IC 4628 (Prawn Nebula) (finally reprocessed!)

[Atmos]

First time in imaging this one, I have a feeling that it is considerably larger than the area that I have done, even if it isn't, there appears to be a fair bit of Ha surrounding this region.
Lost about an hours data throughout the night due to the auto guider dropping out (connection loss). Not sure why it does it, it isn't running through a hub or near any electrical wires. Hopefully won't be an issue in the coming weeks.

It is a bright region so I may just leave it where it is data wise, could add a lot more time to get better SNR but there isn't much faint stuff to worry about. There is only one remotely dark region in the field towards the top left.

Tried adding synthetic RGB stars (combining in HSO), it has worked to an extent on some of the brighter stars but it has also been dulled down a bit as I removed the ringing artefacts. Happy with the result for now though.

Replaced one of two potential flexure areas in the imaging train, tilt has moved from being diagonal (top right to bottom left) to just being at the top now. Looks like I will need to get a Precise Parts part made to remove it. OR it is slight tilt in the sensor as it feels stiff at the moment.

High res on Astrobin (http://www.astrobin.com/full/252166/0/?real=&mod=)

Reprocessed on Astrobin! (http://www.astrobin.com/full/252166/B/)

Equipment:
Sky Rover 130mm Sextuplet
QHY22
Astrodon 3nm narrowband.

[Placidus]

Sharpness, colour, and contrast in the nebulosity are superb.

Wouldn't worry about the image plane tilt - I can't see it myself.

The rings around the stars do need work.

Very best,
Mike

[RickS]

Very sharp indeed, Colin!

[Atmos]

Thanks Mike, totally agree on the stars. I think the best way is to reprocess the colour data again
Just realised that the visible tilt disappeared because of the meridian flip. This means that it isn't flexure as much as it is a repeatable error in the system. Not sure but it may be the compression ring on the rotatable focuser; tighten the knob to stop the rotation. May need to use a screw driver to equally tighten the focuser in place or at least heavily stiffen it.

Thought comes from my previous telescope which suffered from the same problem, the focuser tightening knob caused 0.1-0.2mm tilt. Who knows when the next clear night in Melbourne will be!



Thanks Rick, think I need to play around with your colour mask script a lot more :P

[strongmanmike]

Aaaah run for your lives :scared:...a giant prawn :eyepop: :lol:

Apart form the dark star rings...not bad Col :thumbsup: Given you used 3 filters it is looking a tad yellow-blue bi-colour in appearance maybe you could bring out the third colour a little more? It might look a little less harsh with just a bit less sharpening too :question: anyway you could try just out of interest but I like your FOV there though, just right really for this object...mine is a bit smaller than I would like :sadeyes:

Mike

[glend]

I think it looks fine but i would dial back the magenta so the star halos are dissapated.:thumbsup:

[Atmos]

It is a tad green isn't it, I just used the whole image as the White balance and didn't play with the colour after that. Planning on having another crack at the colour to see if I can reduce the ringing around the stars. Still trying to figure out the best way of dealing with the stars without causing other ring artefacts around the stars. Still so much to learn :P

The very last thing I did on the luminance before combining it with the colour was run a relatively weak UnSharp Mask. I was unsure whether to use it or not but I figured I'd go all out and remove all softness hehe

Your FOV is smaller than mine and I feel mine is too small. If I could afford it I'd love a 16803 ;) One of the reasons I bought this scope is that it has a 55mm corrected circle!



Totally agree Glen, my star processing leaves a bit to be desired :)

[gregbradley]

Not a bad narrowband there. Those blue star halos are they from the O111 layer? You probably need a star mask on your O111 layer so the stars are excluded to prevent the ringing. Narrowband can be prone to odd effects like that. Your O111 may've been a bit out of focus and the stars were larger than in the Ha. With my Astrodons I focus using the luminance and luckily the Ha O111 and S11 seem to be parfocal as focusing narrowbands by themselves is a bit slow and tough to do.

I saw your image last night when I was looking for something to image and I decided to image it so thanks for the inspiration. That vertical wall is an interesting feature.

Greg.

[rustigsmed]

nice prawn (and the little gem) Colin - excellent field. as you have and others have noted, just the stars could do with some attention later.

[DavidU]

Mighty effort there Col.

[Atmos]

The halos are caused by both the SII and OIII, both having larger stars than the Ha. I do focus each filter individually using a bahtinov mask. I have been considering trying to use FWHM values to see if that works better. 1-2s exposures on a bright star are more than enough for narrowband focusing with the bahtinov mask but it doesn't seem to be working for me. They've been in every narrowband image I've taken with this setup so far so it could be something else to try.

Glad it aided to inspiration, wish I had Andy's FOV though :P



Thanks Russell. I did a rework a couple of hours ago, got to the point of saving and then PI crashed on me! GRRRR



Thanks Dave :)

[rustigsmed]

It might be easier to open in photoshop, (as you probably know)
IMAGE -> Adjustment -> Selective Colour - magenta slide down... took me 15 seconds.
i can appreciate you working on your PI workflow/halos, however. :thumbsup: (this wasn't slid all the way down about 75% less magenta)

[Atmos]

Now that just makes me feel bad! :lol:
Having never really used Photoshop I have a MUCH better grasp of PI than I do any other image editing software. Actually use PI for my normal photos as well if that gives you some idea of how little I've used PS :P

[Stevec35]

Not bad Colin apart from the stars (always an issue with NB). One simple way to improve it of course is to select the stars and desaturate the magenta.

Cheers

Steve

[Atmos]

I had tried that, the problem I am getting with that is that I have been trying to preserve the colour of the stars and as the rings are more saturated than the star, the rings are more difficult to remove. Have even tried making ring masks by subtracting a smaller mask from a larger and then multiplying the resulting mask. Had some success but not to a point that I was happy.

At any rate, been through and spent the last few days on and off working on star masks. Think I have finally got there! Have the stars at a point that I am happy and just for you Mike, I have gone and taken away the final Unsharp Mask :P

High res on De Astro Bin (http://www.astrobin.com/full/252166/B/)

[strongmanmike]

Good on ya Col, the sharpening looks better now :thumbsup:....it's got the measles...but otherwise looks good :thumbsup:

Mike

[Atmos]

Thanks Mike :) I assume you mean those fainter purple ones? Or have I left some ringing artefacts?

[strongmanmike]

Nah just some purple stars..a little magenta in stars looks good in narrow band images IMO but it might just be a bit too purple and a little stark here and as you say the small stars are purple.

You're nearly there :thumbsup:

Mike

[Atmos]

I do agree with the magenta stars, they don't bother me, unless they have rings and begin to look like thousands of little eyes peeping back at you! (have the first rendition in mind :P)

DePurpled Version (http://www.astrobin.com/full/252166/C/)

I've gone and done a DePurple/Magenta, dulls them down a bit and makes them less intrusive. Ideally I would like much smaller stars as some of them are quite large but for this I would require a nice tall mountain to get above the atmosphere ;) This version is more of a proof of concept as I was overly kind (no mask, found hue and then desaturate!).

My next goal in star masking is actually to be able to select those tiny little ones. So far, not much success! Luckily they only problem they cause is being a bit overly saturated for personal tastes.

Out of version B and C, I actually prefer B, even disregarding the butchering of C (late night laziness :P). Thanks for your keen eyes Mr Mike :D

[RickS]

It's a bit painful to do but removing all the stars from the NB images for the colour combine produces really nice results, Colin. That's the real core of JP Metsavainio's tone mapping technique. Now that you're a ninja master at star masks it should be easy. I mask the stars then hit them with repeated MMT (remove a few small scale layers) and MT Erosion.

[Atmos]

Does it matter that when using the erosion to remove the stars that there ends up being ringing artefacts caused by the star mask itself? It erodes the area around the stars, not just the stars. I can assume that when the stars are eventually put back in that it covers over these areas again. Or does this mean I just have to make a better star mask :P

Just been having a play, I can remove all but the smallest of stars but there is some residual star mask ringing. This does not to be as big of an issue when removing them but it definitely is when trying to put them back :P

So, more tweaking and learning in star masking me thinks :P

[Atmos]

Well after some new masks I have made some more progress, there is still some evidence of the star masks in the fainter areas but it almost looks like its a part of the nebulosity anyway. Actually tried adding different coloured stars, need better masks for that :P
Link to version D (http://www.astrobin.com/full/252166/D/)

[RickS]

You could just build a magenta mask now and use curves to tone down the blue, red and saturation a little.

You can beat up the colour data pretty badly and it doesn't matter if you have good quality luminance to carry the detail.

Cheers,
Rick.

[Atmos]

Version E (http://www.astrobin.com/full/252166/E/) :P

I think this is what you had in mind, it wasn't until I did a red desaturation that I realised how much of a red overcast there was. I haven't done much other than remove some of the purple and red.

Not sure whether to start the colour data from scratch again, as the bottom left has started turning grey. There is purple around two of the brightest stars, those I may just end up cloning out when everything else is set.

[RickS]

That's cleaned up the stars a bit. Might be worth trying again from scratch. I usually do that at least a couple of times with most images :)

[Atmos]

Usually when I do a reprocess I start from scratch :P It has cleaned up the stars but doing so has lost most of the star colour that I was hoping to preserve :/

[RickS]

You can add RGB star colours or build star colours from the NB data and add them at the end.

[Stevec35]

To me it looks really good now Colin - congratulations. Minor disagreement with the colour- seems to be not enough red but it's still good. I usually use RGB for narrow band stars myself but there are probably better ways. I do agree with Mike - a little magenta is fine, just not too much.

Cheers

Steve

[Atmos]

Very true, just need to learn to build better star masks, need tighter ones for star replacement.



Thanks Steve, I think my colour calibration was a bit off to begin with. Usually the SHO palette ends up being a bit green, looking at it after the red desaturation I think it turned out a bit too red but I do agree, it ended up taking too much of it.

I think the way you do it is the best way, adding RGB stars. I have no idea when we're going to get another clear night down here in Melbourne so I going to do some synthetic colour from the narrowband. I personally find HOS giving some nice star colours but I do need to play with it more.
At the moment my star masks are creating nasty issues when I add them in, another few days of masks make ;)

[RickS]

I only modify the colour (CIELAB a* and b*) with a few gentle blend operations and leave the lightness (CIELAB L*) alone. I find that a lot easier than wholesale star replacement.

[marc4darkskies]

Very nice image Colin. :thumbsup:

If you are in PS, don't overlook the selective colour tool - it's powerful. For your original image invoke it, select magenta and then slide the magenta bar all the way to -100%. See the result in the first image below.

I then took liberties and did a couple more adjustments (in the same invocation of selective colour) to produce the second image :D. Hope you don't mind - I'm learning NB!

PS: Excuse the jpeg artifacts - they're pretty bad, but you get the idea.

PPS: No star mask used because there was no other magenta in the image.

[multiweb]

As Marcus outlined in his post you have to take advantage of the fact there is no purple anywhere else in your picture (or very little) so you can target that color.

Here's a file RAR file here (www.astropic.net/astro/colin/colin.rar) with a stack in PS based on your original picture.

I isolated the two layers of interest, one for the neb and on for the stars. Have a look at the settings. You'll get the gist of things.

In the end it all comes down to proper star masks and how much time you want to spend on it. Mask are easily done but using a difference blend between layers then refine the selection by contracting/expanding then feathering.

Also keep in mind the shadow highlight tool on stars only. It's a flexible way of deringing your stars while controling the cores and colors.

Finally blending layers on top of each other as "color", "lighter color", "Luminosity" helps as well getting rid of hard edges.

[Atmos]

Wasn't something that I'd ever considered, just gave it a quick shot and I think I still need to work on my star masks :P It is not only changing the star colour back the small area around the stars as well, they're not tight enough so its bleeding through. Or maybe my blending is way to harsh :P



Quite like the result Marcus, I do have PS4 on my computer but I have barely used it for anything other than its Save for Web function ;)
It sounds similar to what I did on version C. All I did was open Version B and do a selective colour desaturation on the stars.

Narrowband is a lot of fun ain't it Marcus, has its own challenges though. As you mentioned on your recent first one, it is less about the colour exactness of LRGB but I personally find narrowband more fiddly than LRGB to process. It would also be fair to say that in mind case I butcher RGB a bit as well :P

Tonight is just going to be playing with the RGB data, I think I may have found a better star removal technique that will hopefully leave less artefacts.

[Atmos]

I've downloaded it but not yet had a look at it Marc, hopefully find some time tonight :)

I've done Version G (http://www.astrobin.com/full/252166/G/) now. Tried adding some more red but I don't yet have the time to try to mask all of those minuscule red stars embedded in the nebulosity. That may be a job for the coming days.

Version F was an attempt to replace the stars using the techinque Rick suggested, using the CIEa&b, works well but there is visible issues due to star masks :P It wasn't so obvious in the 32-bit format in PI.
Version G was just keeping the original stars but making them look nicer, think I prefer the Hubble stars moreso.