Introduction

This article aims to illustrate to those who are just starting out with using a consumer digital camera (like a D70) my "quick and dirty" Deep Space image post-processing techniques. I am by no means an expert at this, and much of my knowledge was learned from asking questions on the CCD forum at Cloudy Nights and researching on the internet. However, some techniques I actually learned myself. I hope that the techniques presented here will allow you to get started at processing images and, hopefully, improve upon my technique. As my technique improves, I will update this article or present a new one with the updates.

This article assumes that the image TIF file has already been dark subtracted, aligned, stacked, and combined. I will cover aligning and combining in another article.

Please note that the technique below is far from ideal. In some steps, we attempt to remove the vignetting that is present. However, this does not substitute using flat frames (which I will start doing very soon).

I use two programs for my image processing: Images Plus, and Paint Shop Pro. I use Images Plus to do the initial stretching and Paint Shop Pro to do the final image processing. Images Plus is more than capable at doing all of the image processing by itself, however, I am still learning how to use it and I am pretty comfortable at using Paint Shop Pro at post processing. As I learn more, I will probably use Images Plus more than Paint Shop Pro.

Stretching in Images Plus

The first step after the images have been combined, is to execute digital development in Images Plus. Images Plus does a great job at automatically stretching the image in a way to really bring out the details of an object. Below is a before and after shot of M20 using digital development in Images Plus:

As you can see, in an image that looks pretty bad, automatic digital development did a great job at bringing the details out of the muck. Also, note the overall pink haze in the image. This is from sodium vapor light pollution towards my southern horizon. But we will get to that later. You can fine tweak the parameters in IP, but for now, we will just do the automatic function.

To do this in IP, open the averaged image in Images Plus:

Then select the Color->Brightness Levels and Curves->Digital Development menu.

Select the auto button, then select apply.

Select "Apply" and you will see the image change.

You can play with the sliders and filters in the Digital Development box and select apply and see what changes the image. I usually select a medium or hard filter in this dialog. But for this tutorial, let's keep it simple.

Next, to really bring out the details, I select the Color->Brightness Levels and Curves->Background - Contrast - Stretch... menu which brings up the "Background, Contrast, and Stretch" dialog:

Play around with the Black and White sliders. For this article, I moved the white slider down to about 29308.

The nebula's details and color start to come out more. However, the noise is becoming more apparent in the image which is causing some of the stars to look wrong (wrong colors) and there is more graininess. Because my shot of M20 only had 7 exposures of 30 seconds each (for an effective exposure of only 3.5 minutes), my signal to noise ratio is not high enough. If I would have had more exposures, I would get less noise and more details. But let's see what we can do with this. Here is what we get after the stretch:

Final processing in Paint Shop Pro

Open the stretched image in Paint Shop Pro:

So far, the image looks pretty horrible. We have a lot of noise and the sky glow has become an orange color at the center of the image. Lets get rid of the orange background glow. I came up with a pretty good trick in Paint Shop Pro to do this.

Select the Adjust->Blur->Gaussian Blur menu and change the radius to 100. This effectively diffuses the entire image and gives you a good idea of the overall levels of the orange glow in the back ground.

Select ok. The image now looks like this:

Select the dropper tool (Hit the "E" key on your keyboard) and put the dropper off center of the bright glow in the middle. You do not want to select the very middle as that includes the object itself. I usually go about half way between the dark area and the bright center.

You should see 4 numbers show up in the dropper tool. Write the RGB numbers down. I got R: 141, G: 69, B: 33. Select Edit->Undo (or Control-Z) to undo the Guassian Blur. Now select the Adjust->Brightness and Contrast->Curves... menu.

Select the Channel List box and change it to Red. In the "Input" field, type in the R value you got from the dropper tool. My example I had 141. Leave output to 0.

Do the same for Green and Blue using the cooresponding values for G and B from the dropper tool.

Select ok. WOW! Look what it did to the image:

I do a quick Histogram Stretch by selecting Adjust->Brightness and Contrast->Histogram Stretch (you can also do a Shift-T combination on the keyboard). Now we do some additional stretching by selecting Adjust->Brightness and Contrast->Histogram Adjustment (or Control-Shift-H).

I usually change the midtones to -10 and the gamma to 1.10 to 1.30. It is up to you.

Select ok. Save the image. Now we are done.

Closing Comments

Ok, so this image will most likely not be published in Sky and Telescope or Astronomy. Also, there are those who have very nice equipment including specially-cooled CCD cameras and high end telescopes that produce amazing images (Rob Gendler for example). However, considering that you have not invested thousands into your imaging equipment, you still have a decent image. These techniques don't only apply to using a D70. You can also apply them to using some of the other consumer CCD cameras.

I hope that somehow this simple article can get you started to processing your own images. Even if you live in a light polluted area, you can potentially obtain very nice images that your friends and family, and more importantly, you, will enjoy.

Good luck and happy imaging!

Jason Hissong
07/18/2004