Tutorial:
Siril for pre processing astrophotos

Content:

1. My personal parameters when using Siril
2. Desaturate stars
3. Star Color calibration
4. Calibration tests
5. Dark calibration analyze
6. Adjust the flat calibration with a fake bias
7. Meridian flip and stacking
8. Deconvolution
9. Post Processing
10. Star Synthesis
11. To be continued

Note:
I take no responsibility or liability for what are written here, you use the information at your own risk!

10, Star Synthesis

A star is a point object in small telescopes, normally a few milliarcseconds in diameter. The optics has limitations, the atmosphere blur, camera sensor make other things. On your fine astrophoto it appears as a 4" diameter star or even worse. That's okay, but at the edges can optical aberration cause stars to look elongated or comet shaped. This can be corrected to some degree. Siril can make a new star to you by calculate the objects in your image and replace them with a new synthetic ideal star. Note: it's always better to correct the optics, this is something you do when it can't be done.

Siril version 1.4.2

Siril's Full Resynthesis:

The image must be linear, plate solved, color calibrated and a starless / starmask separation applied.

My optics is of high quality and don't make any elongated stars at the edges. I took a saturated star to do this test on. A line for analyze can be drawn across the star with the tool the arrow point at.

From the drawn line it calculate an intensity profile, mono or color can be selected. We can direct see that the star is saturated, the green spectrum has it's top clipped, that cause the star's edge to look red. We can also see that this star's FWHM (Full Width Half Max) is about 20" (6 pixels), other not saturated stars has a FWHM of 13". What will happen if we replace this saturated blurred star with a synthetic star?

It's very common that bright stars in the green channel has clipped. A color camera is more sensitive in the green wave length, that's why it clip first of the three color channels.

Open the setup for "Full Resynthesis" tool.

Click on the the "Three star" symbol and it look for all stars in the image. Normally we had looked for non round stars, the filter can be adjusted for that here: "Roundness threshold". The pixel scale of my setup is about 3.5" / pixel.

Go back to the "Full Resynthesis" and run the filter, it can take some time.

The red, green and blue dots in the background came from that I didn't dithered the sub images when I took them, it's hot pixels. They are thrown away during the stacking process if it has been taking with dithering. Normally I always do that, but obviously I forgot it this time.

After a couple of minutes an image with these new synthetic stars appear. A new intensity profile line is drown. The synthetic star's profile is Gaussian, that's the profile a point object always give. The star has a little bit tighter and more clean profile. These stars can now be put together with the starless image. This was just a test, normally this tool should be used for correction of elongated stars. Does it sounds interesting for you, more details on Siril's own page: Siril Synthetic Stars.

This line profile tool was new to me, earlier I had always used Fitswork to get an intense profile. Worked very good and now I don't have to open up a special tool for this.