Astronomical Image Processing series
Updated: May 6, 2021
Today I would be presenting you an entry level image processing article which would help beginners to understand how we process and bring out the details in astronomical data ,there are lot of challenges that we face during processing e.g.- gradients light pollution, noise etc. These are the common issues that we face when we have Pre-Processed our data (Pre-Processing stands for Calibrating the raw files with dark, bias and flats) Folks who shoot from LP skies have to capture lots of data to recover the lost signal, I wouldn’t go into much details about capturing as today I would like to show we pull out the details from a nebula , The data was shot by my friend Zhuoqun Wu remotely from Chile it was around 3.7 hrs from SQM 22skies of chile, the data is clean and easy to work so lets begin
Here is our Luminance Tiff, when ever you open your tiff your first step should be stretching the image using levels and curves, though I use levels every time as it is easy and hassle free unlike curves
Levels dialogue box can be opened using ctrl+L or go to image>adjustment>levels
after you open the levels you might notice a mountain like
structure that is the histogram of your tiff file
The second thing you would notice would be 3 small triangles from the left black point middle Gray point and last one is white point, we don’t use the white point the first 2 are enough to stretch your data
Now when you open your data you might notice that your histogram would be narrow unlike the figure 1 it indicates that your data needs stretching and using gray point your have to slide in to the left and press ok , now you have to open the levels again and see the histogram as well as data has changed and its slowly details are coming out , now next step is using black point you have to slide in to right till where the histogram starts (warning do not slide in further it might clip your data and your details might be lost, we have to be careful when stretching) We have to repeat the steps twice or thrice depending upon how much data is hidden there.
Now we have applied stretching to the final image and it looks like this
Now its time to clear some Gradients in the image, to remove the gradients I use Pixinsight Automatic Background Extractor or
ABE, you can find it in < All Process> menu
Here we have the ABE , we don’t have to touch anything except the Target Image correction option ad we have to choose the
Subtraction option and then click on the apply button or press f5
In the Above Image the Gradients have been removed using this tool , removing them is quite important as these glow create lot of hassle in our image
Now you can save the work as Luminance ABE stretch and later apply it on RGB
Now its time to Stretch the R,G,B master tiff , there procedure is exactly same no changes in these , after you stretch the image
its time to combine the Grayscale image into RGB it is easy to do it !
Now Copy any of your red green or blue tiff using ctrl+a select all and ctrl+c then proceed to file> new , the image data is copied here and you do not need to make much adjustments here other than changing mode to RGB like shown in the above picture After you Press OK a new image would open up it would be completely white in color, now you have to go to option channel that is next to layers Now you would spot Red Green and Blue channels there , copy the red tiff and paste in the red channel same will be for Green and blue. your image will be formed like this
Here is our color image formed, but wait it has some weird color bias there, It can be removed by few methods by combining data properly ,by running ABE,DBE or by color balance we would use ABE its not 100% accurate but it works sometimes on such objects.
here is our color bias corrected image looks bit green no problem, we can slightly adjust it using color balance as shown below
Using the color balance tool we can make fine adjustments and its a important tool for maintaining the right color balance in the image now our color balance is set its time to combine the luminance to our RGB, remember be careful in combining the L to RGB as a strong luminance can blow the colors of RGB, so it advise to blend in using 50%first time and then again 50%
Since this was a very basic walkthrough i wont be adding more complicated task like sharpening data pulling techniques right now, i would be releasing series of articles where more important topics will be discussed there like: Noise reduction sharpening ,masking etc if you have more questions you can write directly to me at my email firstname.lastname@example.org i also offer hourly 1-1 Image processing classes to students