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Bloated stars and other problems...


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Ok, so here's my first ever attempt at an RGB image. I have L to go with this too if I want it, though I know Francis recommended against it. I thought it would be interesting to see how it works out myself at some point on one of our rare cloudy nights... Mostly it was just a handy target whilst I tried to make sure everything worked and to get to learn my way around, but I was trying to make a reasonable fist of it.

Anyhow, this is 15 subs of 45 seconds for each colour from my 314L+ and 127Mak, unguided. No darks, flats or anything like tat. I've done little more to the image than crop, apply layers and curves and a very slight sharpen.

m13-01.png

First off there are some fairly badly misaligned stars in the top right, but that's what you get for going unguided at 1500mm I guess. I can live with that for the time being.

What I really don't like is that the yellow stars look rather bloated. Why is that and is there any way I can get them under control in processing?

There are also clearly a very large number of fainter stars outside the core. Should I try to pull those out a bit more? From visual views of this cluster I feel that the core should be really vibrant, but it would be nice to emphasise it's real size and the sheer number of stars.

Is it perhaps worth pushing the sharpening a bit harder?

Are there any other glaring faults or things worth doing in the processing?

James

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I don't use luminance on star images. RGB is easier to control as the stellar cores bringhten.

Yellow s a secondary phenomenon since you shot through RGB, so if you have bloat (not sure that you do...) you need to see which channel(s) it's coming from. Just split the channels and have a look at them. You could then try a star reduction routine (EG Noels Actions) on the offender if there is one.

One stretch does not fit all. The trick is to do a stretch for cores and then a stretch for things faint in order to manage the dynamic range. This would be an easy one to deal with since you could make two layers the top one more fully stretched, then erase the inner core of the over streched top layer to let the good core through. There's a bit of craftsmanship with the eraser brushes here. Lots of feather, low opacity, reduce the diameter between iterations.

Olly

Edit, I think you've a tad too much green in play. Hasta la Vista Green is free/donation from the Deep Sky Colors website of Rogelio Bernal Andreo.

Edited by ollypenrice
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Hmmm. If stretch the histogram really hard at the bottom end there's a clear ring of near-identically coloured but quite dim stars around the core that almost doubles the size of the cluster. With the data I have though I just can't pull those out. Longer exposures required I think :) I wasn't anywhere near saturating the histogram at 45 seconds, so if I can get better alignment and tracking I think there should be a fair bit more to come from M13.

James

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The cropping was really required before stretching I'm afraid. The alignment wasn't great so there were a lot of artefacts around the edge that needed cropping off before I could start any other processing. I'm looking into the possibilities for some simple guiding at the moment and I'll see if I can give that a go when we next get a clear night.

James

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I had another go last night, but had some problems which appear to have resulted in a real mess in the stacking of the lower third of this image. I took the setpoint down to -10C this time however, and it seems to have given me far better detail in the dimmer stars outside the core.

m13-2013-08-05.png

James

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The lover part of the image looks like something else then just a stacking issue to me though? If the stacking hit on all the other stars, i see no obvious reason why it wouldn't hit on the stars at the lower part of the image.

Would be easier to say if you have a single sub you can upload i guess.

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The lover part of the image looks like something else then just a stacking issue to me though? If the stacking hit on all the other stars, i see no obvious reason why it wouldn't hit on the stars at the lower part of the image.

Would be easier to say if you have a single sub you can upload i guess.

It may be stacking problems combined with tracking errors, perhaps. All these are unguided. I had another attempt last night so I'm going to see how that turned out. If it looks ok then I think I'll put it down to experience, otherwise I'll look into it further.

James

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The star sizes aren't bad, in my opinion. With that setup you're imaging at 0.89" / pixel, which is way better than the seeing conditions are likely to be! Globs tend to grow a lot the deeper you go; you're really close up here!

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