M27 - any processing improvements?

[peroni]

Hey All

This processing malarky is a steep learning curve (like most AP areas). The two images below are my first attempts at M27. 

First attempt at processing:

I noticed a horrible green gradient in the image once I'd finished and saved, so I couldn't go back and remove the step that caused it. If you look hard on the right hand part of image you'll see a vertical boundary (where ellipse is) where the sky is greenish on the left and not greenish on the right

And the second:

I could still see the vertical boundary but I've cropped it out this time.

Do you guys and gals have any tips on ways I can improve the image in photoshop?

And any ideas what might have caused the vertical boundary.

[Daniel-K]

to get rid of gradient try http://www.rc-astro.com/resources/GradientXTerminator/ and to get rid of green noise try http://www.deepskycolors.com/tools.html i use both and work great HTH

[peroni]

Thanks. I'll have a look at those.

Sent from my phone :-D

[IanL]

It's not really a gradient so much as a discontinuity or seam.  A gradient would be a change in brightness or colour over some distance, usually due to light pollution, the moon, amp glow or vignetting. What you have is a hard line between a green tone and a reddish tone.

You don't say if this is a stack of multiple images, a mosaic or if this is a single image?

- If a single image then it would be pretty odd to have a discontinuity like that and on the whole I'd assume it was down to a mistake in processing (e.g. masking only part of the image when doing some operation).  You should try to save intermediate steps with different file names as you go, since you can back up and start over if you make a mistake or want to improve things.

- If it is a stack of multiple images then you've covered different parts of the area to different depths (e.g. the stuff on the left might have more exposures covering it than stuff on the right or vice-versa).  It is pretty normal to have a noisy boundary around the edge of a stacked image if you use dithering (deliberately moving the camera a small amount between exposures).  The benefit is that any hot or dead pixels will stack out rather than building up in one spot on the final image, the downside is that the outside edge of the image is more noisy as it will have fewer exposures than the middle.

Normally it is a border maybe 20 or 30 pixels deep depending on the dithering, but if you had markedly different framing for your subs (e.g. different nights) then you could well end up with a much bigger area such as yours due to different amounts of noise, or indeed just because the conditions changes during the night or from one night to the next.  The solution is to crop the image, but you already figured that one out; better framing from night to night is the way to minimise the amount of cropping needed.  If you are using EQMOD rather than the laptop for the mount, go over to the software section of the forum and search for AstroTortilla, which uses the camera and plate solving to get you bang on target from night to night.

- The third (similar) cause of discontinuities is making mosaics.  You have to try to make sure each pane of the mosaic has the same numbers of lights, darks, flats, etc. to achieve similar amounts of noise in the background, and even then colour balancing can be hard if the target is shot over multiple sessions.  In the end you usually have to use blending in PhotoShop or something like the Gradient Merge Mosaic process in PixInsight to hide the seams.

[jaspalchadha]

I agree with Ian

[peroni]

Thanks for the advice Ian. I think the error was introduced on post processing when I used Noel Carboni's actions. One particular action was taking a while and I flicked over to read a forum post. Possibly introducing the problem.

I'll pay more attention next time.

Sent from my phone :-D