First outing of the new year: "The Creations"

(Sebastien Guyader) #1

The story

Twenty four hours pas new year’s eve, I was out for my first photography outing of 2019. I wanted to take some starry sky photos, and decided to go to the Pointe des Châteaux area. My goal was to take pictures of the Milky Way while having some interesting foreground.
I used the photopills application on my smartphone to plan my trip and see where the Milky Way would intersect with the horizon so I could get both the Milky Way and some point of interest in the same frame. The POI I wanted to feature is a big concrete christian cross.
I entitled this post “The Creations” because in the same picture I see the duality between the religious (here Christian) and the scientific viewpoints on the Creation:

  • just above the foreground hill, at the bottom of the Milky Way you can see a reddish area: it is called the Carina nebula, which is an H II region, in which stars are forming along with protoplanetary disks;
  • if you let your eye follow the hill to the left, you will see the 10 meters-high concrete cross on the clifftop, which was first erected by catholic Christians in 1946, and replaced by the one you see here in 2002.

The technical stuff

This image is a stack of many different frames, first processed in Siril 0.9.10-rc2, using:

  • 10 light frames (Samyang 12mm lens, f/2.0, iso 800, 10’’ exposure),
  • 200 bias (offset) frames (1/8000),
  • 25 dark frames (iso 800, 10’’ exposure),
  • 25 flat field frames (f/2.0).

Siril demosaiced and processed the raw files (Fujifilm X-T2) directly, which is great (support for x-trans raw files was added earlier in 2018). I just used one the scripts posted on their website, which takes care of everything.
I ran the script twice : once with star registration, to get the stars aligned, and then without registration for using the stacked foreground.

The 2 resulting 16-bit TIFF files were slightly processed in RawTherapee (exposure, tone curve, contrast, white balance).

The final step was to merge both TIFF’s in the GIMP (masking the foreground of the star-registration image, manual removal of hot/stuck pixels, ‘creative’ midtones dodging in the foreground, cropping and resizing to 1920x1080).

I hope you enjoy the result.

Processing a nightscape in Siril: a tutorial
[Play Raw] Mo and better blending?
(Jonas Wagner) #2

The foreground is still a bit noisy but the image is fantastic! Great job!


that is some fantastic work. I was looking for ways to improve my night/astronomy shots and it really looks like I should give SIril a try! thank you for that…


The more I look at this, the more questions I have. :slight_smile: What are “bias (offset) frames”? How about light vs dark? They seem to be at the same exposure (iso 800 10"), why makes the dark frames darker? f/ stops? What about “flat field frames”?

I guess you used a tripod for this, right? Did you also use a remote? How long did you wait for the camera to settle? Does it matter? I calculate you took at least 60 frames at around 10 seconds each, which means at least 10 minutes delay, during which time the whole sky will have rotated quite significantly (2.5° to be precise) - is that what Siril deals with? How does it deal with the “foreground” (the earth landscape) which doesn’t rotate?

Thank you so much and sorry for flooding you with questions…

(Sebastien Guyader) #5

Thanks anarcat.
Regarding the different types of “calibration” frames (darks, flats, bias), your can have a read here. Regarding the number of frames of each type needed, there’s still some debate. Because people asked me on dpreview why I took so many calibration frames, and that I just used what I saw on websites, I thought I could do some measurements myself. Below is a chart of noise level measurements (done in Siril) as function of the number of dark or bias frames stacked:

And for light frames:

Based on this, I think 20 dark and flat frames are a good number, 10-20 for light frames, and 100 for bias frames. Bias frames don’t take long to be made. Also, when Siril has run the whole process, “master” calibration frames are saved, so you can delete the original frames and just cut the single master frames which contain the information you need, and you can reprocess your light frames at will using those (even for other shootings, as long as the shooting conditions don’t change significantly).

I took only 10 light frames at 10 seconds, using either the integrated intervallometer or the 2 sec. delay timer, on a tripod, with just 1-2 sec of settling time. My camera is mirrorless, so there’s no risk of mirror-induced vibration.
Indeed there’s a visible rotation of the earth, so I ran Siril twice: once with registration (i.e. star detection and aligning), and the second time without alignment to have a stacked foreground. Then I merged the two resulting images in Gimp, using a hand drawn mask, to have both stars aligned and a sharp backround.

Since my original post, I’ve learned more stuff with Siril, and reprocessed my frames. Here’s the current result, this time with different tweaks in Rawtherapee, and without dodging in Gimp, just masking the foreground:


Wow, okay so that’s more involved than I expected. The FAQ’s TL;DR:

  • Light frames: normal pictures with the stars in it. those get stacked
  • Dark frames: same settings, but with the lens cap on (10-20 shots)
  • Bias frames: same as dark frames, but at fastest speed, e.g. 1/8000s (10-20 shots)
  • Flat frames: same focus as light frames, but overexposed (10-20 shots)

Those numbers are according to the FAQ, but you seem to show there are still good improvements to be made above those numbers, particularly for bias frames.

Thanks for the hand-holding! I guess I got more reading to do as well… :slight_smile:

Oh and congrats on the new rendering, it does look much better, especially in the upper-left corner, where the smudge is totally gone…

(Sebastien Guyader) #7

Regarding the mushy corners in the first version of my image, I think the trouble comes from the heavy vignetting introduced by my lens at f/2.0, which requires a heavy correction by the flat field frames, making noise more visible if I try to render the scene brighter.