]

6 minutes ago, Gina said:

Nice start but I think you've lost black level.

Thanks Gina.

Not sure what that means? Have I over stretched it?

[Gina]

Yes, you've clipped the black.  If you redo the processing making sure you keep the whole of the left of the histogram, I'm sure you'll see a lot more in the regions that are currently black in the image above.  Hope that helps - meant as constructive criticism.  Make sure you only crop just up to the start of the main body of the histogram in Levels.  I hope that makes sense - if not, come back and I'll try to find some examples.

[ian_bird]

Gina - that seems to make sense to me. I will give it a try.

Thank you very much!

I have to ask - how can you tell just by looking at an image? Or is it experiece?

Thanks again.

Off to give it a try right now! ;-)

[ian_bird]

Gina

Is this more like it? I have kept well away from the left foot of the histogram. Or should I leave it even more?

I can definitely see more detail!

[PatrickGilliland]

Very nice start, i think you could go further left on the HT still.  SOme of the background in this region is very dark but not all of it should be black.  That aside some super detail and final version will be very nice.

Paddy

[ian_bird]

Thanks Paddy.

" further left on the HT". Is that the same as Gina was saying? Clipping the black point?

Cheers

Ian

[Gina]

Agreed - much better but you need to go a little further - you're getting there   As has been said, none of your image should be absolutely black though you can get away with very small black bits.

Should add...  Do you see how taking the black point further left gives you detail in the dark areas?

[ian_bird]

Gina - thanks for all the help and encouragement.

Not sure I can go much further though - there is no space left on the left side of the histogram.

Should I be thinking about layers? I used that for the bright region just above Sadr (not on this image though).

Perhaps I should do that for the dark areas - in the opposite direction of course.

Or is there a clever usage of curves that will bring the dark areas out without blowing out the main nebulosity?

I am used to processing faint that needs to be bright (galaxies), and luminosity, but not both at the same time! ;-)

Thanks again for the help.

Cheers

Ian

[ian_bird]

Gina

Yes - I did see more detail after I took the black point further left. It's fascinating seeing data seemingly appear out of nothing!

I started out thinking that Narrow Band image processing (from light polluted skies and locale) was a lot easier than LRGB under the same conditions. It is - to be sure. But there are obviously a lot of techniques that I will need to learn to do the images full justice.

On the whole though - I love 3nm filters!

Cheers

Ian

 

[Gina]

Oh yes, 3nm filters make a lot of difference.  If you cannot get further left you may need to capture more data.  But yes, curves can be used to coax more luminosity out of the darker areas - if it's there.  You can make the curve all sorts of funny shapes to do this.  It's a matter of practice and experiment to get this working well.  Good luck and have fun

[ian_bird]

Gina - playing as we speak.

You are right - it is fun. LRGB is usually frustration. 3nm is so much easier (in some ways). The mosaic required no adjustments at all between panels!

Cheers

Ian

[ollypenrice]

What Gina was spotting were large areas of pure black, with no differentiation visible in them. This is the black equivalent of saturation producing all white areas at the other end of the brightness range, but the cause is not the same. You get a 'white out' when regions which are very bright, very very bright and extrememly bright are all hitting the maximum brightness allowed on your screen, so they cannot be distinguished. It comes from either over exposure or over stretching.

At the black end 'black clipping' comes from cutting into the left hand side of the histogram too much between stretches. Like this:

Healthy

Black clipped

In your particular target you have dusty regions which are significantly darker than 'background sky' because the dust is blocking starlight and Ha emission behind it. So you need to be particularly careful to get your background sky up high enough to have room below it for the darker dusty patches. In bringing in the black point look carefully for differences in brightness even in the dust patches themselves. If you clip too hard you will clip away those nice little differences and make the dust one uniform brightness. (Or darkness. You know what I mean!!)

Olly

[ian_bird]

Thanks Olly. (And Gina and Paddy).

Lots to learn here. Perhaps they should rename "Curves" to "Wiggles"!

Thanks again.

Cheers

Ian

[Gina]

You're right Ian there is a heck of a lot to learn.  I'm learning all the time and love it