OK, you found your target and managed to gather some data, now how should you proceed to process your image? Well firstly you need to establish a routine to get your newly captured data in the best possible condition for processing into that beautiful image that you have seen in the lastest astronomy magazine.
Here is a typical step by step imaging route, post image capture.
1. Image calibration, part of the image that you captured is unwanted noise. The unwanted noise has many sources that will be discussed as an advanced topic later. At a bear minimum you need to be shooting BIAS frames and DARK frames. These are then used to cancel out the majority of the camera's noise. To improve image quality further, you should also shoot FLAT frames to record optical defects such as dust motes and vignetting.
Calibration frames
- BIAS
- DARKS
- FLATS
2. Once your LIGHTS have been calibrated they should look a lot cleaner, most of the noise will have been removed along with the HOT & COLD pixels. Now it's time to stack the frames to produce a MASTER frame. Depending upon how many frames that you managed to gather you have several choices to combine them statistically.
- Sum
- Mean
- Median
- Some form or other of Sigma clipping
With a limited number of frames, lets say less than 4, Sum is your best option, up to 6 frames MEAN and MEDIAN come in to play and with 10 or more frames SIGMA CLIP can work its magic. Sigma clipping is a way to further eliminate noise or cosmic ray hit or even satellite tracks. Sigma Clipping only accepts data that is present in all of the frames, everything else is removed.
3. Now that you have your stacked reference image it's time to see just how good it is? Using your favorite image processing tool, stretch the image to reveal all of the data that you have captured. Be very careful here!!! Do not clip the black point as this will kill your background making your stars look over processed and saturated.
Here you can see that there is still some space on the far left of the histogram.
4. Now that your masters are stretched it's time to check for the dreaded gradient! Gradients appear for all sorts of reasons, mostly light pollution is the main culprit. Some software tools are better than others at removing them. However you plan to tackle them, you need to do it at this stage.
5. Combine your nice MASTER images into a colour image using the software tool of your choice.
6. Colour balance the image to ensure a natural look for all of the components: Stars, Nebula and background etc...
7. Perform as many tweaks or enhancements as you like.
I will create an advance routine shortly with more images depicting the various steps in the process.
However, I have just learnt that Sir Patrick Moore has past away peacefully at his home in Selsey, West Sussex this afternoon.
RIP Sir Patrick Moore, an inspiration to many young and now older astronomers alike.
Clear but very sad skies
Neil.
