Processing Messier 33 in PixInsight - A Tutorial
Posted 08 September 2013 - 12:59 PM
This is not an ideal format for this for at least two reasons. First, there is a limit of one picture per post. And secondly, the max size for a picture is 800x800. So I'll have to be reducing the size of some of the results/and or cropping them for display here.
The data itself was taken on the night of September 4th, 2013. A Celestron Powerstar III 8" SCT with a f/6.3 reducer flattener was used so the focal length is close to 1260mm. All subs were 5 minutes in length. Luminosity was unbinned (1x1). R, G, and B were all binned 2x2. Flats were taken for each of those 4 channels individually. Total exposure time for luminosity was 1 hour 55 minutes. Each color channel had 45 minutes of time. I can get by with less than that and sometimes go with just 30 minutes color. However, I like using 45 minutes when I can get it because Winsorized sigma clipping works better.
This tutorial is going to start with the stacked, calibrated images (darks, flats, flat darks). I happen to really like doing the calibration and stacking within PixInsight but I know others still use other tools like Deep Sky Stacker.
The channel stacks are available on DropBox here:
Messier 33 Channel Stacks on Dropbox
I'm a strong believer that you learn better when you actually follow along and do the procedure. Still, it is your choice.
It is also possible I will have false starts and will back up in the processing. That is fine if it occurs. That will show how I think about the processing.
To a certain extent my workflow depends on what happens with the image, but here is a skeleton outline of some of the likely steps:
Crop out stacking artifacts
Dynamic Background Extraction (or Automatic Background Extraction)
Create Point Spread Function for Deconvolution
Create Star Mask for Deconvolution
Create Overall Mask for Deconvolution
Channel Combine the separate color stacks
Dynamic Background Extraction or Automatic Background Extraction to get rid of gradients
Saturation done using the Curves Tool
L and RGB
Register (Align) the separate L and RGB images
Curves (on the luminosity portion) to adjust contrast
Posted 08 September 2013 - 01:12 PM
If you bring up the luminosity stack and do a [ctrl]a (which does an auto-stretch, you will notice that there are black borders on some of the sides. They are caused by when we stack, we are aligning the stars. Unless you got things absolutely perfect when guiding, you have no flexure, and you did not dither, the FOV will be slightly different between the different subs. That mismatch of FOVs will show up as darker borders. In this case, it is most obvious on the left side of the image. This particular image only has minor amounts of it most obvious on the left edge.
Posted 08 September 2013 - 01:22 PM
Posted 08 September 2013 - 01:23 PM
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Posted 08 September 2013 - 01:26 PM
Posted 08 September 2013 - 01:36 PM
This might be a good time to mention another tool that is useful. While [ctrl]a is great for quick looks at the data, sometimes you want more control. The ScreenTransferFunction is used for this.
Posted 08 September 2013 - 01:45 PM
Here are the autostretched results of the Dynamic Crop.
Posted 08 September 2013 - 01:55 PM
Posted 08 September 2013 - 01:57 PM
Posted 08 September 2013 - 02:00 PM
Posted 08 September 2013 - 02:11 PM
Unfortunately, the automatic settings for DBE will also fail miserably. Bring up DBE. Then click on the image. Open up sample generation, and the click on the generate button.
Posted 08 September 2013 - 02:13 PM
Posted 08 September 2013 - 02:14 PM
The trick is to actually place those little samples where we actually need them.
Posted 08 September 2013 - 02:33 PM
Note that I change the tolerance setting under model parameters so that all samples would be good (not red). I also had to change the correction setting under target image correction to subtraction. Otherwise, it won't actually do anything, just show you the background it would subtract.
Posted 08 September 2013 - 02:39 PM
Posted 08 September 2013 - 02:42 PM
Posted 08 September 2013 - 02:52 PM
Thanks for sharing your techniques. One request, could you post at the beginning your 'work flow'? I often don't need the individual frames... but seeing how you tackle going from mono to RGB and linear to non-linear is nice.
Posted 08 September 2013 - 03:17 PM
I've done many a website before, and it is possible this could end up as one or on one if enough people find it useful.
I have added an overall workflow that may or may not need to be tweaked as we go along.
Posted 08 September 2013 - 03:42 PM
Posted 08 September 2013 - 04:48 PM
Posted 08 September 2013 - 07:27 PM
Deconvolution is a way of tightening up the detail in your image. It can make stars smaller, and it can bring out detail in your images.
Perhaps the way to start is to show what happens if you actually use the defaults on an image. It is usually pretty dreadful, and will illustrate why we go to the trouble of all the steps that are about to occur.
If you have followed along, you might want to save your current results. I called mine L_dbe. I save as a 32 bit fit file. I actually save after every operation. There is actually a history explorer, but that won't help if the electric goes out.
Bring up that image and use either [ctrl]a or STF (ScreenTransferFunction) to stretch it so you can see what is happening. Next we want to define a preview. This can be done by pressing [alt]n. Move the cursor to where you want to start. Then press and hold the left mouse button and drag to where you want the preview to end.
Previews are very useful in PixInsight and we will be using them from time to time for a number of purposes.
If successful, you will end up with something like this:
Posted 08 September 2013 - 07:30 PM
Posted 08 September 2013 - 07:35 PM
Now one of the things that makes previews useful, is that because they are not as big as the full image, computationally expensive processing can be tried out before committing the settings to the entire image.
Bring up the deconvolution tool. I have opened mine up to also show the deringing parameters. We aren't currently using them but we will.