M 101 Luminance

[Jim McMillan]

I'm kinda excited about a specific processing step I've been trying recently and thought I'd share it. It's really easy to do and I think it makes a big difference.

Most times, we want to tease out as much detail as possible by applying some type of sharpening process. However, it's really easy to over-do it and introduce artifacts, leaving it look over-cooked and unnatural.

Typical astroimages have a mixture of high and low SNR (signal to noise ratio) areas. High SNR areas can be sharpened relatively aggressively with good results while low SNR areas usually end up looking worse with any kind of sharpening.

I've recently discovered one of the parameters available with MaxIm's unsharp mask that makes it a really effective tool. One can specify the adu range to apply the unsharp mask. What that means is you can sharpen the high SNR areas while ignoring the low SNR areas. Taking it a step farther, one can quite aggressively sharpen the brightest areas and back off on medium SNR areas.

As an example, relatively little processing was done on this image acquired over the weekend. It is 35, 10 min exposures. It would certainly benefit from lots more exposure and improve the SNR in the dimmer areas. However, this demonstrates the process pretty effectively.

The processing steps (all done in MaxIm):

1) Calibrate, remove blooms, auto-gradient removal, remove hot/dead pixels, align, stack.

2) DDP (no sharpening).

3) Unsharp mask 2 times.

The first time was done at a custom level of 20, with the maximum adu at 65000 (max for my camera), the minimum adu at 16000. The lower limit was chosen to include sharpening the brighter areas of the spiral arms, but ignoring the dimmer areas. The custom level of 20 was chosen through experimentation - anything stronger started to look over-done.

The second time I set the minimum adu at 35000 and set the level to "mild" (which is 5). The effect was to sharpen only the galaxy core fairly aggressively.

The only other thing I did was add the one star spike in PhotoShop Elements - just because I wanted to!

Anyway, try it. Once I had the stacked image, it took maybe 5 minutes to produce this "final" image.

Jim