Posted in Astrophoto Log, Technique on April 17th, 2009 by Northwest Astro
Badly Processed M81
Better Processing on M81
Take a quick look at the two images to the right. One is a bad example of how to process M81. Maybe not bad for a first attempt (which it was for me), but not a great galaxy in general. It has very little in the way of features, and you really only get a vague idea of what shape and details it has. One is a better example of how to process the same target. While it is noisy, it still has much more detail visible in the galaxy. You can see the characteristic dust lanes near the core, and more detail in the spiral arms. Overall, it is a much better image.
Both images come from the exact same data. 16 exposures of 5 minutes each, aligned and stacked. In fact, they both come from the same final stacked image. So why is there such a huge difference between the two?
When processing, I’m finding out the hard way that you can’t be too heavy-handed when you do the final stretching work and post-processing. If you start doing that, then you risk losing the very detail you are trying to extract from the noise.
Don’t fall for the temptation to do curves adjustments in one or even two passes. Take time on it, and do it very gradually if you have to. And resist the temptation to ‘clean up’ the image too early. Noise reduction, darkening the sky background, and so on should be left for last once you have exposed the target. Do it too early and you will lose important faint details, or will simply wash out other details.
Now, that isn’t to say you can perform miracles with bad data. While the second image is better processed, I can only do so much due to the noise in the stacked image, and the fact that the galaxy’s data is buried in the noise. My method of collecting Darks for the DSLR seems to not really do much to combat the noise. So I’ll need to take longer exposures to also help get the signal above the noise. On top of that, the focal reducer used tended to bloat the stars and probably had an adverse affect on the galaxy as a result. Since this image was originally taken, I’ve moved to a different focal reducer which has helped there.
The upside is that with extra practice on existing data (one reason to always keep around raw data as long as possible!), I’ve gotten better at processing, and should be able to tackle M81/M82 again in the near future when the weather cooperates and make a second real attempt.
