So here is the continuation to my previous *blog post*. Check it out first if you haven't already. In this post I will focus primarily on the pre and post processing that I used to get the final results.
This is the image straight off of the camera. Not too exciting but with some Deep Sky Stacker and Photoshop you will see a big transformation.
To increase the signal to noise ratio *image stacking* is a must and to do that, I used a free program called *Deep Sky Stacker*. For the stacking program I collected 57 x 1 minute exposures on the target M42 at ISO 800 and f7.1. These are called light frames. I also took 15 dark frames which help to reduce additional noise and 11 flat frames to help correct vignetting and/or any dust motes that appear on the sensor.
I took the dark frames by putting on the lens cap and covering the camera with a black garbage bag to prevent any stray light from reaching the sensor. Exposure time and ISO was set to the same value as the ones used in the lights and the temperature must be within a few degrees Celsius of the temperature during the light frame exposures. Typically it's best to do the dark frames outside at the end of the imaging session. The flat frames are exposures taken of an evenly lit surface at ISO 100, aperture priority, same f stop as the lights, using the same lens and same focus. I shot those frames the next day by pointing up at low uniform layer of stratus clouds. Many people will shoot flats by pointing at the clear sky in the east after sunset before they start imaging. I have also had success taking flats by pointing at the computer LCD on a blank notepad page, making sure to hold the lens right up the the monitor. There are many different methods to investigate.
Sample Dark and Flat File
Note that for the processing of this image, I wanted to stick with software that daytime photographers already have in their arsenol or programs that are free. These days I have been working primarily in *Pixinsight* for my prepocessing but continuing the post processing in Photoshop. Pixinsight however is a really good all in one solution that is popular with many astrophotographers but like any advanced software, it may take some time to get use to and dedication to master.
In Deep Sky Stacker, I load my lights, flats and darks. After, I'll register the images, uncheck poor quality frames, then stack them. I usually follow the recommended settings that the software provides. The website for the software has a really good manual and yahoo discussion group.
After the stacking process (this can take several minutes). The resulting stacked file will be a 32 bit tif that I convert to 16 bits in order to prepare for work in Photoshop. BTW, I do not do any processing or adjustments to the file in deep sky stacker.
Stretching the image (spreading out the histogram) is a process that I do with multiple curve layers. This is what you need to do to get more of the nebula to show up. The data with all the signal from the nebula is there in the 16bit tif file, it just needs to be teased out.
The first stretch is just a simple curve brightening. I always work with layer adjustments.
Second curve will darken the background by bringing in the lower left hand corner of the histogram. I don't want to make the background black or else you could clip dark details. Try to leave at least 20 points on the left side of the histogram
After multiple iterations of curve adjustment layers of brightening and darkening I end up with this. I created this new layer after doing a CTRL-ALT NE. This consolidates everything that I did with the curves.
Since the colour balance is looking imbalanced I will open up another curve adjustment layer and set the mode to color. Then click on the black eye dropper in the curve layer window and select an area on the image that has no stars or nebulosity. Now click on the white eye dropper and select the centre of a white star or in my case I selected the bright blown out core on the nebula. You can see now that the left side of the histogram has aligned RGB colour. The background is neutralized.
I may continue to stretch the image further before doing a sky gradient removal because as you can see, my light pollution is starting to appear from the bottom left of the frame.
Here I created a layer that replicated the gradient with the help of the dust and scratches filter. That filter removed the stars, then I cloned out the nebula and made a gaussian blur. This resultant frame was then subtracted from the affected layer above with the gradient. It's a time consuming process but there is a much much easier and fool proof way to do this by using a plug in called *GradientXTerminator* by Russel Croman.
With M42 being an object with a very bright core and faint outer nebulosity it is usually a popular task to add a layer mask or two on the core to bring back the details that got blown out by the stretching process. To do that I grabbed a layer from below that wasn't stretched too much and added a blurred mask over the non blown out core. Noise reduction was also done with the help of the camera raw filter in the Photoshop CC version.
Now having a little fun with Google's Nik Color Efex Pro plug in for Photoshop. I ran the detail extractor and was surprised to see more extended dust appear, but along with that dust was also a tremendous amount of noise.
I then ran Nik's Define2 plugin to tame the noise, masked out the effects of the detail extractor on the core of the nebula. I then lowered the opacity of the layer to my taste so that it was visible enough but not over the top.
At this point I saved the file as a PSD file and imported it into Lightroom 5 for cropping, finishing tweeks and posting to my website.
For anyone that would like to see me demonstrate the processing, check out this Google Hangout video where I was a guest on the Photoshop Show. The image processing part starts at 1hr 6min 23sec in. It did a slightly differentworkflow but it will be easier to follow for those that are more visual. Enjoy!!