Reworked M33 from Astrofest

[batema]

I am just going over data due to the weather and trying to improve my image processing.
Taken with W/O Flt 110 and Qhy-9 at minus 20. Lum 8x10 min and RGB 6x5 min stacked with DSS and processed in photoshop cs4. My earlier effort is on the left and my latest is on the right.
Any advice more than welcome.
Mark

I am uploading this on an old laptop because my school one is doing very strange things so a sort of experiment.

[gregbradley]

Hi Mark,

You have tamed the noise a lot more in the repro. But the stars have a harsh oversharpened look.

Stars have a distinct sphere of bright light followed by a slightly fuzzy outer halo. When oversharpened the outer halo takes on a harsh edge and loses its fuzziness.

The solution to this is when you are doing sharpening to either:

1. use a layer mask so you control which areas of the image are sharpened and nowhere else - called selective sharpening.

or

2. You select out the stars and then apply the sharpening filter to the rest of the image.

1. above is best as you have total control. In fact it is better to use both and select out the stars then apply the sharpening to the areas of the image only where you want it and nowhere else.

Here's how you do it:

1. Select the brighter stars - select/colour range tool/ set to highlights

2. Expand the selection say 3 pixels. select/modify/expand set to 3

3. Feather the selection of the stars by 2-3 pixels - select/feather set to 3.

4. Now invert the selection so only the rest of the image is affected-
select/inverse (Note before you do this you can use the fact you have selected out the stars to perform so star enhancement like increase saturation, reduce curves near the top of the curve a tad to make them smaller, or use a touch of minimum filter to reduce star sizes).

5. Now make a duplicate layer - layers/make duplicate layer

6. Set it to overlay or soft light mode.

7. Run high pass filter or unsharp mask or smart sharpen.

8. Hide the resulting sharpened result - layer/hide all.

9. Select the brush tool, set the opacity to say 33%, set the foreground colour to white (the tool with the white/black window icon). Rubbing white on the image now reveals the lower sharpened layer only where it is rubbed. Changing the foreground to black now hides the sharpened layer so if you reveal something you don't want switch to black foreground and rub on it.

Now you can control the amount the sharpened layer shows through by 2 ways. Firstly the opacity of the brush tool - 100% reveals everything in one rub, 33% is more gentle and allows several rubs to reveal the sharpened layer. 33% allows more control than 100%.

Also by adjusting the opacity amount of the layer itself with its opacity slider.

Be inconspicuous and make the adjustment like you never did anything so a bit less is better than super sharp and super obvious. 70% may work for that.

Another technique to reduce your processing "footprints" is to blur the mask you have just done by filters/gaussian blur and set to suit to blur the edges of the mask to prevent harsh outlines of sharpened/not sharpened.

10. Once you are happy with the result flatten the image so its
layers/flatten image.

The above is a standard routine on almost any astro image so its worth the effort to learn it.

Its also worth knowing that control/H hides the selection so you can see more clearly what effect you are having on stars
when processing them after selection. Control/H a 2nd time reveals the selection squiggly lines

There is a more advanced sharpening technique I was reading in a Photoshop CS4 book. It had to do with using the luminosity mode for layers. I haven't learnt it myself yet but it may be worth looking into.

Noel Carboni actions have a similar action to the above under - increase star colour, reduce star sizes and increase local contrast. So if you don't want to get into it that hard then use them and they run through pretty similarly to the above to do their job but you will have less control especially with the increase local contrast action. Still, they are awesome value at only US$20 or so.

Greg.