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A quickie in the mist...


ollypenrice

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We had just a brief and hazy window last night but grabbed 2X5 mins per channel of RGB on M35 and NGC2158. (Takahashi FSQ85 at F3.9 and Atik 4000 mono.) The haze threw in a colour gradient and patchy illumination but Pixinsight deals so well with these. I think it goes to show that you can have too much exposure for starfields and that luminance is actually a negative. Shallow exposure keeps more star colour, I reckon. This is the shortest exposure I've ever posted, a hundred and sixty times shorter than my longest! :D

Anyway an old and a young cluster seen together...

M35-RGB-30MNS-TOTAL-XL.jpg

Olly

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I normally forget about luminance on star clusters / fields. With the new scope @ F2.8 I take really short subs, 30 seconds or so and the actually usb download is a killer for me @ 8second per shot :D LOL

Great image Olly, nice and quick in the mist and shows what can be done when you play a bit.

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Very nice for a quicky Olly. I have found the same with my DSLR, I get much better star colour at 3min than at 5 or 8min.

I really think all DSLR imagers should read this carefully and take a set of short subs for star colour, maybe even less than 3 minutes. Applying them is easy. You can just put them as a layer underneath and then select the stars on the top layer and erase them to reveal the better colour beneath. (Select stars using Noel's Actions or by following Matin B's tutorial in the imaging stickies. Expand and feather the selection by experiment.) If you blur and increase the saturation of the bottom layer you spread the colour around the star cores and it comes out better still. I wonder if a short stretch of the image just for the stars might go sme way to getting more colour as well. Depends on how saturated the stars are. In any event you don't need may short star subs because noise is never going to be an issue in star cores!

Olly

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I make a star layer, and put it on top, then increase the saturation and blur it slightly, finally set the blend mode to 'colour' and tweak the layer opacity to suit. You have to be quite careful/tight with your feathered selection though. If you get too much background sky selected round the stars you end up draging a lot of blue into everything.

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I hadn't thought of setting the blend mode to colour. I'd have expected this to preserve the luminosity of the original image - or does it preserve the luminosity of the star layer?

Bringing down the luminosity of the stellar cores enhances their colour, I reckon.

Olly

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Not bad for 30mins Olly.

Thats a good point about the shorter subs. It sounds like it makes sense. You don't want to fill the pixel well of the camera and white out everything.

I usually do a star morp in PI only on the Lum layer, and when I blend the RGB layer in, the RGB stars stand out more. Although having said that, I'm using the Lum on top of RGB layer and the the Lum blend mode in PS tp make an LRGB pic. (You can put RGB on top and set the blend to colour) So I wonder if I put the Lum on top and set the blend to Lum am I really using the Lum star data even though the stars are smaller and dimmer? Maybr I'm doing it wrong!

Tom.

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I end up with background --> stars --> star colour. I think the colour layer preserves only the luminosity of the layer imediately beneath and it doesn't cascade down to the background. I'm using CS3 so I don't know if other packages work differently.

I have seen a curves routine where you pull down the white point a little with each iteration. I gave it a go and it sort of worked but I wasn't convinced. I guess that effectively reduced the luminosity of the cores while retaining the colour. I think I got it wrong and ended up with reduced contrast and flat looking stars. The colour was good but they didn't sparkle and glow.

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